new move analyser2 (#20471)

* produce better code for closure environment creation
* new 'first write' analysis; 
* scope based move analyser
* code cleanup

Co-authored-by: ringabout <43030857+ringabout@users.noreply.github.com>

10 files changed, 439 insertions, 670 deletions

diff --git a/compiler/aliasanalysis.nim b/compiler/aliasanalysis.nim
new file mode 100644
index 000000000..22a66e4df
--- /dev/null
+++ b/compiler/aliasanalysis.nim
@@ -0,0 +1,124 @@
+
+import ast
+
+import std / assertions
+
+const
+  PathKinds0* = {nkDotExpr, nkCheckedFieldExpr,
+                 nkBracketExpr, nkDerefExpr, nkHiddenDeref,
+                 nkAddr, nkHiddenAddr,
+                 nkObjDownConv, nkObjUpConv}
+  PathKinds1* = {nkHiddenStdConv, nkHiddenSubConv}
+
+proc skipConvDfa*(n: PNode): PNode =
+  result = n
+  while true:
+    case result.kind
+    of nkObjDownConv, nkObjUpConv:
+      result = result[0]
+    of PathKinds1:
+      result = result[1]
+    else: break
+
+proc isAnalysableFieldAccess*(orig: PNode; owner: PSym): bool =
+  var n = orig
+  while true:
+    case n.kind
+    of PathKinds0 - {nkHiddenDeref, nkDerefExpr}:
+      n = n[0]
+    of PathKinds1:
+      n = n[1]
+    of nkHiddenDeref, nkDerefExpr:
+      # We "own" sinkparam[].loc but not ourVar[].location as it is a nasty
+      # pointer indirection.
+      # bug #14159, we cannot reason about sinkParam[].location as it can
+      # still be shared for tyRef.
+      n = n[0]
+      return n.kind == nkSym and n.sym.owner == owner and
+         (n.sym.typ.skipTypes(abstractInst-{tyOwned}).kind in {tyOwned})
+    else: break
+  # XXX Allow closure deref operations here if we know
+  # the owner controlled the closure allocation?
+  result = n.kind == nkSym and n.sym.owner == owner and
+    {sfGlobal, sfThread, sfCursor} * n.sym.flags == {} and
+    (n.sym.kind != skParam or isSinkParam(n.sym)) # or n.sym.typ.kind == tyVar)
+  # Note: There is a different move analyzer possible that checks for
+  # consume(param.key); param.key = newValue  for all paths. Then code like
+  #
+  #   let splited = split(move self.root, x)
+  #   self.root = merge(splited.lower, splited.greater)
+  #
+  # could be written without the ``move self.root``. However, this would be
+  # wrong! Then the write barrier for the ``self.root`` assignment would
+  # free the old data and all is lost! Lesson: Don't be too smart, trust the
+  # lower level C++ optimizer to specialize this code.
+
+type AliasKind* = enum
+  yes, no, maybe
+
+proc aliases*(obj, field: PNode): AliasKind =
+  # obj -> field:
+  # x -> x: true
+  # x -> x.f: true
+  # x.f -> x: false
+  # x.f -> x.f: true
+  # x.f -> x.v: false
+  # x -> x[0]: true
+  # x[0] -> x: false
+  # x[0] -> x[0]: true
+  # x[0] -> x[1]: false
+  # x -> x[i]: true
+  # x[i] -> x: false
+  # x[i] -> x[i]: maybe; Further analysis could make this return true when i is a runtime-constant
+  # x[i] -> x[j]: maybe; also returns maybe if only one of i or j is a compiletime-constant
+  template collectImportantNodes(result, n) =
+    var result: seq[PNode]
+    var n = n
+    while true:
+      case n.kind
+      of PathKinds0 - {nkDotExpr, nkBracketExpr}:
+        n = n[0]
+      of PathKinds1:
+        n = n[1]
+      of nkDotExpr, nkBracketExpr:
+        result.add n
+        n = n[0]
+      of nkSym:
+        result.add n
+        break
+      else: return no
+
+  collectImportantNodes(objImportantNodes, obj)
+  collectImportantNodes(fieldImportantNodes, field)
+
+  # If field is less nested than obj, then it cannot be part of/aliased by obj
+  if fieldImportantNodes.len < objImportantNodes.len: return no
+
+  result = yes
+  for i in 1..objImportantNodes.len:
+    # We compare the nodes leading to the location of obj and field
+    # with each other.
+    # We continue until they diverge, in which case we return no, or
+    # until we reach the location of obj, in which case we do not need
+    # to look further, since field must be part of/aliased by obj now.
+    # If we encounter an element access using an index which is a runtime value,
+    # we simply return maybe instead of yes; should further nodes not diverge.
+    let currFieldPath = fieldImportantNodes[^i]
+    let currObjPath = objImportantNodes[^i]
+
+    if currFieldPath.kind != currObjPath.kind:
+      return no
+
+    case currFieldPath.kind
+    of nkSym:
+      if currFieldPath.sym != currObjPath.sym: return no
+    of nkDotExpr:
+      if currFieldPath[1].sym != currObjPath[1].sym: return no
+    of nkBracketExpr:
+      if currFieldPath[1].kind in nkLiterals and currObjPath[1].kind in nkLiterals:
+        if currFieldPath[1].intVal != currObjPath[1].intVal:
+          return no
+      else:
+        result = maybe
+    else: assert false # unreachable
+
diff --git a/compiler/ast.nim b/compiler/ast.nim
index 399d7e561..e1d982a43 100644
--- a/compiler/ast.nim
+++ b/compiler/ast.nim
@@ -508,7 +508,8 @@ type
                        # the flag is applied to proc default values and to calls
     nfExecuteOnReload  # A top-level statement that will be executed during reloads
     nfLastRead  # this node is a last read
-    nfFirstWrite# this node is a first write
+    nfFirstWrite # this node is a first write
+    nfFirstWrite2 # alternative first write implementation
     nfHasComment # node has a comment
 
   TNodeFlags* = set[TNodeFlag]
@@ -1075,7 +1076,8 @@ const
                                       nfDotSetter, nfDotField,
                                       nfIsRef, nfIsPtr, nfPreventCg, nfLL,
                                       nfFromTemplate, nfDefaultRefsParam,
-                                      nfExecuteOnReload, nfLastRead, nfFirstWrite}
+                                      nfExecuteOnReload, nfLastRead,
+                                      nfFirstWrite, nfFirstWrite2}
   namePos* = 0
   patternPos* = 1    # empty except for term rewriting macros
   genericParamsPos* = 2
diff --git a/compiler/ccgcalls.nim b/compiler/ccgcalls.nim
index ed5a5b079..81e74d089 100644
--- a/compiler/ccgcalls.nim
+++ b/compiler/ccgcalls.nim
@@ -313,7 +313,7 @@ proc genArgNoParam(p: BProc, n: PNode; result: var Rope; needsTmp = false) =
     initLocExprSingleUse(p, n, a)
     addRdLoc(withTmpIfNeeded(p, a, needsTmp), result)
 
-from dfa import aliases, AliasKind
+import aliasanalysis
 
 proc potentialAlias(n: PNode, potentialWrites: seq[PNode]): bool =
   for p in potentialWrites:
diff --git a/compiler/dfa.nim b/compiler/dfa.nim
index 669207151..57fd3ae2b 100644
--- a/compiler/dfa.nim
+++ b/compiler/dfa.nim
@@ -9,27 +9,20 @@
 
 ## Data flow analysis for Nim.
 ## We transform the AST into a linear list of instructions first to
-## make this easier to handle: There are only 2 different branching
+## make this easier to handle: There are only 3 different branching
 ## instructions: 'goto X' is an unconditional goto, 'fork X'
 ## is a conditional goto (either the next instruction or 'X' can be
-## taken). Exhaustive case statements are translated
+## taken), 'loop X' is the only jump that jumps back.
+##
+## Exhaustive case statements are translated
 ## so that the last branch is transformed into an 'else' branch.
 ## ``return`` and ``break`` are all covered by 'goto'.
 ##
-## Control flow through exception handling:
-## Contrary to popular belief, exception handling doesn't cause
-## many problems for this DFA representation, ``raise`` is a statement
-## that ``goes to`` the outer ``finally`` or ``except`` if there is one,
-## otherwise it is the same as ``return``. Every call is treated as
-## a call that can potentially ``raise``. However, without a surrounding
-## ``try`` we don't emit these ``fork ReturnLabel`` instructions in order
-## to speed up the dataflow analysis passes.
-##
 ## The data structures and algorithms used here are inspired by
 ## "A Graph–Free Approach to Data–Flow Analysis" by Markus Mohnen.
 ## https://link.springer.com/content/pdf/10.1007/3-540-45937-5_6.pdf
 
-import ast, intsets, lineinfos, renderer
+import ast, intsets, lineinfos, renderer, aliasanalysis
 import std/private/asciitables
 
 when defined(nimPreviewSlimSystem):
@@ -37,12 +30,12 @@ when defined(nimPreviewSlimSystem):
 
 type
   InstrKind* = enum
-    goto, fork, def, use
+    goto, loop, fork, def, use
   Instr* = object
-    n*: PNode # contains the def/use location.
     case kind*: InstrKind
-    of goto, fork: dest*: int
-    else: discard
+    of goto, fork, loop: dest*: int
+    of def, use:
+      n*: PNode # contains the def/use location.
 
