cmd/gc, runtime: optimize map[string] lookup from []byte key

Brad has been asking for this for a while.
I have resisted because I wanted to find a more general way to
do this, one that would keep the performance of code introducing
variables the same as the performance of code that did not.
(See golang.org/issue/3512#c20).

I have not found the more general way, and recent changes to
remove ambiguously live temporaries have blown away the
property I was trying to preserve, so that's no longer a reason
not to make the change.

Fixes #3512.

LGTM=iant
R=iant
CC=bradfitz, golang-codereviews, khr, r
https://golang.org/cl/83740044

src/cmd/gc/builtin.c

@@ -33,6 +33,7 @@ char *runtimeimport =
 	"func @\"\".eqstring (? string, ? string) (? bool)\n"
 	"func @\"\".intstring (? int64) (? string)\n"
 	"func @\"\".slicebytetostring (? []byte) (? string)\n"
+	"func @\"\".slicebytetostringtmp (? []byte) (? string)\n"
 	"func @\"\".slicerunetostring (? []rune) (? string)\n"
 	"func @\"\".stringtoslicebyte (? string) (? []byte)\n"
 	"func @\"\".stringtoslicerune (? string) (? []rune)\n"

src/cmd/gc/go.h

@@ -450,6 +450,7 @@ enum
 	OANDAND,	// b0 && b1
 	OAPPEND,	// append
 	OARRAYBYTESTR,	// string(bytes)
+	OARRAYBYTESTRTMP, // string(bytes) ephemeral
 	OARRAYRUNESTR,	// string(runes)
 	OSTRARRAYBYTE,	// []byte(s)
 	OSTRARRAYRUNE,	// []rune(s)

src/cmd/gc/order.c

@@ -565,9 +565,13 @@ orderstmt(Node *n, Order *order)
 		// and make sure OINDEXMAP is not copied out.
 		t = marktemp(order);
 		orderexprlist(n->list, order);
-		orderexpr(&n->rlist->n->left, order);
-		orderexpr(&n->rlist->n->right, order);
-		orderaddrtemp(&n->rlist->n->right, order);
+		r = n->rlist->n;
+		orderexpr(&r->left, order);
+		orderexpr(&r->right, order);
+		// See case OINDEXMAP below.
+		if(r->right->op == OARRAYBYTESTR)
+			r->right->op = OARRAYBYTESTRTMP;
+		orderaddrtemp(&r->right, order);
 		ordermapassign(n, order);
 		cleantemp(t, order);
 		break;
@@ -935,6 +939,20 @@ orderexpr(Node **np, Order *order)
 		// key must be addressable
 		orderexpr(&n->left, order);
 		orderexpr(&n->right, order);
+
+		// For x = m[string(k)] where k is []byte, the allocation of
+		// backing bytes for the string can be avoided by reusing
+		// the []byte backing array. This is a special case that it
+		// would be nice to handle more generally, but because
+		// there are no []byte-keyed maps, this specific case comes
+		// up in important cases in practice. See issue 3512.
+		// Nothing can change the []byte we are not copying before
+		// the map index, because the map access is going to
+		// be forced to happen immediately following this
+		// conversion (by the ordercopyexpr a few lines below).
+		if(n->etype == 0 && n->right->op == OARRAYBYTESTR)
+			n->right->op = OARRAYBYTESTRTMP;
+
 		orderaddrtemp(&n->right, order);
 		if(n->etype == 0) {
 			// use of value (not being assigned);

src/cmd/gc/runtime.go

@@ -47,6 +47,7 @@ func cmpstring(string, string) int
 func eqstring(string, string) bool
 func intstring(int64) string
 func slicebytetostring([]byte) string
+func slicebytetostringtmp([]byte) string
 func slicerunetostring([]rune) string
 func stringtoslicebyte(string) []byte
 func stringtoslicerune(string) []rune

src/cmd/gc/walk.c

@@ -1316,6 +1316,11 @@ walkexpr(Node **np, NodeList **init)
 		n = mkcall("slicebytetostring", n->type, init, n->left);
 		goto ret;
 
+	case OARRAYBYTESTRTMP:
+		// slicebytetostringtmp([]byte) string;
+		n = mkcall("slicebytetostringtmp", n->type, init, n->left);
+		goto ret;
+
 	case OARRAYRUNESTR:
 		// slicerunetostring([]rune) string;
 		n = mkcall("slicerunetostring", n->type, init, n->left);

src/pkg/runtime/map_test.go

@@ -438,3 +438,40 @@ func TestMapIterOrder(t *testing.T) {
 		}
 	}
 }
+
+func TestMapStringBytesLookup(t *testing.T) {
+	// Use large string keys to avoid small-allocation coalescing,
+	// which can cause AllocsPerRun to report lower counts than it should.
+	m := map[string]int{
+		"1000000000000000000000000000000000000000000000000": 1,
+		"2000000000000000000000000000000000000000000000000": 2,
+	}
+	buf := []byte("1000000000000000000000000000000000000000000000000")
+	if x := m[string(buf)]; x != 1 {
+		t.Errorf(`m[string([]byte("1"))] = %d, want 1`, x)
+	}
+	buf[0] = '2'
+	if x := m[string(buf)]; x != 2 {
+		t.Errorf(`m[string([]byte("2"))] = %d, want 2`, x)
+	}
+
+	var x int
+	n := testing.AllocsPerRun(100, func() {
+		x += m[string(buf)]
+	})
+	if n != 0 {
+		t.Errorf("AllocsPerRun for m[string(buf)] = %v, want 0", n)
+	}
+
+	x = 0
+	n = testing.AllocsPerRun(100, func() {
+		y, ok := m[string(buf)]
+		if !ok {
+			panic("!ok")
+		}
+		x += y
+	})
+	if n != 0 {
+		t.Errorf("AllocsPerRun for x,ok = m[string(buf)] = %v, want 0", n)
+	}
+}

src/pkg/runtime/string.goc

@@ -294,6 +294,25 @@ func slicebytetostring(b Slice) (s String) {
 	runtime·memmove(s.str, b.array, s.len);
 }
 
+func slicebytetostringtmp(b Slice) (s String) {
+	void *pc;
+
+	if(raceenabled) {
+		pc = runtime·getcallerpc(&b);
+		runtime·racereadrangepc(b.array, b.len, pc, runtime·slicebytetostringtmp);
+	}
+
+	// Return a "string" referring to the actual []byte bytes.
+	// This is only for use by internal compiler optimizations
+	// 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
+	// m is a string-keyed map and k is a []byte.
+	s.str = b.array;
+	s.len = b.len;
+}
+
 func stringtoslicebyte(s String) (b Slice) {
 	uintptr cap;