cmd/compile: please cherry pick internal compiler error fix for #23504 for 1.10.4 [1.10 backport]

[cwedgwood]

@randall77 It looks perhaps like the fix for #23504 didn't make it to 1.10.x releases (it's not in 1.9.x either, but I already patch those locally and am upgrading so less concerned).

Cherry picking the fix (4313d77) and rebuilding 1.10.3 works as expected for me.

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

go version go1.10.3 linux/amd64

What did you do?

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

go build of

package main

func f(op string) {
        for len(op) > 1 && !(op[0] >= '0' || op[0] <= '9') {
        }
}

func main() {
        f("x")
}

What did you expect to see?

... no output ...

What did you see instead?

prog.go:4:16: internal compiler error: panic during layout while compiling f:

runtime error: index out of range

goroutine 7 [running]:
cmd/compile/internal/ssa.Compile.func1(0xc4203f3288, 0xc42000e500)
	/usr/local/go/src/cmd/compile/internal/ssa/compile.go:38 +0xc8
panic(0xbbc700, 0xf9d830)
	/usr/local/go/src/runtime/panic.go:502 +0x229
cmd/compile/internal/ssa.layout(0xc42000e500)
	/usr/local/go/src/cmd/compile/internal/ssa/layout.go:68 +0xdca
cmd/compile/internal/ssa.Compile(0xc42000e500)
	/usr/local/go/src/cmd/compile/internal/ssa/compile.go:70 +0x2bb
cmd/compile/internal/gc.buildssa(0xc420001200, 0x1, 0x0)
	/usr/local/go/src/cmd/compile/internal/gc/ssa.go:223 +0xb32
cmd/compile/internal/gc.compileSSA(0xc420001200, 0x1)
	/usr/local/go/src/cmd/compile/internal/gc/pgen.go:239 +0x39
cmd/compile/internal/gc.compileFunctions.func2(0xc4203e8180, 0xc4200125a0, 0x1)
	/usr/local/go/src/cmd/compile/internal/gc/pgen.go:289 +0x49
created by cmd/compile/internal/gc.compileFunctions
	/usr/local/go/src/cmd/compile/internal/gc/pgen.go:287 +0x11c



Please file a bug report including a short program that triggers the error.
https://golang.org/issue/new

(apologies, this is the output from the playground as i now have it fixed locally so no error and am too lazy to rebuild it 'broken' right now)

[odeke-em]

Thank you @cwedgwood for reporting this!

I believe @randall77 has been looking at backporting to 1.10.* but also /cc @FiloSottile @andybons for auto-cherry picking/backporting.

[randall77]

This is not a regression since it's been failing since Go 1.7. Normally we do not backport to fix such issues.
The fix is super simple, though. I'll put this issue in the queue for consideration.

[andybons]

@ianlancetaylor

[ianlancetaylor]

I'm OK with backporting https://golang.org/cl/88955 to 1.10. I'll send a CL.

[gopherbot]

Change https://golang.org/cl/128855 mentions this issue: [release-branch.go1.10] cmd/compile: reset branch prediction when deleting a branch

[cwedgwood]

@ianlancetaylor if this is deemed unsuitable for 1.10.x close this out as-is and it becomes a non-issue for 1.11.x in a little bit.

[gopherbot]

Closed by merging dea961e to release-branch.go1.10.