Golang cmd/asm: amd64 PCALIGN between labels generates wrong jump targets

Go version

go version go1.24.4 windows/amd64

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

λ go env
set AR=ar
set CC=gcc
set CGO_CFLAGS=-O2 -g
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-O2 -g
set CGO_ENABLED=0
set CGO_FFLAGS=-O2 -g
set CGO_LDFLAGS=-O2 -g
set CXX=g++
set GCCGO=gccgo
set GO111MODULE=
set GOAMD64=v1
set GOARCH=amd64
set GOAUTH=netrc
set GOBIN=
set GOCACHE=C:\Users\klaus\AppData\Local\go-build
set GOCACHEPROG=
set GODEBUG=
set GOENV=C:\Users\klaus\AppData\Roaming\go\env
set GOEXE=.exe
set GOEXPERIMENT=
set GOFIPS140=off
set GOFLAGS=
set GOGCCFLAGS=-m64 -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=c:\temp\wintemp\go-build338002192=/tmp/go-build -gno-record-gcc-switches
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMOD=c:\gopath\src\github.com\minio\minlz\go.mod
set GOMODCACHE=c:\gopath\pkg\mod
set GONOPROXY=github.com/miniohq
set GONOSUMDB=github.com/miniohq
set GOOS=windows
set GOPATH=c:\gopath
set GOPRIVATE=github.com/miniohq
set GOPROXY=https://proxy.golang.org,direct
set GOROOT=C:\Go
set GOSUMDB=sum.golang.org
set GOTELEMETRY=local
set GOTELEMETRYDIR=C:\Users\klaus\AppData\Roaming\go\telemetry
set GOTMPDIR=
set GOTOOLCHAIN=auto
set GOTOOLDIR=C:\Go\pkg\tool\windows_amd64
set GOVCS=
set GOVERSION=go1.24.4
set GOWORK=
set PKG_CONFIG=pkg-config

What did you do?

main.go

package main

import "fmt"

func repro()

func main() {
    repro()
    fmt.Println("ok")
}

repro_amd64.s

TEXT ·repro(SB), $0
   JMP prealigned
   INT $3 // should never be reached

prealigned:
   PCALIGN $0x10

aligned:
   RET

What did you see happen?

λ go run .
fatal error: fault
[signal 0x80000003 code=0x0 addr=0x0 pc=0x88a993]

goroutine 1 gp=0xc0000021c0 m=0 mp=0x960600 [running]:
runtime.throw({0x8aee23?, 0xc000053ee8?})
        C:/Go/src/runtime/panic.go:1101 +0x4d fp=0xc000053eb8 sp=0xc000053e88 pc=0x85780d
runtime.sigpanic()
        C:/Go/src/runtime/signal_windows.go:426 +0x1f9 fp=0xc000053f00 sp=0xc000053eb8 pc=0x83d879
main.repro()
        c:/gopath/src/github.com/minio/minlz/testdata/repro/repro_amd64.s:3 +0x3 fp=0xc000053f08 sp=0xc000053f00 pc=0x88a993
main.main()
        c:/gopath/src/github.com/minio/minlz/testdata/repro/main.go:8 +0x13 fp=0xc000053f50 sp=0xc000053f08 pc=0x88a933
runtime.main()
        C:/Go/src/runtime/proc.go:283 +0x27d fp=0xc000053fe0 sp=0xc000053f50 pc=0x82b09d
runtime.goexit({})
        C:/Go/src/runtime/asm_amd64.s:1700 +0x1 fp=0xc000053fe8 sp=0xc000053fe0 pc=0x85e141

goroutine 2 gp=0xc0000028c0 m=nil [force gc (idle)]:
runtime.gopark(0x0?, 0x0?, 0x0?, 0x0?, 0x0?)
        C:/Go/src/runtime/proc.go:435 +0xce fp=0xc00004ffa8 sp=0xc00004ff88 pc=0x8578ee
runtime.goparkunlock(...)
        C:/Go/src/runtime/proc.go:441
runtime.forcegchelper()
        C:/Go/src/runtime/proc.go:348 +0xb8 fp=0xc00004ffe0 sp=0xc00004ffa8 pc=0x82b3b8
runtime.goexit({})
        C:/Go/src/runtime/asm_amd64.s:1700 +0x1 fp=0xc00004ffe8 sp=0xc00004ffe0 pc=0x85e141
created by runtime.init.7 in goroutine 1
        C:/Go/src/runtime/proc.go:336 +0x1a