   ControlFlowGraph* = seq[Instr]
 
@@ -59,9 +52,10 @@ type
 
   Con = object
     code: ControlFlowGraph
-    inTryStmt: int
+    inTryStmt, interestingInstructions: int
     blocks: seq[TBlock]
     owner: PSym
+    root: PSym
 
 proc codeListing(c: ControlFlowGraph, start = 0; last = -1): string =
   # for debugging purposes
@@ -69,7 +63,7 @@ proc codeListing(c: ControlFlowGraph, start = 0; last = -1): string =
   var jumpTargets = initIntSet()
   let last = if last < 0: c.len-1 else: min(last, c.len-1)
   for i in start..last:
-    if c[i].kind in {goto, fork}:
+    if c[i].kind in {goto, fork, loop}:
       jumpTargets.incl(i+c[i].dest)
   var i = start
   while i <= last:
@@ -80,12 +74,12 @@ proc codeListing(c: ControlFlowGraph, start = 0; last = -1): string =
     case c[i].kind
     of def, use:
       result.add renderTree(c[i].n)
-    of goto, fork:
+      result.add("\t#")
+      result.add($c[i].n.info.line)
+      result.add("\n")
+    of goto, fork, loop:
       result.add "L"
       result.addInt c[i].dest+i
-    result.add("\t#")
-    result.add($c[i].n.info.line)
-    result.add("\n")
     inc i
   if i in jumpTargets: result.add("L" & $i & ": End\n")
 
@@ -93,181 +87,13 @@ proc echoCfg*(c: ControlFlowGraph; start = 0; last = -1) {.deprecated.} =
   ## echos the ControlFlowGraph for debugging purposes.
   echo codeListing(c, start, last).alignTable
 
-proc forkI(c: var Con; n: PNode): TPosition =
+proc forkI(c: var Con): TPosition =
   result = TPosition(c.code.len)
-  c.code.add Instr(n: n, kind: fork, dest: 0)
+  c.code.add Instr(kind: fork, dest: 0)
 
-proc gotoI(c: var Con; n: PNode): TPosition =
+proc gotoI(c: var Con): TPosition =
   result = TPosition(c.code.len)
-  c.code.add Instr(n: n, kind: goto, dest: 0)
-
-#[
-
-Join is no more
-===============
-Instead of generating join instructions we adapt our traversal of the CFG.
-
-When encountering a fork we split into two paths, we follow the path
-starting at "pc + 1" until it encounters the joinpoint: "pc + forkInstr.dest".
-If we encounter gotos that would jump further than the current joinpoint,
-as can happen with gotos generated by unstructured controlflow such as break, raise or return,
-we simply suspend following the current path, and follow the other path until the new joinpoint
-which is simply the instruction pointer returned to us by the now suspended path.
-If the path we are following now, also encounters a goto that exceeds the joinpoint
-we repeat the process; suspending the current path and evaluating the other one with a new joinpoint.
-If we eventually reach a common joinpoint we join the two paths.
-This new "ping-pong" approach has the obvious advantage of not requiring join instructions, as such
-cutting down on the CFG size but is also mandatory for correctly handling complicated cases
-of unstructured controlflow.
-
-
-Design of join
-==============
-
-block:
-  if cond: break
-  def(x)
-
-use(x)
-
-Generates:
-
-L0: fork lab1
-  join L0  # patched.
-  goto Louter
-lab1:
-  def x
-  join L0
-Louter:
-  use x
-
-
-block outer:
-  while a:
-    while b:
-      if foo:
-        if bar:
-          break outer # --> we need to 'join' every pushed 'fork' here
-
-
-This works and then our abstract interpretation needs to deal with 'fork'
-differently. It really causes a split in execution. Two threads are
-"spawned" and both need to reach the 'join L' instruction. Afterwards
-the abstract interpretations are joined and execution resumes single
-threaded.
-
-
-Abstract Interpretation
------------------------
-
-proc interpret(pc, state, comesFrom): state =
-  result = state
-  # we need an explicit 'create' instruction (an explicit heap), in order
-  # to deal with 'var x = create(); var y = x; var z = y; destroy(z)'
-  while true:
-    case pc
-    of fork:
-      let a = interpret(pc+1, result, pc)
-      let b = interpret(forkTarget, result, pc)
-      result = a ++ b # ++ is a union operation
-      inc pc
-    of join:
-      if joinTarget == comesFrom: return result
-      else: inc pc
-    of use X:
-      if not result.contains(x):
-        error "variable not initialized " & x
-      inc pc
-    of def X:
-      if not result.contains(x):
-        result.incl X
-      else:
-        error "overwrite of variable causes memory leak " & x
-      inc pc
-    of destroy X:
-      result.excl X
-
-This is correct but still can lead to false positives:
-
-proc p(cond: bool) =
-  if cond:
-    new(x)
-  otherThings()
-  if cond:
-    destroy x
-
-Is not a leak. We should find a way to model *data* flow, not just
-control flow. One solution is to rewrite the 'if' without a fork
-instruction. The unstructured aspect can now be easily dealt with
-the 'goto' and 'join' instructions.
-
-proc p(cond: bool) =
-  L0: fork Lend
-    new(x)
-    # do not 'join' here!
-
-  Lend:
-    otherThings()
-    join L0  # SKIP THIS FOR new(x) SOMEHOW
-  destroy x
-  join L0 # but here.
-
-
-
-But if we follow 'goto Louter' we will never come to the join point.
-We restore the bindings after popping pc from the stack then there
-"no" problem?!
-
-
-while cond:
-  prelude()
-  if not condB: break
-  postlude()
-
---->
-var setFlag = true
-while cond and not setFlag:
-  prelude()
-  if not condB:
-    setFlag = true   # BUT: Dependency
-  if not setFlag:    # HERE
-    postlude()
-
---->
-var setFlag = true
-while cond and not setFlag:
-  prelude()
-  if not condB:
-    postlude()
-    setFlag = true
-
-
--------------------------------------------------
-
-while cond:
-  prelude()
-  if more:
-    if not condB: break
-    stuffHere()
-  postlude()
-
--->
-var setFlag = true
-while cond and not setFlag:
-  prelude()
-  if more:
-    if not condB:
-      setFlag = false
-    else:
-      stuffHere()
-      postlude()
-  else:
-    postlude()
-
-This is getting complicated. Instead we keep the whole 'join' idea but
-duplicate the 'join' instructions on breaks and return exits!
-
-]#
+  c.code.add Instr(kind: goto, dest: 0)
 
 proc genLabel(c: Con): TPosition = TPosition(c.code.len)
 
@@ -275,8 +101,8 @@ template checkedDistance(dist): int =
   doAssert low(int) div 2 + 1 < dist and dist < high(int) div 2
   dist
 
-proc jmpBack(c: var Con, n: PNode, p = TPosition(0)) =
-  c.code.add Instr(n: n, kind: goto, dest: checkedDistance(p.int - c.code.len))
+proc jmpBack(c: var Con, p = TPosition(0)) =
+  c.code.add Instr(kind: loop, dest: checkedDistance(p.int - c.code.len))
 
 proc patch(c: var Con, p: TPosition) =
   # patch with current index
@@ -286,12 +112,12 @@ proc gen(c: var Con; n: PNode)
 
 proc popBlock(c: var Con; oldLen: int) =
   var exits: seq[TPosition]
-  exits.add c.gotoI(newNode(nkEmpty))
+  exits.add c.gotoI()
   for f in c.blocks[oldLen].breakFixups:
     c.patch(f[0])
     for finale in f[1]:
       c.gen(finale)
-    exits.add c.gotoI(newNode(nkEmpty))
+    exits.add c.gotoI()
   for e in exits:
     c.patch e
   c.blocks.setLen(oldLen)
@@ -306,90 +132,28 @@ proc isTrue(n: PNode): bool =
   n.kind == nkSym and n.sym.kind == skEnumField and n.sym.position != 0 or
     n.kind == nkIntLit and n.intVal != 0
 
