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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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@cespare
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@cespare 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

@mvdan mvdan added the Performance 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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@gopherbot gopherbot commented Feb 12, 2021

Change https://golang.org/cl/291749 mentions this issue: cmd/compile: improve bit test code

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