added system.newSeqOfCap for improved efficiency

2 files changed, 20 insertions, 0 deletions

diff --git a/lib/system.nim b/lib/system.nim
index 40607bdb3..00eae7337 100644
--- a/lib/system.nim
+++ b/lib/system.nim
@@ -669,6 +669,12 @@ proc newSeq*[T](len = 0.Natural): seq[T] =
   ##   #inputStrings[3] = "out of bounds"
   newSeq(result, len)
 
+proc newSeqOfCap*[T](cap: Natural): seq[T] {.
+  magic: "NewSeqOfCap", noSideEffect.} =
+  ## creates a new sequence of type ``seq[T]`` with length 0 and capacity
+  ## ``cap``.
+  discard
+
 proc len*[TOpenArray: openArray|varargs](x: TOpenArray): int {.
   magic: "LengthOpenArray", noSideEffect.}
 proc len*(x: string): int {.magic: "LengthStr", noSideEffect.}
diff --git a/lib/system/mmdisp.nim b/lib/system/mmdisp.nim
index 7ef26a7c1..186349152 100644
--- a/lib/system/mmdisp.nim
+++ b/lib/system/mmdisp.nim
@@ -354,6 +354,12 @@ elif defined(gogc):
     cast[PGenericSeq](result).reserved = len
     cast[PGenericSeq](result).elemSize = typ.base.size
 
+  proc nimNewSeqOfCap(typ: PNimType, cap: int): pointer {.compilerproc.} =
+    result = newObj(typ, cap * typ.base.size + GenericSeqSize)
+    cast[PGenericSeq](result).len = 0
+    cast[PGenericSeq](result).reserved = cap
+    cast[PGenericSeq](result).elemSize = typ.base.size
+
   proc growObj(old: pointer, newsize: int): pointer =
     # the Go GC doesn't have a realloc
     var
@@ -447,6 +453,7 @@ elif defined(nogc) and defined(useMalloc):
     result = newObj(typ, addInt(mulInt(len, typ.base.size), GenericSeqSize))
     cast[PGenericSeq](result).len = len
     cast[PGenericSeq](result).reserved = len
+
   proc newObjNoInit(typ: PNimType, size: int): pointer =
     result = alloc(size)
 
@@ -506,6 +513,7 @@ elif defined(nogc):
     result = newObj(typ, addInt(mulInt(len, typ.base.size), GenericSeqSize))
     cast[PGenericSeq](result).len = len
     cast[PGenericSeq](result).reserved = len
+
   proc growObj(old: pointer, newsize: int): pointer =
     result = realloc(old, newsize)
 
@@ -545,4 +553,10 @@ else:
   else:
     include "system/gc"
 
+when not declared(nimNewSeqOfCap):
+  proc nimNewSeqOfCap(typ: PNimType, cap: int): pointer {.compilerproc.} =
+    result = newObj(typ, addInt(mulInt(cap, typ.base.size), GenericSeqSize))
+    cast[PGenericSeq](result).len = 0
+    cast[PGenericSeq](result).reserved = cap
+
 {.pop.}