-when true:
-  proc genWhile(c: var Con; n: PNode) =
-    # We unroll every loop 3 times. We emulate 0, 1, 2 iterations
-    # through the loop. We need to prove this is correct for our
-    # purposes. But Herb Sutter claims it is. (Proof by authority.)
-    #
-    # EDIT: Actually, we only need to unroll 2 times
-    # because Nim doesn't have a way of breaking/goto-ing into
-    # a loop iteration. Unrolling 2 times is much better for compile
-    # times of nested loops than 3 times, so we do that here.
-    #[
-    while cond:
-      body
-
-    Becomes:
-
-    block:
-      if cond:
-        body
-        if cond:
-          body
-          if cond:
-            body
-
-    We still need to ensure 'break' resolves properly, so an AST to AST
-    translation is impossible.
-
-    So the code to generate is:
-
-      cond
-      fork L4  # F1
-      body
-      cond
-      fork L5  # F2
-      body
-      cond
-      fork L6  # F3
-      body
-    L6:
-      join F3
-    L5:
-      join F2
-    L4:
-      join F1
-    ]#
+template forkT(body) =
+  let lab1 = c.forkI()
+  body
+  c.patch(lab1)
+
+proc genWhile(c: var Con; n: PNode) =
+  # lab1:
+  #   cond, tmp
+  #   fork tmp, lab2
+  #   body
+  #   jmp lab1
+  # lab2:
+  let lab1 = c.genLabel
+  withBlock(nil):
     if isTrue(n[0]):
-      # 'while true' is an idiom in Nim and so we produce
-      # better code for it:
-      withBlock(nil):
-        for i in 0..1:
-          c.gen(n[1])
+      c.gen(n[1])
+      c.jmpBack(lab1)
     else:
-      withBlock(nil):
-        var endings: array[2, TPosition]
-        for i in 0..1:
-          c.gen(n[0])
-          endings[i] = c.forkI(n)
-          c.gen(n[1])
-        for i in countdown(endings.high, 0):
-          c.patch(endings[i])
-
-else:
-  proc genWhile(c: var Con; n: PNode) =
-    # lab1:
-    #   cond, tmp
-    #   fork tmp, lab2
-    #   body
-    #   jmp lab1
-    # lab2:
-    let lab1 = c.genLabel
-    withBlock(nil):
-      if isTrue(n[0]):
+      c.gen(n[0])
+      forkT:
         c.gen(n[1])
-        c.jmpBack(n, lab1)
-      else:
-        c.gen(n[0])
-        forkT(n):
-          c.gen(n[1])
-          c.jmpBack(n, lab1)
-
-template forkT(n, body) =
-  let lab1 = c.forkI(n)
-  body
-  c.patch(lab1)
+        c.jmpBack(lab1)
 
 proc genIf(c: var Con, n: PNode) =
   #[
@@ -429,15 +193,22 @@ proc genIf(c: var Con, n: PNode) =
 
   ]#
   var endings: seq[TPosition] = @[]
+  let oldInteresting = c.interestingInstructions
+  let oldLen = c.code.len
+
   for i in 0..<n.len:
     let it = n[i]
     c.gen(it[0])
     if it.len == 2:
-      forkT(it[1]):
-        c.gen(it[1])
-        endings.add c.gotoI(it[1])
-  for i in countdown(endings.high, 0):
-    c.patch(endings[i])
+      forkT:
+        c.gen(it.lastSon)
+        endings.add c.gotoI()
+
+  if oldInteresting == c.interestingInstructions:
+    setLen c.code, oldLen
+  else:
+    for i in countdown(endings.high, 0):
+      c.patch(endings[i])
 
 proc genAndOr(c: var Con; n: PNode) =
   #   asgn dest, a
@@ -446,7 +217,7 @@ proc genAndOr(c: var Con; n: PNode) =
   # lab1:
   #   join F1
   c.gen(n[1])
-  forkT(n):
+  forkT:
     c.gen(n[2])
 
 proc genCase(c: var Con; n: PNode) =
@@ -463,32 +234,32 @@ proc genCase(c: var Con; n: PNode) =
   let isExhaustive = skipTypes(n[0].typ,
     abstractVarRange-{tyTypeDesc}).kind notin {tyFloat..tyFloat128, tyString, tyCstring}
 
-  # we generate endings as a set of chained gotos, this is a bit awkward but it
-  # ensures when recursively traversing the CFG for various analysis, we don't
-  # artificially extended the life of each branch (for the purposes of DFA)
-  # beyond the minimum amount.
   var endings: seq[TPosition] = @[]
   c.gen(n[0])
+  let oldInteresting = c.interestingInstructions
+  let oldLen = c.code.len
   for i in 1..<n.len:
     let it = n[i]
     if it.len == 1 or (i == n.len-1 and isExhaustive):
       # treat the last branch as 'else' if this is an exhaustive case statement.
       c.gen(it.lastSon)
-      if endings.len != 0:
-        c.patch(endings[^1])
     else:
-      forkT(it.lastSon):
+      forkT:
         c.gen(it.lastSon)
-        if endings.len != 0:
-          c.patch(endings[^1])
-        endings.add c.gotoI(it.lastSon)
+        endings.add c.gotoI()
+
+  if oldInteresting == c.interestingInstructions:
+    setLen c.code, oldLen
+  else:
+    for i in countdown(endings.high, 0):
+      c.patch(endings[i])
 
 proc genBlock(c: var Con; n: PNode) =
   withBlock(n[0].sym):
     c.gen(n[1])
 
 proc genBreakOrRaiseAux(c: var Con, i: int, n: PNode) =
-  let lab1 = c.gotoI(n)
+  let lab1 = c.gotoI()
   if c.blocks[i].isTryBlock:
     c.blocks[i].raiseFixups.add lab1
   else:
@@ -501,6 +272,7 @@ proc genBreakOrRaiseAux(c: var Con, i: int, n: PNode) =
     c.blocks[i].breakFixups.add (lab1, trailingFinales)
 
 proc genBreak(c: var Con; n: PNode) =
+  inc c.interestingInstructions
   if n[0].kind == nkSym:
     for i in countdown(c.blocks.high, 0):
       if not c.blocks[i].isTryBlock and c.blocks[i].label == n[0].sym:
@@ -531,9 +303,9 @@ proc genTry(c: var Con; n: PNode) =
   for i in 1..<n.len:
     let it = n[i]
     if it.kind != nkFinally:
-      forkT(it):
+      forkT:
         c.gen(it.lastSon)
-        endings.add c.gotoI(it)
+        endings.add c.gotoI()
   for i in countdown(endings.high, 0):
     c.patch(endings[i])
 
@@ -541,11 +313,12 @@ proc genTry(c: var Con; n: PNode) =
   if fin.kind == nkFinally:
     c.gen(fin[0])
 
-template genNoReturn(c: var Con; n: PNode) =
+template genNoReturn(c: var Con) =
   # leave the graph
-  c.code.add Instr(n: n, kind: goto, dest: high(int) - c.code.len)
+  c.code.add Instr(kind: goto, dest: high(int) - c.code.len)
 
 proc genRaise(c: var Con; n: PNode) =
+  inc c.interestingInstructions
   gen(c, n[0])
   if c.inTryStmt > 0:
     for i in countdown(c.blocks.high, 0):
@@ -554,13 +327,14 @@ proc genRaise(c: var Con; n: PNode) =
         return
     assert false #Unreachable
   else:
-    genNoReturn(c, n)
+    genNoReturn(c)
 
 proc genImplicitReturn(c: var Con) =
   if c.owner.kind in {skProc, skFunc, skMethod, skIterator, skConverter} and resultPos < c.owner.ast.len:
     gen(c, c.owner.ast[resultPos])
 
 proc genReturn(c: var Con; n: PNode) =
+  inc c.interestingInstructions
   if n[0].kind != nkEmpty:
     gen(c, n[0])
   else:
@@ -569,122 +343,6 @@ proc genReturn(c: var Con; n: PNode) =
 
