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cmd/compile: Go 1.27 regression in BigEndian.Uint32 load combining with array copy #81839
Description
Go version
go version go1.27.1 linux/amd64
Output of go env in your module/workspace:
n/aWhat did you do?
We noticed a substantial slowdown in microbenchmarks for a specialized hash table implementation.
Here's a minimized reproducer:
package slowload
import (
"encoding/binary"
"testing"
)
//go:noinline
func f(k [4]byte) uint32 {
q := k
return binary.BigEndian.Uint32(q[:])
}
func BenchmarkIndex(b *testing.B) {
k := [4]byte{1, 2, 3, 4}
for b.Loop() {
f(k)
}
}
What did you see happen?
There's a slowdown on amd64:
goos: linux
goarch: amd64
pkg: github.com/cespare/misc/slowload
cpu: AMD Ryzen 9 3900X 12-Core Processor
│ amd64/126.txt │ amd64/127.txt │
│ sec/op │ sec/op vs base │
Index-24 1.906n ± 3% 2.185n ± 3% +14.66% (p=0.000 n=10)
If we look at the generated code, we see:
1.26.7:
MOVL 0x4(SP), CX
BSWAP CX
1.27.1:
MOVZX 0x22(SP), BX
ROLW $0x8, BX
MOVZX BX, BX
MOVZX 0x21(SP), SI
MOVQ CX, 0x4(SP)
MOVL DX, 0xc(SP)
SHLL $0x10, SI
ORL SI, BX
MOVZX 0x4(SP), CX
SHLL $0x18, CX
ORL BX, CX
On arm64 (my test machine is Graviton 5) we see a much more dramatic slowdown:
goos: linux
goarch: arm64
pkg: github.com/cespare/misc/slowload
│ arm64/126.txt │ arm64/127.txt │
│ sec/op │ sec/op vs base │
Index-48 1.856n ± 0% 4.229n ± 1% +127.86% (p=0.000 n=10)
The generated code:
1.26.7:
MOVWU 40(RSP), R1
MOVW R1, 20(RSP)
MOVWU 20(RSP), R1
REVW R1, R0
1.27.1:
MOVWU 40(RSP), R1
MOVHU 42(RSP), R2
REV16W R2, R2
UBFX $0, R2, $16, R2
MOVBU 41(RSP), R3
MOVW R1, 20(RSP)
ORR R3<<16, R2, R1
MOVBU 20(RSP), R2
ORR R2<<24, R1, R0
What did you expect to see?
No regression.
The weird-looking array copy (q := k) is important. In our real code, the copy happens due to a function call.
I bisected this regression to CL 748200 ("cmd/compile: forward small Load through Move to avoid redundant copies"). Issue #80980 is also complaining about a regression from that CL (I'm not sure if this should be considered a dup of #80980 or not).
/cc @Dorbmon @randall77
Activity
gopherbot commentedon Sep 29, 2026
Change https://go.dev/cl/841105 mentions this issue:
cmd/compile: forward the whole of a copy's loads through a Movecmd/compile: forward the whole of a copy's loads through a Move