cmd/compile: avoid runtime call during switch string(byteslice)

This triggers three times while building std,
once in image/png and twice in go/internal/gccgoimporter.

There are no instances in std in which a more aggressive
optimization would have triggered.

This doesn't necessarily avoid an allocation,
because escape analysis is already able in many cases
to use a temporary backing for the string,
but it does at a minimum avoid the runtime call and copy.

Fixes #24937

Change-Id: I7019e85638ba8cd7e2f03890e672558b858579bc
Reviewed-on: https://go-review.googlesource.com/108035
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/swt.go

@@ -254,6 +254,34 @@ func (s *exprSwitch) walk(sw *Node) {
 		}
 	}
 
+	// Given "switch string(byteslice)",
+	// with all cases being constants (or the default case),
+	// use a zero-cost alias of the byte slice.
+	// In theory, we could be more aggressive,
+	// allowing any side-effect-free expressions in cases,
+	// but it's a bit tricky because some of that information
+	// is unavailable due to the introduction of temporaries during order.
+	// Restricting to constants is simple and probably powerful enough.
+	// Do this before calling walkexpr on cond,
+	// because walkexpr will lower the string
+	// conversion into a runtime call.
+	// See issue 24937 for more discussion.
+	if cond.Op == OARRAYBYTESTR {
+		ok := true
+		for _, cas := range sw.List.Slice() {
+			if cas.Op != OCASE {
+				Fatalf("switch string(byteslice) bad op: %v", cas.Op)
+			}
+			if cas.Left != nil && !Isconst(cas.Left, CTSTR) {
+				ok = false
+				break
+			}
+		}
+		if ok {
+			cond.Op = OARRAYBYTESTRTMP
+		}
+	}
+
 	cond = walkexpr(cond, &sw.Ninit)
 	t := sw.Type
 	if t == nil {

test/fixedbugs/issue24937.go

@@ -0,0 +1,15 @@
+// run
+
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+func main() {
+	x := []byte{'a'}
+	switch string(x) {
+	case func() string { x[0] = 'b'; return "b" }():
+		panic("FAIL")
+	}
+}