 const
   InterestingSyms = {skVar, skResult, skLet, skParam, skForVar, skTemp}
-  PathKinds0 = {nkDotExpr, nkCheckedFieldExpr,
-                nkBracketExpr, nkDerefExpr, nkHiddenDeref,
-                nkAddr, nkHiddenAddr,
-                nkObjDownConv, nkObjUpConv}
-  PathKinds1 = {nkHiddenStdConv, nkHiddenSubConv}
-
-proc skipConvDfa*(n: PNode): PNode =
-  result = n
-  while true:
-    case result.kind
-    of nkObjDownConv, nkObjUpConv:
-      result = result[0]
-    of PathKinds1:
-      result = result[1]
-    else: break
-
-type AliasKind* = enum
-  yes, no, maybe
-
-proc aliases*(obj, field: PNode): AliasKind =
-  # obj -> field:
-  # x -> x: true
-  # x -> x.f: true
-  # x.f -> x: false
-  # x.f -> x.f: true
-  # x.f -> x.v: false
-  # x -> x[0]: true
-  # x[0] -> x: false
-  # x[0] -> x[0]: true
-  # x[0] -> x[1]: false
-  # x -> x[i]: true
-  # x[i] -> x: false
-  # x[i] -> x[i]: maybe; Further analysis could make this return true when i is a runtime-constant
-  # x[i] -> x[j]: maybe; also returns maybe if only one of i or j is a compiletime-constant
-  template collectImportantNodes(result, n) =
-    var result: seq[PNode]
-    var n = n
-    while true:
-      case n.kind
-      of PathKinds0 - {nkDotExpr, nkBracketExpr}:
-        n = n[0]
-      of PathKinds1:
-        n = n[1]
-      of nkDotExpr, nkBracketExpr:
-        result.add n
-        n = n[0]
-      of nkSym:
-        result.add n; break
-      else: return no
-
-  collectImportantNodes(objImportantNodes, obj)
-  collectImportantNodes(fieldImportantNodes, field)
-
-  # If field is less nested than obj, then it cannot be part of/aliased by obj
-  if fieldImportantNodes.len < objImportantNodes.len: return no
-
-  result = yes
-  for i in 1..objImportantNodes.len:
-    # We compare the nodes leading to the location of obj and field
-    # with each other.
-    # We continue until they diverge, in which case we return no, or
-    # until we reach the location of obj, in which case we do not need
-    # to look further, since field must be part of/aliased by obj now.
-    # If we encounter an element access using an index which is a runtime value,
-    # we simply return maybe instead of yes; should further nodes not diverge.
-    let currFieldPath = fieldImportantNodes[^i]
-    let currObjPath = objImportantNodes[^i]
-
-    if currFieldPath.kind != currObjPath.kind:
-      return no
-
-    case currFieldPath.kind
-    of nkSym:
-      if currFieldPath.sym != currObjPath.sym: return no
-    of nkDotExpr:
-      if currFieldPath[1].sym != currObjPath[1].sym: return no
-    of nkBracketExpr:
-      if currFieldPath[1].kind in nkLiterals and currObjPath[1].kind in nkLiterals:
-        if currFieldPath[1].intVal != currObjPath[1].intVal:
-          return no
-      else:
-        result = maybe
-    else: assert false # unreachable
-
-proc isAnalysableFieldAccess*(orig: PNode; owner: PSym): bool =
-  var n = orig
-  while true:
-    case n.kind
-    of PathKinds0 - {nkHiddenDeref, nkDerefExpr}:
-      n = n[0]
-    of PathKinds1:
-      n = n[1]
-    of nkHiddenDeref, nkDerefExpr:
-      # We "own" sinkparam[].loc but not ourVar[].location as it is a nasty
-      # pointer indirection.
-      # bug #14159, we cannot reason about sinkParam[].location as it can
-      # still be shared for tyRef.
-      n = n[0]
-      return n.kind == nkSym and n.sym.owner == owner and
-         (n.sym.typ.skipTypes(abstractInst-{tyOwned}).kind in {tyOwned})
-    else: break
-  # XXX Allow closure deref operations here if we know
-  # the owner controlled the closure allocation?
-  result = n.kind == nkSym and n.sym.owner == owner and
-    {sfGlobal, sfThread, sfCursor} * n.sym.flags == {} and
-    (n.sym.kind != skParam or isSinkParam(n.sym)) # or n.sym.typ.kind == tyVar)
-  # Note: There is a different move analyzer possible that checks for
-  # consume(param.key); param.key = newValue  for all paths. Then code like
-  #
-  #   let splited = split(move self.root, x)
-  #   self.root = merge(splited.lower, splited.greater)
-  #
-  # could be written without the ``move self.root``. However, this would be
-  # wrong! Then the write barrier for the ``self.root`` assignment would
-  # free the old data and all is lost! Lesson: Don't be too smart, trust the
-  # lower level C++ optimizer to specialize this code.
 
 proc skipTrivials(c: var Con, n: PNode): PNode =
   result = n
@@ -703,8 +361,9 @@ proc genUse(c: var Con; orig: PNode) =
   let n = c.skipTrivials(orig)
 
   if n.kind == nkSym:
-    if n.sym.kind in InterestingSyms:
-      c.code.add Instr(n: orig, kind: use)
+    if n.sym.kind in InterestingSyms and n.sym == c.root:
+      c.code.add Instr(kind: use, n: orig)
+      inc c.interestingInstructions
   else:
     gen(c, n)
 
@@ -712,7 +371,9 @@ proc genDef(c: var Con; orig: PNode) =
   let n = c.skipTrivials(orig)
 
   if n.kind == nkSym and n.sym.kind in InterestingSyms:
-    c.code.add Instr(n: orig, kind: def)
+    if n.sym == c.root:
+      c.code.add Instr(kind: def, n: orig)
+      inc c.interestingInstructions
 
 proc genCall(c: var Con; n: PNode) =
   gen(c, n[0])
@@ -725,13 +386,13 @@ proc genCall(c: var Con; n: PNode) =
         # Pass by 'out' is a 'must def'. Good enough for a move optimizer.
         genDef(c, n[i])
   # every call can potentially raise:
-  if c.inTryStmt > 0 and canRaiseConservative(n[0]):
+  if false: # c.inTryStmt > 0 and canRaiseConservative(n[0]):
     # we generate the instruction sequence:
     # fork lab1
     # goto exceptionHandler (except or finally)
     # lab1:
     # join F1
-    forkT(n):
+    forkT:
       for i in countdown(c.blocks.high, 0):
         if c.blocks[i].isTryBlock:
           genBreakOrRaiseAux(c, i, n)
@@ -769,7 +430,7 @@ proc gen(c: var Con; n: PNode) =
       else:
         genCall(c, n)
       if sfNoReturn in n[0].sym.flags:
-        genNoReturn(c, n)
+        genNoReturn(c)
     else:
       genCall(c, n)
   of nkCharLit..nkNilLit: discard
@@ -810,13 +471,32 @@ proc gen(c: var Con; n: PNode) =
   of nkDefer: doAssert false, "dfa construction pass requires the elimination of 'defer'"
   else: discard
 
-proc constructCfg*(s: PSym; body: PNode): ControlFlowGraph =
+when false:
+  proc optimizeJumps(c: var ControlFlowGraph) =
+    for i in 0..<c.len:
+      case c[i].kind
+      of goto, fork:
+        var pc = i + c[i].dest
+        if pc < c.len and c[pc].kind == goto:
+          while pc < c.len and c[pc].kind == goto:
+            let newPc = pc + c[pc].dest
+            if newPc > pc:
+              pc = newPc
+            else:
+              break
+          c[i].dest = pc - i
+      of loop, def, use: discard
+
+proc constructCfg*(s: PSym; body: PNode; root: PSym): ControlFlowGraph =
   ## constructs a control flow graph for ``body``.
-  var c = Con(code: @[], blocks: @[], owner: s)
+  var c = Con(code: @[], blocks: @[], owner: s, root: root)
   withBlock(s):
     gen(c, body)
-    genImplicitReturn(c)
+    if root.kind == skResult:
+      genImplicitReturn(c)
   when defined(gcArc) or defined(gcOrc):
     result = c.code # will move
   else:
     shallowCopy(result, c.code)
+  when false:
+    optimizeJumps result
diff --git a/compiler/injectdestructors.nim b/compiler/injectdestructors.nim
index 63cfbbd9f..298bc7711 100644
--- a/compiler/injectdestructors.nim
+++ b/compiler/injectdestructors.nim
@@ -15,9 +15,9 @@
 
 import
   intsets, strtabs, ast, astalgo, msgs, renderer, magicsys, types, idents,
-  strutils, options, dfa, lowerings, tables, modulegraphs,
+  strutils, options, lowerings, tables, modulegraphs,
   lineinfos, parampatterns, sighashes, liftdestructors, optimizer,
-  varpartitions
+  varpartitions, aliasanalysis, dfa
 
 when defined(nimPreviewSlimSystem):
   import std/assertions
@@ -27,12 +27,14 @@ from trees import exprStructuralEquivalent, getRoot
 type
   Con = object
     owner: PSym
-    g: ControlFlowGraph
+    when true:
+      g: ControlFlowGraph
     graph: ModuleGraph
     inLoop, inSpawn, inLoopCond: int
     uninit: IntSet # set of uninit'ed vars
-    uninitComputed: bool
     idgen: IdGenerator
+    body: PNode
+    otherUsage: TLineInfo
 
   Scope = object # we do scope-based memory management.
     # a scope is comparable to an nkStmtListExpr like
@@ -40,6 +42,8 @@ type
     vars: seq[PSym]
     wasMoved: seq[PNode]
     final: seq[PNode] # finally section
+    locals: seq[PSym]
+    body: PNode
     needsTry: bool
     parent: ptr Scope
 
@@ -70,160 +74,95 @@ proc getTemp(c: var Con; s: var Scope; typ: PType; info: TLineInfo): PNode =
   s.vars.add(sym)
   result = newSymNode(sym)
 
-proc nestedScope(parent: var Scope): Scope =
-  Scope(vars: @[], wasMoved: @[], final: @[], needsTry: false, parent: addr(parent))
+proc nestedScope(parent: var Scope; body: PNode): Scope =
+  Scope(vars: @[], locals: @[], wasMoved: @[], final: @[], body: body, needsTry: false, parent: addr(parent))
 
 proc p(n: PNode; c: var Con; s: var Scope; mode: ProcessMode): PNode
 proc moveOrCopy(dest, ri: PNode; c: var Con; s: var Scope; isDecl = false): PNode
 
-import sets, hashes
+when false:
+  var
+    perfCounters: array[InstrKind, int]
 
-proc hash(n: PNode): Hash = hash(cast[pointer](n))
+  proc showCounters*() =
+    for i in low(InstrKind)..high(InstrKind):
+      echo "INSTR ", i, " ", perfCounters[i]
 
