cmd/compile: flagalloc doesn't delete dead instructions

[dominikh]

Go version

go version devel go1.22-b7c630dc3a Tue Jan 9 01:36:54 2024 +0000 linux/amd64

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/home/dominikh/.cache/go-build'
GOENV='/home/dominikh/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/dominikh/prj/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/dominikh/prj'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/home/dominikh/prj/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/home/dominikh/prj/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='devel go1.22-b7c630dc3a Tue Jan 9 01:36:54 2024 +0000'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/home/dominikh/prj/src/example.com/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build1294566987=/tmp/go-build -gno-record-gcc-switches'

What did you do?

Compile https://go.dev/play/p/4RMX7fsEHw9

package pkg

import (
	"math/bits"
)

var Sink uint

func Foo(x uint64) {
	log2 := uint(63 - bits.LeadingZeros64(x))
	if x&(x-1) != 0 {
		log2++
	}
	Sink = log2
}

What did you see happen?

The generated assembly contains a dead TESTQ instruction at 0x0005

command-line-arguments.Foo STEXT nosplit size=42 args=0x8 locals=0x0 funcid=0x0 align=0x0
	0x0000 00000 (/home/dominikh/prj/src/example.com/foo.go:9)	TEXT	command-line-arguments.Foo(SB), NOSPLIT|NOFRAME|ABIInternal, $0-8
	0x0000 00000 (/home/dominikh/prj/src/example.com/foo.go:9)	FUNCDATA	$0, gclocals·g2BeySu+wFnoycgXfElmcg==(SB)
	0x0000 00000 (/home/dominikh/prj/src/example.com/foo.go:9)	FUNCDATA	$1, gclocals·g2BeySu+wFnoycgXfElmcg==(SB)
	0x0000 00000 (/home/dominikh/prj/src/example.com/foo.go:9)	FUNCDATA	$5, command-line-arguments.Foo.arginfo1(SB)
	0x0000 00000 (/home/dominikh/prj/src/example.com/foo.go:9)	FUNCDATA	$6, command-line-arguments.Foo.argliveinfo(SB)
	0x0000 00000 (/home/dominikh/prj/src/example.com/foo.go:9)	PCDATA	$3, $1
	0x0000 00000 (/home/dominikh/prj/src/example.com/foo.go:11)	LEAQ	-1(AX), CX
	0x0004 00004 (/home/dominikh/prj/src/example.com/foo.go:10)	XCHGL	AX, AX
	0x0005 00005 (/home/dominikh/prj/src/example.com/foo.go:11)	TESTQ	AX, CX
	0x0008 00008 (/home/dominikh/prj/go/src/math/bits/bits.go:37)	BSRQ	AX, DX
	0x000c 00012 (/home/dominikh/prj/go/src/math/bits/bits.go:37)	MOVQ	$-1, BX
	0x0013 00019 (/home/dominikh/prj/go/src/math/bits/bits.go:37)	CMOVQEQ	BX, DX
	0x0017 00023 (/home/dominikh/prj/src/example.com/foo.go:12)	LEAQ	1(DX), BX
	0x001b 00027 (/home/dominikh/prj/src/example.com/foo.go:11)	TESTQ	AX, CX
	0x001e 00030 (/home/dominikh/prj/src/example.com/foo.go:14)	CMOVQNE	BX, DX
	0x0022 00034 (/home/dominikh/prj/src/example.com/foo.go:14)	MOVQ	DX, command-line-arguments.Sink(SB)
	0x0029 00041 (/home/dominikh/prj/src/example.com/foo.go:11)	RET

What did you expect to see?

CMOVQNE at 0x001e uses ZF set by TESTQ AX, CX. The ZF set by TESTQ at 0x0005 gets immediately clobbered by the BSRQ that follows. flagalloc inserts a second TESTQ at 0x001b, but doesn't clean up the old TESTQ, which is now dead.

[dominikh]

Related: #24537 (comment)

[dmitshur]

CC @golang/compiler.

[kungfukennyg]

Has anyone claimed this or can I take a stab at it?

[randall77]

@kungfukennyg go for it.