bytes: comparing long byte slices on s390x gives incorrect results

[billotosyr]

Please answer these questions before submitting your issue. Thanks!

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

master tip, plus 1.10, 1.9, 1.8, etc

Does this issue reproduce with the latest release?

yes

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

GOARCH="s390x"
GOBIN=""
GOEXE=""
GOHOSTARCH="s390x"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/gsa/tlbgsa/home/b/i/billo/work"
GORACE=""
GOROOT="/sandbox/billo/master/go"
GOTOOLDIR="/sandbox/billo/master/go/pkg/tool/linux_s390x"
GCCGO="gccgo"
CC="s390x-linux-gnu-gcc"
GOGCCFLAGS="-fPIC -m64 -march=z196 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build416940965=/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?

https://play.golang.org/p/A_j0ONgu6hS

What did you expect to see?

-1

What did you see instead?

0

This showed up in a leveldb application. I have a fix -- will submit the CL shortly. Also hoping for a backport to 1.10 and 1.9 if possible.

[bcmills]

@gopherbot, please consider backport to 1.10 and 1.9.

[gopherbot]

Backport issue(s) opened: #26117 (for 1.10), #26118 (for 1.9).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://golang.org/wiki/MinorReleases.

[gopherbot]

Change https://golang.org/cl/121495 mentions this issue: bytes: fix comparison of long byte slices on s390x

[odeke-em]

@billotosyr @bcmills might we know the cause/bissect of this regression?

[mundaym]

@odeke-em The incorrect assembly was part of the original port added in this CL: https://go-review.googlesource.com/c/go/+/20946 so it isn't a regression per se, but since the bug is nasty and the fix is trivial it makes sense to backport it.

[odeke-em]

Cool, thanks @mundaym!