goroutine 3 gp=0xc000002c40 m=nil [GC sweep wait]:
runtime.gopark(0x0?, 0x0?, 0x0?, 0x0?, 0x0?)
        C:/Go/src/runtime/proc.go:435 +0xce fp=0xc000051f80 sp=0xc000051f60 pc=0x8578ee
runtime.goparkunlock(...)
        C:/Go/src/runtime/proc.go:441
runtime.bgsweep(0xc00005c000)
        C:/Go/src/runtime/mgcsweep.go:276 +0x94 fp=0xc000051fc8 sp=0xc000051f80 pc=0x815594
runtime.gcenable.gowrap1()
        C:/Go/src/runtime/mgc.go:204 +0x25 fp=0xc000051fe0 sp=0xc000051fc8 pc=0x809c25
runtime.goexit({})
        C:/Go/src/runtime/asm_amd64.s:1700 +0x1 fp=0xc000051fe8 sp=0xc000051fe0 pc=0x85e141
created by runtime.gcenable in goroutine 1
        C:/Go/src/runtime/mgc.go:204 +0x66

goroutine 4 gp=0xc000002e00 m=nil [GC scavenge wait]:
runtime.gopark(0xc00005c000?, 0x8d55c8?, 0x1?, 0x0?, 0xc000002e00?)
        C:/Go/src/runtime/proc.go:435 +0xce fp=0xc000063f78 sp=0xc000063f58 pc=0x8578ee
runtime.goparkunlock(...)
        C:/Go/src/runtime/proc.go:441
runtime.(*scavengerState).park(0x95f820)
        C:/Go/src/runtime/mgcscavenge.go:425 +0x49 fp=0xc000063fa8 sp=0xc000063f78 pc=0x813049
runtime.bgscavenge(0xc00005c000)
        C:/Go/src/runtime/mgcscavenge.go:653 +0x3c fp=0xc000063fc8 sp=0xc000063fa8 pc=0x8135bc
runtime.gcenable.gowrap2()
        C:/Go/src/runtime/mgc.go:205 +0x25 fp=0xc000063fe0 sp=0xc000063fc8 pc=0x809bc5
runtime.goexit({})
        C:/Go/src/runtime/asm_amd64.s:1700 +0x1 fp=0xc000063fe8 sp=0xc000063fe0 pc=0x85e141
created by runtime.gcenable in goroutine 1
        C:/Go/src/runtime/mgc.go:205 +0xa5

goroutine 5 gp=0xc000003180 m=nil [finalizer wait]:
runtime.gopark(0x0?, 0x0?, 0x0?, 0x0?, 0x0?)
        C:/Go/src/runtime/proc.go:435 +0xce fp=0xc000065e30 sp=0xc000065e10 pc=0x8578ee
runtime.runfinq()
        C:/Go/src/runtime/mfinal.go:196 +0x107 fp=0xc000065fe0 sp=0xc000065e30 pc=0x808c27
runtime.goexit({})
        C:/Go/src/runtime/asm_amd64.s:1700 +0x1 fp=0xc000065fe8 sp=0xc000065fe0 pc=0x85e141
created by runtime.createfing in goroutine 1
        C:/Go/src/runtime/mfinal.go:166 +0x3d
exit status 2

Inspecting disassembly, it seems like the jump always points to the next instruction if it is pointing to a label before a PCALIGN, regardless of the distance. An INT3 is inserted, that should always be skipped, but any amount of instructions can be present.

What did you expect to see?

λ go run .
ok

(removing the PCALIGN)

I am generating assembly, which can sometimes cause labels to be placed this way.

Comment From: zveinn

nice find

Comment From: Analistarjx

Add a comment , what ? I lost my new computer in a cloud.

Comment From: zveinn

Add a comment , what ? I lost my new computer in a cloud.

machine learning github bot ? profile looks pretty generated.

Comment From: cherrymui

Thanks for the report. Looks like a bug in the assembler. cc @golang/compiler