cmd/compile: panic during fuse with if true block containing a goto and a return

[helloPiers]

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

Does this issue reproduce with the latest release?

Reproduces on playground, master branch, and Go v1.11.2.
Doesn't reproduce on Go v1.10.5.

What did you do?

Tried to compile code which I've boiled down to:
https://play.golang.org/p/2fEsQtceadQ

What did you expect to see?

Compilation success

What did you see instead?

prog.go:5:2: internal compiler error: 'f1': panic during fuse while compiling f1:

runtime error: index out of range

goroutine 20 [running]:
cmd/compile/internal/ssa.Compile.func1(0xc000403218, 0xc00034c6e0)
	/usr/local/go/src/cmd/compile/internal/ssa/compile.go:38 +0xc6
panic(0xc445e0, 0x12dc7e0)
	/usr/local/go/src/runtime/panic.go:513 +0x1b9
cmd/compile/internal/ssa.fuseBlockPlain(0xc0003dfce8, 0x47be00)
	/usr/local/go/src/cmd/compile/internal/ssa/fuse.go:123 +0x781
...

[ianlancetaylor]

CC @randall77

[ianlancetaylor]

@gopherbot please open a backport for 1.11

[gopherbot]

Backport issue(s) opened: #28617 (for 1.11).

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

[randall77]

@quasilyte I believe your CL https://go-review.googlesource.com/c/go/+/91056 is the cause.
In cmd/compile/internal/gc/typecheck.go:deadcode, we remove everything from a function after an if true { ... return } pattern. But we can't do that if, inside that if, there is a goto to a label which occurs after the if.

[gopherbot]

Change https://golang.org/cl/147817 mentions this issue: cmd/compile: don't deadcode eliminate labels

[quasilyte]

@randall77, thanks for quickly fixing it.
My mistake without a doubt.

@helloPiers, thanks for reporting it! 👍