cmd/asm: incorrectly signaled 'R15 as clobbered' in MULXQ #58632
Description
What version of Go are you using (go version)?
$ go version go version go1.20.1 linux/amd64
Does this issue reproduce with the latest release?
Yes
What operating system and processor architecture are you using (go env)?
go env Output
$ go envGO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/user/.cache/go-build"
GOENV="/home/user/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/user/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/user/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.20.1"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
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 -fdebug-prefix-map=/tmp/go-build1448370878=/tmp/go-build -gno-record-gcc-switches"
What did you do?
main.go
package main
var myGlobal = []byte("helloworld")
//go:noescape
func a1()
//go:noescape
func a2()
//go:noescape
func a3()
//go:noescape
func a4()
func main() {
_ = myGlobal
a1(); a2(); a3(); a4()
}mulx_amd64.s
TEXT ·a1(SB),0,$0-0
CMPL ·myGlobal(SB), $0
MULXQ R15, AX, DX
RET
TEXT ·a2(SB),0,$0-0
CMPL ·myGlobal(SB), $0
MULXQ AX, R15, DX
RET
TEXT ·a3(SB),0,$0-0
CMPL ·myGlobal(SB), $0
MULXQ AX, DX, R15
RET
TEXT ·a4(SB),0,$0-0
CMPL ·myGlobal(SB), $0
MOVQ AX, R15
RETCommand:
$ go mod init example.com/mulx
$ go build -buildmode=pluginWhat did you expect to see?
Compiler triggers 1 error. This is a true error, since R15 is being used as an input of MULX.
asm: mulx_amd64.s:3: when dynamic linking, R15 is clobbered by a global variable access and is used here: 00003 (/home/user/mulx_amd64.s:3) MULXQ R15, AX, DX
asm: assembly failedWhat did you see instead?
The compiler signals 3 errors. The first one is a true error. The second and third are false positives, since R15 is being used as an output of MULX.
To confirm this, note that the a4 function has no error as R15 is used as output of MOVQ.
asm: mulx_amd64.s:3: when dynamic linking, R15 is clobbered by a global variable access and is used here: 00003 (/home/user/mulx_amd64.s:3) MULXQ R15, AX, DX
asm: mulx_amd64.s:8: when dynamic linking, R15 is clobbered by a global variable access and is used here: 00007 (/home/user/mulx_amd64.s:8) MULXQ AX, R15, DX
asm: mulx_amd64.s:13: when dynamic linking, R15 is clobbered by a global variable access and is used here: 00011 (/home/user/mulx_amd64.s:13) MULXQ AX, DX, R15
asm: assembly failedOther info:
The closest CL related to this issue is: https://go-review.googlesource.com/c/go/+/283474
I found this issue while working in cloudflare/circl#407