cmd/internal/obj: rewriting MOVQ to XOR causes bugs and confusion

[hdevalence]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go version go1.8.3 linux/amd64

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/hdevalence/go"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build126311645=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"
PKG_CONFIG="pkg-config"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"

What did you do?

I put MOVQ $0, AX in a Go assembly file.

What did you expect to see?

A move instruction, such as

48 b8 00 00 00 00 00 	movabs rax,0x0
00 00 00

or any other equivalent instruction.

What did you see instead?

An XOR, which clobbers the carry flags, breaking the program.

31 c0                	xor    eax,eax

This is essentially a duplicate of #12405, which is now locked. That issue notes that MOV is now "marked as clobbering flags" (presumably in the compiler internals?) but this fact seems not to be documented in either the Go assembly documentation or in the Plan 9 assembler documentation.

[mvdan]

Have you tried on tip or 1.9beta2?

[bradfitz]

/cc @randall77 @josharian @mdempsky

[josharian]

I don't think there's anything to say beyond what was said in #12405. Unless Russ has changed his mind, this is a documentation issue.

[aclements]

More information about the pain this "optimization" has caused: #22325.

FWIW, I think @rsc has changed his mind at this point and agrees that we should just take this out of obj.

[aclements]

I just chatted about this with several runtime/compiler people and we all agreed that this should be removed from obj (and done explicitly in the compiler where it makes sense). Now someone just has to do it. :)

[gopherbot]

Change https://golang.org/cl/73072 mentions this issue: cmd/internal/obj/x86: stop rewriting MOV to XOR

[gopherbot]

Change https://golang.org/cl/73073 mentions this issue: cmd/internal/obj/x86: remove PRESERVEFLAGS