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cmd/compile: suboptimal code generation for bit test (regression in Go 1.15) #44228

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cespare opened this issue Feb 12, 2021 · 1 comment
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cmd/compile: suboptimal code generation for bit test (regression in Go 1.15) #44228

cespare opened this issue Feb 12, 2021 · 1 comment
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FrozenDueToAge NeedsFix The path to resolution is known, but the work has not been done. Performance
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Go1.17

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@cespare
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cespare commented Feb 12, 2021

What version of Go are you using (go version)?

Go 1.15.8 and tip

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

linux/amd64

We're upgrading from an old Go version (1.13.x) to Go 1.15 and I'm looking at a noticeable benchmark regression.

Here's a repro:

https://github.com/cespare/misc/blob/00c5f6e9852290b26ea32fc437c69eaa47fe63dd/btregression/btregression.go

The interesting bit is simply

var charSet [4]uint64

func inCharSet(b byte) bool {
	return charSet[b>>6]&(1<<(b&63)) != 0
}

It just does a lookup in a small global bitset.

On my machine (i7-8700K), here's the benchmark results comparing Go 1.14.15 and Go 1.15.8:

name          old time/op  new time/op  delta
InCharSet-12  10.5ns ± 1%  15.7ns ± 0%  +49.90%  (p=0.008 n=5+5)

The results are similar on tip.

The generated code used to use a BT instruction, but now it uses a longer sequence instead.

Go 1.14.15:

btregression.go:6       MOVL BX, SI
btregression.go:6       SHRL $0x6, BL
btregression.go:6       MOVZX BL, BX
btregression.go:6       LEAQ github.com/cespare/misc/btregression.charSet(SB), DI
btregression.go:6       MOVQ 0(DI)(BX*8), BX
btregression.go:6       BTQ SI, BX
btregression_test.go:9  JAE 0x4fd5ed

Go 1.15.8

btregression.go:6       MOVL BX, SI
btregression.go:6       SHRL $0x6, BL
btregression.go:6       MOVZX BL, BX
btregression_test.go:6  MOVQ CX, DI
btregression.go:6       MOVL SI, CX
btregression.go:6       MOVL $0x1, R8
btregression.go:6       SHLQ CL, R8
btregression.go:6       LEAQ github.com/cespare/misc/btregression.charSet(SB), SI
btregression.go:6       ANDQ 0(SI)(BX*8), R8
btregression.go:6       TESTQ R8, R8
btregression_test.go:9  JE 0x50a28d

I bisected the change back to 98cb767 (CL 217097).

/cc @randall77 @josharian
@ALTree ALTree added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Feb 12, 2021
@randall77 randall77 added this to the Go1.17 milestone Feb 12, 2021
@randall77 randall77 self-assigned this Feb 12, 2021
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Change https://golang.org/cl/291749 mentions this issue: cmd/compile: improve bit test code

@dmitshur dmitshur added NeedsFix The path to resolution is known, but the work has not been done. and removed NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. labels Feb 13, 2021
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@cespare @dmitshur @mvdan @ALTree @randall77 @gopherbot