[go] math/big: only use pool for large allocations
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Keith Randall (Gerrit)
Jun 5, 2025, 9:19:15 PM6/5/25
to Russ Cox, goph...@pubsubhelper.golang.org, Keith Randall, golang-co...@googlegroups.com
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Commit message
math/big: only use pool for large allocations
The native allocator seems faster for small things.
Only start accessing the pool if we need something large.
Currently "large" is more than 4 words. It seems a reasonable
threshold, although I didn't do much experimentation to pick
a number.
Fixes #73999
1.24.2 to tip:
goos: darwin
goarch: arm64
pkg: github.com/dustin/go-humanize
cpu: Apple M2 Ultra
│ base │ pre │
│ sec/op │ sec/op vs base │
ParseBigBytes-24 625.0n ± 1% 665.8n ± 0% +6.53% (p=0.000 n=10)
1.24.2 to tip+this CL:
goos: darwin
goarch: arm64
pkg: github.com/dustin/go-humanize
cpu: Apple M2 Ultra
│ base │ post │
│ sec/op │ sec/op vs base │
ParseBigBytes-24 625.0n ± 1% 626.8n ± 0% ~ (p=0.470 n=10)
Change-Id: Ic071e6d82d4aa4a0d3a6ec6e026f513c83cb0b37
Change diff
diff --git a/src/math/big/nat.go b/src/math/big/nat.go
index 43e36d3..ca966ca 100644
--- a/src/math/big/nat.go
+++ b/src/math/big/nat.go
@@ -274,24 +274,24 @@
// so that they can handle trivial stack-free cases without forcing the
// caller to obtain and free a stack that will be unused. These functions
// document that they accept a nil *stack in their doc comments.
-type stack struct {
+type stackInner struct {
w []Word
}
-var stackPool sync.Pool
+var stackPool sync.Pool // pool of *stackInner
// getStack returns a temporary stack.
// The caller must call [stack.free] to give up use of the stack when finished.
-func getStack() *stack {
- s, _ := stackPool.Get().(*stack)
+func getStackInner() *stackInner {
+ s, _ := stackPool.Get().(*stackInner)
if s == nil {
- s = new(stack)
+ s = new(stackInner)
}
return s
}
// free returns the stack for use by another calculation.
-func (s *stack) free() {
+func (s *stackInner) free() {
s.w = s.w[:0]
stackPool.Put(s)
}
@@ -299,7 +299,7 @@
// save returns the current stack pointer.
// A future call to restore with the same value
// frees any temporaries allocated on the stack after the call to save.
-func (s *stack) save() int {
+func (s *stackInner) save() int {
return len(s.w)
}
@@ -310,12 +310,12 @@
//
// which makes sure to pop any temporaries allocated in the current function
// from the stack before returning.
-func (s *stack) restore(n int) {
+func (s *stackInner) restore(n int) {
s.w = s.w[:n]
}
// nat returns a nat of n words, allocated on the stack.
-func (s *stack) nat(n int) nat {
+func (s *stackInner) nat(n int) nat {
nr := (n + 3) &^ 3 // round up to multiple of 4
off := len(s.w)
s.w = slices.Grow(s.w, nr)
@@ -327,6 +327,56 @@
return x
}
+type stack struct {
+ si *stackInner
+}
+
+func getStack() *stack {
+ return &stack{}
+}
+func (s *stack) free() {
+ si := s.si
+ if si != nil {
+ si.free()
+ }
+}
+func (s *stack) save() int {
+ si := s.si
+ if si == nil {
+ return 0
+ }
+ return si.save()
+}
+func (s *stack) restore(n int) {
+ si := s.si
+ if si == nil {
+ return
+ }
+ si.restore(n)
+}
+func (s *stack) nat(n int) nat {
+ si := s.si
+ if si == nil {
+ if n <= 4 {
+ // For small allocations, just ask the allocator.
+ // It isn't worth pooling these allocations.
+ // See issue 73999.
+ r := slices.Grow(nat(nil), n)
+ r = r[:n]
+ if n > 0 {
+ r[0] = 0xabcdef
+ }
+ return r
+ }
+ si, _ = stackPool.Get().(*stackInner)
+ if si == nil {
+ si = new(stackInner)
+ }
+ s.si = si
+ }
+ return si.nat(n)
+}
+
// bitLen returns the length of x in bits.
// Unlike most methods, it works even if x is not normalized.
func (x nat) bitLen() int {
Change information
Files:
- M src/math/big/nat.go
Change size: M
Delta: 1 file changed, 59 insertions(+), 9 deletions(-)
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Keith Randall (Gerrit)
Apr 27, 2026, 3:48:30 PMApr 27
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Keith Randall (Gerrit)
May 18, 2026, 4:17:19 PM (16 hours ago) May 18
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Keith Randall (Gerrit)
May 18, 2026, 4:17:37 PM (16 hours ago) May 18
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Cuong Manh Le (Gerrit)
1:06 AM (7 hours ago) 1:06 AM
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1 comment
File src/math/big/nat.go
Line 268, Patchset 3 (Latest):
// A stack provides temporary storage for complex calculationsCuong Manh Le . unresolved
This comment needs updating.
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