Golang cmd/compile: regalloc can leave unused instructions after performing rematerialization and removing the only dependent

Go version

go version go1.22.4 linux/amd64

Output of `go env` in your module/workspace:

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/home/ren/.cache/go-build'
GOENV='/home/ren/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/ren/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/ren/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.22.4'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/home/ren/dev/junk/bugXXX/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-build1837702511=/tmp/go-build -gno-record-gcc-switches'

What did you do?

func isIn(c byte, lo, hi uint64) bool {
    var m uint64
    if c < 128 {
        m = hi
        if c < 64 {
            m = lo
        }
    }
    return (1<<(c%64))&m != 0
}

https://go.godbolt.org/z/EMn16qvPY

What did you see happen?

On amd64, an unnecessary CMPB instruction is generated, immediately followed by another CMPB.

CMPB    AL, $-128
CMPB    AL, $64
CMOVQCS BX, CX
CMPB    AL, $-128
MOVL    $0, DX
CMOVQCS CX, DX
BTQ AX, DX
SETCS   AL
RET

Similarly on arm64, two CMPWs one after the other.

MOVBU   R0, R3
MOVD    $1, R4
LSL R0, R4, R4
CMPW    $128, R3
CMPW    $64, R3
CSEL    LO, R1, R2, R1
CMPW    $128, R3
CSEL    LO, R1, ZR, R1
TST R1, R4
CSET    NE, R0
RET (R30)

What did you expect to see?

No unnecessary compares.

Comment From: Jorropo

Thx for the bug report. Minimized example:

func isIn(c byte, lo, hi uint64) uint64 {
    var m uint64
    if c < 128 {
        m = hi
        if c < 64 {
            m = lo
        }
    }
    return m
}

This is probably fine, as in completely free, the compare are fast and this one does not havet have any dependencies. There is still cost in fetching and caching bytecodes and maybe sometime it makes loops unaligned but that chance.

Comment From: gabyhelp

Similar Issues

https://github.com/golang/go/issues/53599
https://github.com/golang/go/issues/14761
https://github.com/golang/go/issues/43357
https://github.com/golang/go/issues/50325
https://github.com/golang/go/issues/65039

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