-type
-  State = ref object
-    lastReads: IntSet
-    potentialLastReads: IntSet
-    notLastReads: IntSet
-    alreadySeen: HashSet[PNode]
-
-proc preprocessCfg(cfg: var ControlFlowGraph) =
-  for i in 0..<cfg.len:
-    if cfg[i].kind in {goto, fork} and i + cfg[i].dest > cfg.len:
-      cfg[i].dest = cfg.len - i
-
-proc mergeStates(a: var State, b: sink State) =
-  # Inplace for performance:
-  #   lastReads = a.lastReads + b.lastReads
-  #   potentialLastReads = (a.potentialLastReads + b.potentialLastReads) - (a.notLastReads + b.notLastReads)
-  #   notLastReads = a.notLastReads + b.notLastReads
-  #   alreadySeen = a.alreadySeen + b.alreadySeen
-  # b is never nil
-  if a == nil:
-    a = b
+proc isLastReadImpl(n: PNode; c: var Con; scope: var Scope): bool =
+  let root = parampatterns.exprRoot(n, allowCalls=false)
+  if root == nil: return false
+
+  var s = addr(scope)
+  while s != nil:
+    if s.locals.contains(root): break
+    s = s.parent
+
+  c.g = constructCfg(c.owner, if s != nil: s.body else: c.body, root)
+  dbg:
+    echo "\n### ", c.owner.name.s, ":\nCFG:"
+    echoCfg(c.g)
+    #echo c.body
+
+  var j = 0
+  while j < c.g.len:
+    if c.g[j].kind == use and c.g[j].n == n: break
+    inc j
+  c.otherUsage = unknownLineInfo
+  if j < c.g.len:
+    var pcs = @[j+1]
+    var marked = initIntSet()
+    result = true
+    while pcs.len > 0:
+      var pc = pcs.pop()
+      if not marked.contains(pc):
+        let oldPc = pc
+        while pc < c.g.len:
+          dbg:
+            echo "EXEC ", c.g[pc].kind, " ", pc, " ", n
+          when false:
+            inc perfCounters[c.g[pc].kind]
+          case c.g[pc].kind
+          of loop:
+            let back = pc + c.g[pc].dest
+            if not marked.containsOrIncl(back):
+              pc = back
+            else:
+              break
+          of goto:
+            pc = pc + c.g[pc].dest
+          of fork:
+            if not marked.contains(pc+1):
+              pcs.add pc + 1
+            pc = pc + c.g[pc].dest
+          of use:
+            if c.g[pc].n.aliases(n) != no or n.aliases(c.g[pc].n) != no:
+              c.otherUsage = c.g[pc].n.info
+              return false
+            inc pc
+          of def:
+            if c.g[pc].n.aliases(n) == yes:
+              # the path leads to a redefinition of 's' --> sink 's'.
+              break
+            elif n.aliases(c.g[pc].n) != no:
+              # only partially writes to 's' --> can't sink 's', so this def reads 's'
+              # or maybe writes to 's' --> can't sink 's'
+              c.otherUsage = c.g[pc].n.info
+              return false
+            inc pc
+        marked.incl oldPc
   else:
-    a.lastReads.incl b.lastReads
-    a.potentialLastReads.incl b.potentialLastReads
-    a.potentialLastReads.excl a.notLastReads
-    a.potentialLastReads.excl b.notLastReads
-    a.notLastReads.incl b.notLastReads
-    a.alreadySeen.incl b.alreadySeen
-
-proc computeLastReadsAndFirstWrites(cfg: ControlFlowGraph) =
-  template aliasesCached(obj, field: PNode): AliasKind =
-    aliases(obj, field)
-
-  var cfg = cfg
-  preprocessCfg(cfg)
-
-  var states = newSeq[State](cfg.len + 1)
-  states[0] = State()
-
-  for pc in 0..<cfg.len:
-    template state: State = states[pc]
-    if state != nil:
-      case cfg[pc].kind
-      of def:
-        var potentialLastReadsCopy = state.potentialLastReads
-        for r in potentialLastReadsCopy:
-          if cfg[pc].n.aliasesCached(cfg[r].n) == yes:
-            # the path leads to a redefinition of 's' --> sink 's'.
-            state.lastReads.incl r
-            state.potentialLastReads.excl r
-          elif cfg[r].n.aliasesCached(cfg[pc].n) != no:
-            # only partially writes to 's' --> can't sink 's', so this def reads 's'
-            # or maybe writes to 's' --> can't sink 's'
-            cfg[r].n.comment = '\n' & $pc
-            state.potentialLastReads.excl r
-            state.notLastReads.incl r
-
-        var alreadySeenThisNode = false
-        for s in state.alreadySeen:
-          if cfg[pc].n.aliasesCached(s) != no or s.aliasesCached(cfg[pc].n) != no:
-            alreadySeenThisNode = true; break
-        if alreadySeenThisNode: cfg[pc].n.flags.excl nfFirstWrite
-        else: cfg[pc].n.flags.incl nfFirstWrite
-
-        state.alreadySeen.incl cfg[pc].n
-
-        mergeStates(states[pc + 1], move(states[pc]))
-      of use:
-        var potentialLastReadsCopy = state.potentialLastReads
-        for r in potentialLastReadsCopy:
-          if cfg[pc].n.aliasesCached(cfg[r].n) != no or cfg[r].n.aliasesCached(cfg[pc].n) != no:
-            cfg[r].n.comment = '\n' & $pc
-            state.potentialLastReads.excl r
-            state.notLastReads.incl r
-
-        state.potentialLastReads.incl pc
-
-        state.alreadySeen.incl cfg[pc].n
-
-        mergeStates(states[pc + 1], move(states[pc]))
-      of goto:
-        mergeStates(states[pc + cfg[pc].dest], move(states[pc]))
-      of fork:
-        var copy = State()
-        copy[] = states[pc][]
-        mergeStates(states[pc + cfg[pc].dest], copy)
-        mergeStates(states[pc + 1], move(states[pc]))
-
-  let lastReads = (states[^1].lastReads + states[^1].potentialLastReads) - states[^1].notLastReads
-  var lastReadTable: Table[PNode, seq[int]]
-  for position, node in cfg:
-    if node.kind == use:
-      lastReadTable.mgetOrPut(node.n, @[]).add position
-  for node, positions in lastReadTable:
-    block checkIfAllPosLastRead:
-      for p in positions:
-        if p notin lastReads: break checkIfAllPosLastRead
-      node.flags.incl nfLastRead
-
-proc isLastRead(n: PNode; c: var Con): bool =
-  let m = dfa.skipConvDfa(n)
-  (m.kind == nkSym and sfSingleUsedTemp in m.sym.flags) or nfLastRead in m.flags
+    result = false
+
+proc isLastRead(n: PNode; c: var Con; s: var Scope): bool =
+  if not hasDestructor(c, n.typ): return true
+
+  let m = skipConvDfa(n)
+  result = (m.kind == nkSym and sfSingleUsedTemp in m.sym.flags) or
+      isLastReadImpl(n, c, s)
 
 proc isFirstWrite(n: PNode; c: var Con): bool =
-  let m = dfa.skipConvDfa(n)
-  nfFirstWrite in m.flags
-
-proc initialized(code: ControlFlowGraph; pc: int,
-                 init, uninit: var IntSet; until: int): int =
-  ## Computes the set of definitely initialized variables across all code paths
-  ## as an IntSet of IDs.
-  var pc = pc
-  while pc < code.len:
-    case code[pc].kind
-    of goto:
-      pc += code[pc].dest
-    of fork:
-      var initA = initIntSet()
-      var initB = initIntSet()
-      var variantA = pc + 1
-      var variantB = pc + code[pc].dest
-      while variantA != variantB:
-        if max(variantA, variantB) > until:
-          break
-        if variantA < variantB:
-          variantA = initialized(code, variantA, initA, uninit, min(variantB, until))
-        else:
-          variantB = initialized(code, variantB, initB, uninit, min(variantA, until))
-      pc = min(variantA, variantB)
-      # we add vars if they are in both branches:
-      for v in initA:
-        if v in initB:
-          init.incl v
-    of use:
-      let v = code[pc].n.sym
-      if v.kind != skParam and v.id notin init:
-        # attempt to read an uninit'ed variable
-        uninit.incl v.id
-      inc pc
-    of def:
-      let v = code[pc].n.sym
-      init.incl v.id
-      inc pc
-  return pc
+  let m = skipConvDfa(n)
+  result = nfFirstWrite2 in m.flags
 
