tree-ssa-strlen vs. zero-length strings

[Ian Lance Taylor <iant at google dot com>]

The file tree-ssa-strlen.c causes several warnings when compiling the Go
library.  The warnings look like:

../../../gccgo2/libgo/go/http/transport.go: In function âhttp.String.pN29_libgo_http.http.connectMethodâ:
../../../gccgo2/libgo/go/http/transport.go:437:1: warning: offset outside bounds of constant string [enabled by default]

They are occurring because Go uses strings which are not null
terminated.  Some of those strings are zero length.  The code in
tree-ssa-srlen in strlen_enter_block calls get_stridx on the PHI
arguments.  get_stridx calls c_strlen.  c_strlen issues the above
warning.

I don't think it is possible to trigger this using C because I don't
think it is possible to have a zero-length STRING_CST in C.

Here is a Go file which will trigger the warning when compiling with
-O2.

package p
func A(b bool) string {
	s := ""
	if b {
		s = "a"
	}
	return s
}

foo.go: In function âp.Aâ:
foo.go:2:1: warning: offset outside bounds of constant string [enabled by default]

I'm not quite sure what get_stridx is doing here.  Perhaps the fix is as
simple as checking TREE_STRING_LENGTH before calling c_strlen.

Ian