cmd/compile: flagalloc doesn't delete dead instructions #65039
Description
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.