From 205ae07cd3c39dbb17948e3c957a785212a8c5d1 Mon Sep 17 00:00:00 2001
From: Dmitry Vyukov <dvyukov@google.com>
Date: Thu, 22 Jan 2015 17:56:12 +0300
Subject: [PATCH] cmd/gc: don't copy []byte during string concatenation

Consider the following code:

s := "(" + string(byteSlice) + ")"

Currently we allocate a new string during []byte->string conversion,
and pass it to concatstrings. String allocation is unnecessary in
this case, because concatstrings does memorize the strings for later use.
This change uses slicebytetostringtmp to construct temp string directly
from []byte buffer and passes it to concatstrings.

I've found few such cases in std lib:

	s += string(msg[off:off+c]) + "."
	buf.WriteString("Sec-WebSocket-Accept: " + string(c.accept) + "\r\n")
	bw.WriteString("Sec-WebSocket-Key: " + string(nonce) + "\r\n")
	err = xml.Unmarshal([]byte("<Top>"+string(data)+"</Top>"), &logStruct)
	d.err = d.syntaxError("invalid XML name: " + string(b))
	return m, ProtocolError("malformed MIME header line: " + string(kv))

But there are much more in our internal code base.

Change-Id: I42f401f317131237ddd0cb9786b0940213af16fb
Reviewed-on: https://go-review.googlesource.com/3163
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
---
 src/cmd/gc/order.c         | 29 ++++++++++++++++++++++++++++-
 src/runtime/malloc_test.go | 17 +++++++++++++++++
 src/runtime/string.go      |  3 ++-
 3 files changed, 47 insertions(+), 2 deletions(-)

diff --git a/src/cmd/gc/order.c b/src/cmd/gc/order.c
index a1aa1bd300..2cd155d4d7 100644
--- a/src/cmd/gc/order.c
+++ b/src/cmd/gc/order.c
@@ -58,6 +58,13 @@ static void	orderexprlistinplace(NodeList*, Order*);
 void
 order(Node *fn)
 {
+	char s[50];
+
+	if(debug['W'] > 1) {
+		snprint(s, sizeof(s), "\nbefore order %S", fn->nname->sym);
+		dumplist(s, fn->nbody);
+	}
+
 	orderblock(&fn->nbody);
 }
 
@@ -974,7 +981,7 @@ orderexpr(Node **np, Order *order)
 	Node *n;
 	NodeList *mark, *l;
 	Type *t;
-	int lno;
+	int lno, haslit, hasbyte;
 
 	n = *np;
 	if(n == N)
@@ -1002,6 +1009,26 @@ orderexpr(Node **np, Order *order)
 			t->type = types[TSTRING];
 			n->alloc = ordertemp(t, order, 0);
 		}
+
+		// Mark string(byteSlice) arguments to reuse byteSlice backing
+		// buffer during conversion. String concatenation does not
+		// memorize the strings for later use, so it is safe.
+		// However, we can do it only if there is at least one non-empty string literal.
+		// Otherwise if all other arguments are empty strings,
+		// concatstrings will return the reference to the temp string
+		// to the caller.
+		hasbyte = 0;
+		haslit = 0;
+		for(l=n->list; l != nil; l=l->next) {
+			hasbyte |= l->n->op == OARRAYBYTESTR;
+			haslit |= l->n->op == OLITERAL && l->n->val.u.sval->len != 0;
+		}
+		if(haslit && hasbyte) {
+			for(l=n->list; l != nil; l=l->next) {
+				if(l->n->op == OARRAYBYTESTR)
+					l->n->op = OARRAYBYTESTRTMP;
+			}
+		}
 		break;
 
 	case OINDEXMAP:
diff --git a/src/runtime/malloc_test.go b/src/runtime/malloc_test.go
index b7795aa1d6..454b3c36fe 100644
--- a/src/runtime/malloc_test.go
+++ b/src/runtime/malloc_test.go
@@ -46,6 +46,23 @@ func TestMemStats(t *testing.T) {
 	}
 }
 
+func TestStringConcatenationAllocs(t *testing.T) {
+	n := testing.AllocsPerRun(1e3, func() {
+		b := make([]byte, 10)
+		for i := 0; i < 10; i++ {
+			b[i] = byte(i) + '0'
+		}
+		s := "foo" + string(b)
+		if want := "foo0123456789"; s != want {
+			t.Fatalf("want %v, got %v", want, s)
+		}
+	})
+	// Only string concatenation allocates.
+	if n != 1 {
+		t.Fatalf("want 1 allocation, got %v", n)
+	}
+}
+
 var mallocSink uintptr
 
 func BenchmarkMalloc8(b *testing.B) {
diff --git a/src/runtime/string.go b/src/runtime/string.go
index 96f9579624..8aa0dd076d 100644
--- a/src/runtime/string.go
+++ b/src/runtime/string.go
@@ -73,8 +73,9 @@ func slicebytetostringtmp(b []byte) string {
 	// that know that the string form will be discarded before
 	// the calling goroutine could possibly modify the original
 	// slice or synchronize with another goroutine.
-	// Today, the only such case is a m[string(k)] lookup where
+	// First such case is a m[string(k)] lookup where
 	// m is a string-keyed map and k is a []byte.
+	// Second such case is "<"+string(b)+">" concatenation where b is []byte.
 
 	if raceenabled && len(b) > 0 {
 		racereadrangepc(unsafe.Pointer(&b[0]),