tree 67fe627a9cf3fe90e7222e90159d46545734ebbc
parent 9c8581187b1c1a30036263728370f31cb846a274
author Cherry Zhang <cherryyz@google.com> 1554319602 -0400
committer Ian Lance Taylor <iant@golang.org> 1557436882 +0000

compiler: avoid copy for string([]byte) conversion used in map keys

If a string([]byte) conversion is used immediately as a key for a
map read, we don't need to copy the backing store of the byte
slice, as mapaccess does not keep a reference to it.

The gc compiler does more than this: it also avoids the copy if
the map key is a composite literal that contains the conversion
as a field, like, T{ ... { ..., string(b), ... }, ... }. For now,
we just optimize the simple case, which is probably most common.

A test could be added in the GCC testsuite:

Index: gcc/testsuite/go.dg/mapstring.go
===================================================================
--- gcc/testsuite/go.dg/mapstring.go	(nonexistent)
+++ gcc/testsuite/go.dg/mapstring.go	(working copy)
@@ -0,0 +1,11 @@
+// { dg-do compile }
+// { dg-options "-fgo-debug-optimization" }
+
+package p
+
+func F(m map[string]int, a, b []byte) int {
+	x := m[string(a)]     // { dg-error "no copy string\\(\\\[\\\]byte\\)" }
+	y, ok := m[string(b)] // { dg-error "no copy string\\(\\\[\\\]byte\\)" }
+	_ = ok
+	return x + y
+}

Change-Id: Iccb0dafa2780dfb398d2db946e86b2634984527c
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/176197
Reviewed-by: Ian Lance Taylor <iant@golang.org>