 proc isCursor(n: PNode): bool =
   case n.kind
@@ -247,9 +186,11 @@ proc checkForErrorPragma(c: Con; t: PType; ri: PNode; opname: string) =
     m.add "; requires a copy because it's not the last read of '"
     m.add renderTree(ri)
     m.add '\''
-    if ri.comment.startsWith('\n'):
+    if c.otherUsage != unknownLineInfo:
+       # ri.comment.startsWith('\n'):
       m.add "; another read is done here: "
-      m.add c.graph.config $ c.g[parseInt(ri.comment[1..^1])].n.info
+      m.add c.graph.config $ c.otherUsage
+      #m.add c.graph.config $ c.g[parseInt(ri.comment[1..^1])].n.info
     elif ri.kind == nkSym and ri.sym.kind == skParam and not isSinkType(ri.sym.typ):
       m.add "; try to make "
       m.add renderTree(ri)
@@ -625,7 +566,7 @@ template handleNestedTempl(n, processCall: untyped, willProduceStmt = false) =
       var branch = shallowCopy(it)
       for j in 0 ..< it.len-1:
         branch[j] = copyTree(it[j])
-      var ofScope = nestedScope(s)
+      var ofScope = nestedScope(s, it.lastSon)
       branch[^1] = if it[^1].typ.isEmptyType or willProduceStmt:
                      processScope(c, ofScope, maybeVoid(it[^1], ofScope))
                    else:
@@ -638,7 +579,7 @@ template handleNestedTempl(n, processCall: untyped, willProduceStmt = false) =
     result = copyNode(n)
     result.add p(n[0], c, s, normal)
     dec c.inLoopCond
-    var bodyScope = nestedScope(s)
+    var bodyScope = nestedScope(s, n[1])
     let bodyResult = p(n[1], c, bodyScope, normal)
     result.add processScope(c, bodyScope, bodyResult)
     dec c.inLoop
@@ -650,7 +591,7 @@ template handleNestedTempl(n, processCall: untyped, willProduceStmt = false) =
     for i in 0..<last-1:
       result[i] = n[i]
     result[last-1] = p(n[last-1], c, s, normal)
-    var bodyScope = nestedScope(s)
+    var bodyScope = nestedScope(s, n[1])
     let bodyResult = p(n[last], c, bodyScope, normal)
     result[last] = processScope(c, bodyScope, bodyResult)
     dec c.inLoop
@@ -658,7 +599,7 @@ template handleNestedTempl(n, processCall: untyped, willProduceStmt = false) =
   of nkBlockStmt, nkBlockExpr:
     result = copyNode(n)
     result.add n[0]
-    var bodyScope = nestedScope(s)
+    var bodyScope = nestedScope(s, n[1])
     result.add if n[1].typ.isEmptyType or willProduceStmt:
                  processScope(c, bodyScope, processCall(n[1], bodyScope))
                else:
@@ -669,7 +610,7 @@ template handleNestedTempl(n, processCall: untyped, willProduceStmt = false) =
     for i in 0..<n.len:
       let it = n[i]
       var branch = shallowCopy(it)
-      var branchScope = nestedScope(s)
+      var branchScope = nestedScope(s, it.lastSon)
       if it.kind in {nkElifBranch, nkElifExpr}:
         #Condition needs to be destroyed outside of the condition/branch scope
         branch[0] = p(it[0], c, s, normal)
@@ -682,7 +623,7 @@ template handleNestedTempl(n, processCall: untyped, willProduceStmt = false) =
 
   of nkTryStmt:
     result = copyNode(n)
-    var tryScope = nestedScope(s)
+    var tryScope = nestedScope(s, n[0])
     result.add if n[0].typ.isEmptyType or willProduceStmt:
                  processScope(c, tryScope, maybeVoid(n[0], tryScope))
                else:
@@ -691,7 +632,7 @@ template handleNestedTempl(n, processCall: untyped, willProduceStmt = false) =
     for i in 1..<n.len:
       let it = n[i]
       var branch = copyTree(it)
-      var branchScope = nestedScope(s)
+      var branchScope = nestedScope(s, it[^1])
       branch[^1] = if it[^1].typ.isEmptyType or willProduceStmt or it.kind == nkFinally:
                      processScope(c, branchScope, if it.kind == nkFinally: p(it[^1], c, branchScope, normal)
                                                   else: maybeVoid(it[^1], branchScope))
@@ -744,7 +685,7 @@ proc p(n: PNode; c: var Con; s: var Scope; mode: ProcessMode): PNode =
          nkCallKinds + nkLiterals:
       result = p(n, c, s, consumed)
     elif ((n.kind == nkSym and isSinkParam(n.sym)) or isAnalysableFieldAccess(n, c.owner)) and
-        isLastRead(n, c) and not (n.kind == nkSym and isCursor(n)):
+        isLastRead(n, c, s) and not (n.kind == nkSym and isCursor(n)):
       # Sinked params can be consumed only once. We need to reset the memory
       # to disable the destructor which we have not elided
       result = destructiveMoveVar(n, c, s)
@@ -864,6 +805,8 @@ proc p(n: PNode; c: var Con; s: var Scope; mode: ProcessMode): PNode =
       for it in n:
         var ri = it[^1]
         if it.kind == nkVarTuple and hasDestructor(c, ri.typ):
+          for i in 0..<it.len-2:
+            if it[i].kind == nkSym: s.locals.add it[i].sym
           let x = lowerTupleUnpacking(c.graph, it, c.idgen, c.owner)
           result.add p(x, c, s, consumed)
         elif it.kind == nkIdentDefs and hasDestructor(c, skipPragmaExpr(it[0]).typ):
@@ -871,6 +814,7 @@ proc p(n: PNode; c: var Con; s: var Scope; mode: ProcessMode): PNode =
             let v = skipPragmaExpr(it[j])
             if v.kind == nkSym:
               if sfCompileTime in v.sym.flags: continue
+              s.locals.add v.sym
               pVarTopLevel(v, c, s, result)
             if ri.kind != nkEmpty:
               result.add moveOrCopy(v, ri, c, s, isDecl = v.kind == nkSym)
@@ -943,7 +887,7 @@ proc p(n: PNode; c: var Con; s: var Scope; mode: ProcessMode): PNode =
       for i in 1 ..< n.len:
         result[i] = n[i]
       if mode == sinkArg and hasDestructor(c, n.typ):
-        if isAnalysableFieldAccess(n, c.owner) and isLastRead(n, c):
+        if isAnalysableFieldAccess(n, c.owner) and isLastRead(n, c, s):
           s.wasMoved.add c.genWasMoved(n)
         else:
           result = passCopyToSink(result, c, s)
@@ -953,7 +897,7 @@ proc p(n: PNode; c: var Con; s: var Scope; mode: ProcessMode): PNode =
       for i in 0 ..< n.len:
         result[i] = p(n[i], c, s, normal)
       if mode == sinkArg and hasDestructor(c, n.typ):
-        if isAnalysableFieldAccess(n, c.owner) and isLastRead(n, c):
+        if isAnalysableFieldAccess(n, c.owner) and isLastRead(n, c, s):
           # consider 'a[(g; destroy(g); 3)]', we want to say 'wasMoved(a[3])'
           # without the junk, hence 'c.genWasMoved(n)'
           # and not 'c.genWasMoved(result)':
@@ -1054,7 +998,7 @@ proc moveOrCopy(dest, ri: PNode; c: var Con; s: var Scope, isDecl = false): PNod
       if isUnpackedTuple(ri[0]):
         # unpacking of tuple: take over the elements
         result = c.genSink(dest, p(ri, c, s, consumed), isDecl)
-      elif isAnalysableFieldAccess(ri, c.owner) and isLastRead(ri, c):
+      elif isAnalysableFieldAccess(ri, c.owner) and isLastRead(ri, c, s):
         if aliases(dest, ri) == no:
           # Rule 3: `=sink`(x, z); wasMoved(z)
           if isAtom(ri[1]):
@@ -1079,12 +1023,12 @@ proc moveOrCopy(dest, ri: PNode; c: var Con; s: var Scope, isDecl = false): PNod
     of nkObjConstr, nkTupleConstr, nkClosure, nkCharLit..nkNilLit:
       result = c.genSink(dest, p(ri, c, s, consumed), isDecl)
     of nkSym:
-      if isSinkParam(ri.sym) and isLastRead(ri, c):
+      if isSinkParam(ri.sym) and isLastRead(ri, c, s):
         # Rule 3: `=sink`(x, z); wasMoved(z)
         let snk = c.genSink(dest, ri, isDecl)
         result = newTree(nkStmtList, snk, c.genWasMoved(ri))
       elif ri.sym.kind != skParam and ri.sym.owner == c.owner and
-          isLastRead(ri, c) and canBeMoved(c, dest.typ) and not isCursor(ri):
+          isLastRead(ri, c, s) and canBeMoved(c, dest.typ) and not isCursor(ri):
         # Rule 3: `=sink`(x, z); wasMoved(z)
         let snk = c.genSink(dest, ri, isDecl)
         result = newTree(nkStmtList, snk, c.genWasMoved(ri))
@@ -1101,7 +1045,7 @@ proc moveOrCopy(dest, ri: PNode; c: var Con; s: var Scope, isDecl = false): PNod
     of nkRaiseStmt:
       result = pRaiseStmt(ri, c, s)
     else:
-      if isAnalysableFieldAccess(ri, c.owner) and isLastRead(ri, c) and
+      if isAnalysableFieldAccess(ri, c.owner) and isLastRead(ri, c, s) and
           canBeMoved(c, dest.typ):
         # Rule 3: `=sink`(x, z); wasMoved(z)
         let snk = c.genSink(dest, ri, isDecl)
@@ -1111,49 +1055,44 @@ proc moveOrCopy(dest, ri: PNode; c: var Con; s: var Scope, isDecl = false): PNod
         result.add p(ri, c, s, consumed)
         c.finishCopy(result, dest, isFromSink = false)
 
-proc computeUninit(c: var Con) =
-  if not c.uninitComputed:
-    c.uninitComputed = true
-    c.uninit = initIntSet()
-    var init = initIntSet()
-    discard initialized(c.g, pc = 0, init, c.uninit, int.high)
+when false:
+  proc computeUninit(c: var Con) =
+    if not c.uninitComputed:
+      c.uninitComputed = true
+      c.uninit = initIntSet()
+      var init = initIntSet()
+      discard initialized(c.g, pc = 0, init, c.uninit, int.high)
 
