cmd/compile: test/fixedbugs/issue6036.go fails to compile for arm64

[rahulchaudhry]

Please answer these questions before submitting your issue. Thanks!

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

go version go1.8rc3 linux/amd64

What did you do?

The test is in the go repo: "test/fixedbugs/issue6036.go"

$ GOARCH=arm64 go tool compile $GOROOT/test/fixedbugs/issue6036.go
test/fixedbugs/issue6036.go:45: negative large offset
00024 (test/fixedbugs/issue6036.go:20) MOVD 2147483648(R0), R1
test/fixedbugs/issue6036.go:45: negative large offset
00032 (test/fixedbugs/issue6036.go:20) MOVD R1, 2147483656(R0)
test/fixedbugs/issue6036.go:45: negative large offset
00028 (test/fixedbugs/issue6036.go:26) MOVB R1, 2147483649(R0)

The test compiles with go1.7.5, so this is a regression.

A quick bisect points to https://go-review.googlesource.com/#/c/26950: [dev.ssa] cmd/compile, etc.: more ARM64 optimizations, and enable SSA by default

[bradfitz]

Um, but the build dashboard is happy for the arm64 column:

https://build.golang.org/?branch=release-branch.go1.8

... which runs that code presumably?

Oh, that file is // +build amd64.

Not sure why.

/cc @randall77 @cherrymui @minux @josharian

[ianlancetaylor]

The test won't build on 32-bit targets, but should ideally be compiled on all 64-bit targets.

Probably the //build line should be written as a negative constraint which will be easy to find and modify for any new 32-bit target.

[gopherbot]

CL https://golang.org/cl/36318 mentions this issue.

[cherrymui]

Are we targeting this for Go 1.8? This is a regression due to SSA beckend.

[bradfitz]

@cherrymui, your call. You comfortable with it (safe enough?) and worth it? Does this impact real code?

[cherrymui]

Ok. It only makes optimization more conservative, so it should be safe. It would affect real code if the code does use such a huge array... Will send a CL for Go 1.8.

[gopherbot]

CL https://golang.org/cl/36413 mentions this issue.