Constant folding for integer casts (#8095)

1 files changed, 25 insertions, 7 deletions

diff --git a/compiler/semfold.nim b/compiler/semfold.nim
index 444940144..fe1871881 100644
--- a/compiler/semfold.nim
+++ b/compiler/semfold.nim
@@ -450,21 +450,38 @@ proc rangeCheck(n: PNode, value: BiggestInt; g: ModuleGraph) =
     localError(g.config, n.info, "cannot convert " & $value &
                                      " to " & typeToString(n.typ))
 
-proc foldConv*(n, a: PNode; g: ModuleGraph; check = false): PNode =
+proc foldConv(n, a: PNode; g: ModuleGraph; check = false): PNode =
+  let dstTyp = skipTypes(n.typ, abstractRange)
+  let srcTyp = skipTypes(a.typ, abstractRange)
+
   # XXX range checks?
-  case skipTypes(n.typ, abstractRange).kind
-  of tyInt..tyInt64, tyUInt..tyUInt64:
-    case skipTypes(a.typ, abstractRange).kind
+  case dstTyp.kind
+  of tyInt..tyInt64, tyUint..tyUInt64:
+    case srcTyp.kind
     of tyFloat..tyFloat64:
       result = newIntNodeT(int(getFloat(a)), n, g)
-    of tyChar: result = newIntNodeT(getOrdValue(a), n, g)
+    of tyChar:
+      result = newIntNodeT(getOrdValue(a), n, g)
+    of tyUInt8..tyUInt32, tyInt8..tyInt32:
+      let fromSigned = srcTyp.kind in tyInt..tyInt64
+      let toSigned = dstTyp.kind in tyInt..tyInt64
+
+      let mask = lastOrd(g.config, dstTyp, fixedUnsigned=true)
+
+      var val =
+        if toSigned:
+          a.getOrdValue mod mask
+        else:
+          a.getOrdValue and mask
+
+      result = newIntNodeT(val, n, g)
     else:
       result = a
       result.typ = n.typ
     if check and result.kind in {nkCharLit..nkUInt64Lit}:
       rangeCheck(n, result.intVal, g)
   of tyFloat..tyFloat64:
-    case skipTypes(a.typ, abstractRange).kind
+    case srcTyp.kind
     of tyInt..tyInt64, tyEnum, tyBool, tyChar:
       result = newFloatNodeT(toBiggestFloat(getOrdValue(a)), n, g)
     else:
@@ -742,7 +759,8 @@ proc getConstExpr(m: PSym, n: PNode; g: ModuleGraph): PNode =
   of nkHiddenStdConv, nkHiddenSubConv, nkConv:
     var a = getConstExpr(m, n.sons[1], g)
     if a == nil: return
-    result = foldConv(n, a, g, check=n.kind == nkHiddenStdConv)
+    # XXX: we should enable `check` for other conversion types too
+    result = foldConv(n, a, g, check=n.kind == nkHiddenSubConv)
   of nkCast:
     var a = getConstExpr(m, n.sons[1], g)
     if a == nil: return