-proc injectDefaultCalls(n: PNode, c: var Con) =
-  case n.kind
-  of nkVarSection, nkLetSection:
-    for it in n:
-      if it.kind == nkIdentDefs and it[^1].kind == nkEmpty:
-        computeUninit(c)
-        for j in 0..<it.len-2:
-          let v = skipPragmaExpr(it[j])
-          doAssert v.kind == nkSym
-          if c.uninit.contains(v.sym.id):
-            it[^1] = genDefaultCall(v.sym.typ, c, v.info)
-            break
-  of nkNone..nkNilLit, nkTypeSection, nkProcDef, nkConverterDef, nkMethodDef,
-      nkIteratorDef, nkMacroDef, nkTemplateDef, nkLambda, nkDo, nkFuncDef:
-    discard
-  else:
-    for i in 0..<n.safeLen:
-      injectDefaultCalls(n[i], c)
+  proc injectDefaultCalls(n: PNode, c: var Con) =
+    case n.kind
+    of nkVarSection, nkLetSection:
+      for it in n:
+        if it.kind == nkIdentDefs and it[^1].kind == nkEmpty:
+          computeUninit(c)
+          for j in 0..<it.len-2:
+            let v = skipPragmaExpr(it[j])
+            doAssert v.kind == nkSym
+            if c.uninit.contains(v.sym.id):
+              it[^1] = genDefaultCall(v.sym.typ, c, v.info)
+              break
+    of nkNone..nkNilLit, nkTypeSection, nkProcDef, nkConverterDef, nkMethodDef,
+        nkIteratorDef, nkMacroDef, nkTemplateDef, nkLambda, nkDo, nkFuncDef:
+      discard
+    else:
+      for i in 0..<n.safeLen:
+        injectDefaultCalls(n[i], c)
 
 proc injectDestructorCalls*(g: ModuleGraph; idgen: IdGenerator; owner: PSym; n: PNode): PNode =
   when toDebug.len > 0:
     shouldDebug = toDebug == owner.name.s or toDebug == "always"
   if sfGeneratedOp in owner.flags or (owner.kind == skIterator and isInlineIterator(owner.typ)):
     return n
-  var c = Con(owner: owner, graph: g, g: constructCfg(owner, n), idgen: idgen)
-  dbg:
-    echo "\n### ", owner.name.s, ":\nCFG:"
-    echoCfg(c.g)
-    echo n
+  var c = Con(owner: owner, graph: g, idgen: idgen, body: n, otherUsage: unknownLineInfo)
 
   if optCursorInference in g.config.options:
     computeCursors(owner, n, g)
 
-  computeLastReadsAndFirstWrites(c.g)
-
-  var scope: Scope
+  var scope = Scope(body: n)
   let body = p(n, c, scope, normal)
 
   if owner.kind in {skProc, skFunc, skMethod, skIterator, skConverter}:
diff --git a/compiler/int128.nim b/compiler/int128.nim
index afa07094b..d85f96084 100644
--- a/compiler/int128.nim
+++ b/compiler/int128.nim
@@ -57,15 +57,9 @@ proc toInt128*[T: SomeInteger | bool](arg: T): Int128 =
 template isNegative(arg: Int128): bool =
   arg.sdata(3) < 0
 
-template isNegative(arg: int32): bool =
-  arg < 0
-
 proc bitconcat(a, b: uint32): uint64 =
   (uint64(a) shl 32) or uint64(b)
 
-proc bitsplit(a: uint64): (uint32, uint32) =
-  (cast[uint32](a shr 32), cast[uint32](a))
-
 proc toInt64*(arg: Int128): int64 =
   if isNegative(arg):
     assert(arg.sdata(3) == -1, "out of range")
@@ -211,12 +205,6 @@ proc `==`*(a, b: Int128): bool =
   if a.udata[3] != b.udata[3]: return false
   return true
 
-proc inplaceBitnot(a: var Int128) =
-  a.udata[0] = not a.udata[0]
-  a.udata[1] = not a.udata[1]
-  a.udata[2] = not a.udata[2]
-  a.udata[3] = not a.udata[3]
-
 proc bitnot*(a: Int128): Int128 =
   result.udata[0] = not a.udata[0]
   result.udata[1] = not a.udata[1]
@@ -357,18 +345,6 @@ proc low64(a: Int128): uint64 =
   bitconcat(a.udata[1], a.udata[0])
 
 proc `*`*(lhs, rhs: Int128): Int128 =
-  let
-    a = cast[uint64](lhs.udata[0])
-    b = cast[uint64](lhs.udata[1])
-    c = cast[uint64](lhs.udata[2])
-    d = cast[uint64](lhs.udata[3])
-
-    e = cast[uint64](rhs.udata[0])
-    f = cast[uint64](rhs.udata[1])
-    g = cast[uint64](rhs.udata[2])
-    h = cast[uint64](rhs.udata[3])
-
-
   let a32 = cast[uint64](lhs.udata[1])
   let a00 = cast[uint64](lhs.udata[0])
   let b32 = cast[uint64](rhs.udata[1])
@@ -444,11 +420,11 @@ proc divMod*(dividend, divisor: Int128): tuple[quotient, remainder: Int128] =
     result.remainder = dividend
 
 proc `div`*(a, b: Int128): Int128 =
-  let (a, b) = divMod(a, b)
+  let (a, _) = divMod(a, b)
   return a
 
 proc `mod`*(a, b: Int128): Int128 =
-  let (a, b) = divMod(a, b)
+  let (_, b) = divMod(a, b)
   return b
 
 proc addInt128*(result: var string; value: Int128) =
diff --git a/compiler/lambdalifting.nim b/compiler/lambdalifting.nim
index 96edba8c8..986879f8f 100644
--- a/compiler/lambdalifting.nim
+++ b/compiler/lambdalifting.nim
@@ -245,6 +245,13 @@ proc createTypeBoundOpsLL(g: ModuleGraph; refType: PType; info: TLineInfo; idgen
     if tfHasAsgn in refType.flags or optSeqDestructors in g.config.globalOptions:
       owner.flags.incl sfInjectDestructors
 
+proc genCreateEnv(env: PNode): PNode =
+  var c = newNodeIT(nkObjConstr, env.info, env.typ)
+  c.add newNodeIT(nkType, env.info, env.typ)
+  let e = copyTree(env)
+  e.flags.incl nfFirstWrite2
+  result = newAsgnStmt(e, c)
+
 proc liftIterSym*(g: ModuleGraph; n: PNode; idgen: IdGenerator; owner: PSym): PNode =
   # transforms  (iter)  to  (let env = newClosure[iter](); (iter, env))
   if liftingHarmful(g.config, owner): return n
@@ -268,7 +275,8 @@ proc liftIterSym*(g: ModuleGraph; n: PNode; idgen: IdGenerator; owner: PSym): PN
     addVar(v, env)
     result.add(v)
   # add 'new' statement:
-  result.add newCall(getSysSym(g, n.info, "internalNew"), env)
+  #result.add newCall(getSysSym(g, n.info, "internalNew"), env)
+  result.add genCreateEnv(env)
   createTypeBoundOpsLL(g, env.typ, n.info, idgen, owner)
   result.add makeClosure(g, idgen, iter, env, n.info)
 
@@ -596,7 +604,7 @@ proc rawClosureCreation(owner: PSym;
         addVar(v, unowned)
 
     # add 'new' statement:
-    result.add(newCall(getSysSym(d.graph, env.info, "internalNew"), env))
+    result.add genCreateEnv(env)
     if optOwnedRefs in d.graph.config.globalOptions:
       let unowned = c.unownedEnvVars[owner.id]
       assert unowned != nil
@@ -658,7 +666,7 @@ proc closureCreationForIter(iter: PNode;
     var vs = newNodeI(nkVarSection, iter.info)
     addVar(vs, vnode)
     result.add(vs)
-  result.add(newCall(getSysSym(d.graph, iter.info, "internalNew"), vnode))
+  result.add genCreateEnv(vnode)
   createTypeBoundOpsLL(d.graph, vnode.typ, iter.info, d.idgen, owner)
 
   let upField = lookupInRecord(v.typ.skipTypes({tyOwned, tyRef, tyPtr}).n, getIdent(d.graph.cache, upName))
@@ -943,7 +951,7 @@ proc liftForLoop*(g: ModuleGraph; body: PNode; idgen: IdGenerator; owner: PSym):
     addVar(v, newSymNode(env))
     result.add(v)
     # add 'new' statement:
-    result.add(newCall(getSysSym(g, env.info, "internalNew"), env.newSymNode))
+    result.add genCreateEnv(env.newSymNode)
     createTypeBoundOpsLL(g, env.typ, body.info, idgen, owner)
 
   elif op.kind == nkStmtListExpr:
diff --git a/compiler/parampatterns.nim b/compiler/parampatterns.nim
index 38b7faa07..534c3b5d1 100644
--- a/compiler/parampatterns.nim
+++ b/compiler/parampatterns.nim
@@ -183,7 +183,7 @@ type
     arLentValue,              # lent value
     arStrange                 # it is a strange beast like 'typedesc[var T]'
 
