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fixes #25718; setLenUnit slow (#25743)

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fixes #25718

This pull request optimizes sequence allocation in the Nim standard
library by introducing a way to create uninitialized sequence payloads
for element types that don't require zero-initialization. The changes
allow for more efficient memory allocation when initializing sequences
with types that have no references, avoiding unnecessary zeroing of
memory.

Sequence allocation and initialization improvements:

* Added the `newSeqUninitRaw` procedure to create sequence payloads with
a specified length without forcing zero-initialization for element types
marked as `ntfNoRefs`. (`lib/system/sysstr.nim`,
[lib/system/sysstr.nimR277-R292](diffhunk://#diff-bcaa1967f436ad03877f353823c08a8b4a719fe387629d33aab4bddf16534b5eR277-R292))
* Modified the `extendCapacityRaw` procedure and the `setLengthSeqImpl`
template to use `newSeqUninitRaw` when zero-initialization is not
required, controlled by the `doInit` static parameter.
(`lib/system/sysstr.nim`,
[[1]](diffhunk://#diff-bcaa1967f436ad03877f353823c08a8b4a719fe387629d33aab4bddf16534b5eR277-R292)
[[2]](diffhunk://#diff-bcaa1967f436ad03877f353823c08a8b4a719fe387629d33aab4bddf16534b5eL316-R335)
This commit is contained in:
ringabout
2026-04-20 02:12:01 +08:00
committed by GitHub
parent 98131a9fa1
commit 5948dbbeed
1 changed files with 19 additions and 5 deletions
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24
lib/system/sysstr.nim
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@@ -299,12 +299,22 @@ proc incrSeqV3(s: PGenericSeq, typ: PNimType): PGenericSeq {.compilerproc.} =
# since we steal the content from 's', it's crucial to set s's len to 0.
s.len = 0
proc newSeqUninitRaw(typ: PNimType; len: int): pointer {.inline.} =
## Creates a sequence payload with capacity and length `len` without
## forcing zero-initialization for `ntfNoRefs` element types.
result = nimNewSeqOfCap(typ, len)
cast[PGenericSeq](result).len = len
proc extendCapacityRaw(src: PGenericSeq; typ: PNimType;
elemSize, elemAlign, newLen: int): PGenericSeq {.inline.} =
elemSize, elemAlign, newLen: int;
doInit: static bool): PGenericSeq {.inline.} =
## Reallocs `src` to fit `newLen` elements without any checks.
## Capacity always increases to at least next `resize` step.
let newCap = max(resize(src.space), newLen)
result = cast[PGenericSeq](newSeq(typ, newCap))
when doInit:
result = cast[PGenericSeq](newSeq(typ, newCap))
else:
result = cast[PGenericSeq](newSeqUninitRaw(typ, newCap))
copyMem(dataPointer(result, elemAlign), dataPointer(src, elemAlign), src.len * elemSize)
# since we steal the content from 's', it's crucial to set s's len to 0.
src.len = 0
@@ -335,15 +345,19 @@ proc truncateRaw(src: PGenericSeq; baseFlags: set[TNimTypeFlag]; isTrivial: bool
((result.len-%newLen) *% elemSize))
template setLengthSeqImpl(s: PGenericSeq, typ: PNimType, newLen: int; isTrivial: bool;
doInit: static bool) =
doInit: static bool) =
if s == nil:
if newLen == 0: return s
else: return cast[PGenericSeq](newSeq(typ, newLen)) # newSeq zeroes!
else:
when doInit:
return cast[PGenericSeq](newSeq(typ, newLen)) # newSeq zeroes!
else:
return cast[PGenericSeq](newSeqUninitRaw(typ, newLen))
else:
let elemSize = typ.base.size
let elemAlign = typ.base.align
result = if newLen > s.space:
s.extendCapacityRaw(typ, elemSize, elemAlign, newLen)
s.extendCapacityRaw(typ, elemSize, elemAlign, newLen, doInit)
elif newLen < s.len:
s.truncateRaw(typ.base.flags, isTrivial, elemSize, elemAlign, newLen)
else: