cmd/compile: crash when compiling function named "_"

[alandonovan]

$ go version
go version devel +cad6d1fef5 Mon Jan 21 23:31:09 2019 +0000 linux/amd64
$ cat test/test.go 
package test

import "log"

func a() {
        log.Println("%2F")
}
$ go build test
$ sed -i -e 's/func a/func _/' test/test.go
$ go build test
# test
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x60 pc=0xc4e259]

goroutine 50 [running]:
cmd/compile/internal/gc.emitStackObjects(0xc000538000, 0xc0005ea0e0)
        ~/goroot/src/cmd/compile/internal/gc/ssa.go:5057 +0x1c9
cmd/compile/internal/gc.genssa(0xc00053c000, 0xc0005ea0e0)
        ~/goroot/src/cmd/compile/internal/gc/ssa.go:5092 +0xe1
cmd/compile/internal/gc.compileSSA(0xc0000cc420, 0x1)
        ~/goroot/src/cmd/compile/internal/gc/pgen.go:301 +0x378
cmd/compile/internal/gc.compileFunctions.func2(0xc0004fcf60, 0xc0004fe1d0, 0x1)
        ~/goroot/src/cmd/compile/internal/gc/pgen.go:356 +0x49
created by cmd/compile/internal/gc.compileFunctions
        ~/goroot/src/cmd/compile/internal/gc/pgen.go:354 +0x129

[bcmills]

CC @randall77 @josharian @martisch

[gopherbot]

Change https://golang.org/cl/158845 mentions this issue: cmd/compile: don't bother compiling functions named "_"

[randall77]

This was introduced as part of adding stack objects. Fix out for review.

Do we care about functions named "_"? Is there any legitimate use for them? If so, we should fix this in 1.12. If not, we can leave for 1.13.

[bcmills]

I could imagine someone using them to contain static assertions: that is, a function named _ that fails to compile if certain conditions (such as equality of sizes) are not met.

I don't know whether anyone does that in practice.

[alandonovan]

I could imagine someone using them to contain static assertions.

Exactly. They must be type-checked, but there's no need to generate code for them.

[gopherbot]

Change https://golang.org/cl/158820 mentions this issue: cmd/compile: don't bother compiling functions named "_"

[randall77]

The latter CL is for 1.12. The former CL has more clean up in it and will go in for 1.13.