-proc exprRoot*(n: PNode): PSym =
+proc exprRoot*(n: PNode; allowCalls = true): PSym =
   var it = n
   while true:
     case it.kind
@@ -204,7 +204,7 @@ proc exprRoot*(n: PNode): PSym =
       if it.len > 0 and it.typ != nil: it = it.lastSon
       else: break
     of nkCallKinds:
-      if it.typ != nil and it.typ.kind in {tyVar, tyLent} and it.len > 1:
+      if allowCalls and it.typ != nil and it.typ.kind in {tyVar, tyLent} and it.len > 1:
         # See RFC #7373, calls returning 'var T' are assumed to
         # return a view into the first argument (if there is one):
         it = it[1]
diff --git a/compiler/sempass2.nim b/compiler/sempass2.nim
index 5d701c496..94b3f4fbd 100644
--- a/compiler/sempass2.nim
+++ b/compiler/sempass2.nim
@@ -67,10 +67,11 @@ type
     exc: PNode  # stack of exceptions
     tags: PNode # list of tags
     forbids: PNode # list of tags
-    bottom, inTryStmt, inExceptOrFinallyStmt, leftPartOfAsgn, inIfStmt: int
+    bottom, inTryStmt, inExceptOrFinallyStmt, leftPartOfAsgn, inIfStmt, currentBlock: int
     owner: PSym
     ownerModule: PSym
     init: seq[int] # list of initialized variables
+    scopes: Table[int, int] # maps var-id to its scope (see also `currentBlock`).
     guards: TModel # nested guards
     locked: seq[PNode] # locked locations
     gcUnsafe, isRecursive, isTopLevel, hasSideEffect, inEnforcedGcSafe: bool
@@ -189,6 +190,10 @@ proc makeVolatile(a: PEffects; s: PSym) {.inline.} =
   if a.inTryStmt > 0 and a.config.exc == excSetjmp:
     incl(s.flags, sfVolatile)
 
+proc varDecl(a: PEffects; n: PNode) {.inline.} =
+  if n.kind == nkSym:
+    a.scopes[n.sym.id] = a.currentBlock
+
 proc initVar(a: PEffects, n: PNode; volatileCheck: bool) =
   if n.kind != nkSym: return
   let s = n.sym
@@ -197,6 +202,23 @@ proc initVar(a: PEffects, n: PNode; volatileCheck: bool) =
     for x in a.init:
       if x == s.id: return
     a.init.add s.id
+    if a.scopes.getOrDefault(s.id) == a.currentBlock:
+      #[ Consider this case:
+
+      var x: T
+      while true:
+        if cond:
+          x = T() #1
+        else:
+          x = T() #2
+        use x
+
+      Even though both #1 and #2 are first writes we must use the `=copy`
+      here so that the old value is destroyed because `x`'s destructor is
+      run outside of the while loop. This is why we need the check here that
+      the assignment is done in the same logical block as `x` was declared in.
+      ]#
+      n.flags.incl nfFirstWrite2
 
 proc initVarViaNew(a: PEffects, n: PNode) =
   if n.kind != nkSym: return
@@ -1102,18 +1124,22 @@ proc track(tracked: PEffects, n: PNode) =
             createTypeBoundOps(tracked, child[i].typ, child.info)
         else:
           createTypeBoundOps(tracked, skipPragmaExpr(child[0]).typ, child.info)
-      if child.kind == nkIdentDefs and last.kind != nkEmpty:
+      if child.kind == nkIdentDefs:
         for i in 0..<child.len-2:
           let a = skipPragmaExpr(child[i])
-          initVar(tracked, a, volatileCheck=false)
-          addAsgnFact(tracked.guards, a, last)
-          notNilCheck(tracked, last, a.typ)
-      elif child.kind == nkVarTuple and last.kind != nkEmpty:
+          varDecl(tracked, a)
+          if last.kind != nkEmpty:
+            initVar(tracked, a, volatileCheck=false)
+            addAsgnFact(tracked.guards, a, last)
+            notNilCheck(tracked, last, a.typ)
+      elif child.kind == nkVarTuple:
         for i in 0..<child.len-1:
           if child[i].kind == nkEmpty or
             child[i].kind == nkSym and child[i].sym.name.s == "_":
             continue
-          initVar(tracked, child[i], volatileCheck=false)
+          varDecl(tracked, child[i])
+          if last.kind != nkEmpty:
+            initVar(tracked, child[i], volatileCheck=false)
           if last.kind in {nkPar, nkTupleConstr}:
             addAsgnFact(tracked.guards, child[i], last[i])
             notNilCheck(tracked, last[i], child[i].typ)
@@ -1128,6 +1154,7 @@ proc track(tracked: PEffects, n: PNode) =
   of nkBlockStmt, nkBlockExpr: trackBlock(tracked, n[1])
   of nkWhileStmt:
     # 'while true' loop?
+    inc tracked.currentBlock
     if isTrue(n[0]):
       trackBlock(tracked, n[1])
     else:
@@ -1139,8 +1166,10 @@ proc track(tracked: PEffects, n: PNode) =
       track(tracked, n[1])
       setLen(tracked.init, oldState)
       setLen(tracked.guards.s, oldFacts)
+    dec tracked.currentBlock
   of nkForStmt, nkParForStmt:
     # we are very conservative here and assume the loop is never executed:
+    inc tracked.currentBlock
     let oldState = tracked.init.len
 
     let oldFacts = tracked.guards.s.len
@@ -1182,6 +1211,8 @@ proc track(tracked: PEffects, n: PNode) =
     track(tracked, loopBody)
     setLen(tracked.init, oldState)
     setLen(tracked.guards.s, oldFacts)
+    dec tracked.currentBlock
+
   of nkObjConstr:
     when false: track(tracked, n[0])
     let oldFacts = tracked.guards.s.len
@@ -1435,6 +1466,7 @@ proc initEffects(g: ModuleGraph; effects: PNode; s: PSym; t: var TEffects; c: PC
   t.graph = g
   t.config = g.config
   t.c = c
+  t.currentBlock = 1
 
 proc hasRealBody(s: PSym): bool =
   ## also handles importc procs with runnableExamples, which requires `=`,
@@ -1455,6 +1487,12 @@ proc trackProc*(c: PContext; s: PSym, body: PNode) =
   var t: TEffects
   initEffects(g, inferredEffects, s, t, c)
   rawInitEffects g, effects
+
+  if not isEmptyType(s.typ[0]) and
+     s.kind in {skProc, skFunc, skConverter, skMethod}:
+    var res = s.ast[resultPos].sym # get result symbol
+    t.scopes[res.id] = t.currentBlock
+
   track(t, body)
 
   if s.kind != skMacro:
diff --git a/compiler/transf.nim b/compiler/transf.nim
index 7ab67873b..dd0d546eb 100644
--- a/compiler/transf.nim
+++ b/compiler/transf.nim
@@ -104,9 +104,11 @@ proc transformSons(c: PTransf, n: PNode): PNode =
   for i in 0..<n.len:
     result[i] = transform(c, n[i])
 
-proc newAsgnStmt(c: PTransf, kind: TNodeKind, le: PNode, ri: PNode): PNode =
+proc newAsgnStmt(c: PTransf, kind: TNodeKind, le: PNode, ri: PNode; isFirstWrite: bool): PNode =
   result = newTransNode(kind, ri.info, 2)
   result[0] = le
+  if isFirstWrite:
+    le.flags.incl nfFirstWrite2
   result[1] = ri
 
 proc transformSymAux(c: PTransf, n: PNode): PNode =
@@ -365,9 +367,9 @@ proc transformYield(c: PTransf, n: PNode): PNode =
     case lhs.kind
     of nkSym:
       internalAssert c.graph.config, lhs.sym.kind == skForVar
-      result = newAsgnStmt(c, nkFastAsgn, lhs, rhs)
+      result = newAsgnStmt(c, nkFastAsgn, lhs, rhs, false)
     of nkDotExpr:
-      result = newAsgnStmt(c, nkAsgn, lhs, rhs)
+      result = newAsgnStmt(c, nkAsgn, lhs, rhs, false)
     else:
       internalAssert c.graph.config, false
   result = newTransNode(nkStmtList, n.info, 0)
@@ -734,7 +736,7 @@ proc transformFor(c: PTransf, n: PNode): PNode =
       # generate a temporary and produce an assignment statement:
       var temp = newTemp(c, t, formal.info)
       addVar(v, temp)
-      stmtList.add(newAsgnStmt(c, nkFastAsgn, temp, arg))
+      stmtList.add(newAsgnStmt(c, nkFastAsgn, temp, arg, true))
       idNodeTablePut(newC.mapping, formal, temp)
     of paVarAsgn:
       assert(skipTypes(formal.typ, abstractInst).kind in {tyVar})
@@ -744,7 +746,7 @@ proc transformFor(c: PTransf, n: PNode): PNode =
       # arrays will deep copy here (pretty bad).
       var temp = newTemp(c, arg.typ, formal.info)
       addVar(v, temp)
-      stmtList.add(newAsgnStmt(c, nkFastAsgn, temp, arg))
+      stmtList.add(newAsgnStmt(c, nkFastAsgn, temp, arg, true))
       idNodeTablePut(newC.mapping, formal, temp)
 
   let body = transformBody(c.graph, c.idgen, iter, true)