cmd/gc: allocate buffers for non-escaping string conversions on stack

Support the following conversions in escape analysis:
[]rune("foo")
[]byte("foo")
string([]rune{})

If the result does not escape, allocate temp buffer on stack
and pass it to runtime functions.

Change-Id: I1d075907eab8b0109ad7ad1878104b02b3d5c690
Reviewed-on: https://go-review.googlesource.com/3590Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/gc/builtin.c

@@ -36,10 +36,10 @@ char *runtimeimport =
 	"func @\"\".intstring (? *[4]byte, ? int64) (? string)\n"
 	"func @\"\".slicebytetostring (? *[32]byte, ? []byte) (? string)\n"
 	"func @\"\".slicebytetostringtmp (? []byte) (? string)\n"
-	"func @\"\".slicerunetostring (? []rune) (? string)\n"
-	"func @\"\".stringtoslicebyte (? string) (? []byte)\n"
+	"func @\"\".slicerunetostring (? *[32]byte, ? []rune) (? string)\n"
+	"func @\"\".stringtoslicebyte (? *[32]byte, ? string) (? []byte)\n"
 	"func @\"\".stringtoslicebytetmp (? string) (? []byte)\n"
-	"func @\"\".stringtoslicerune (? string) (? []rune)\n"
+	"func @\"\".stringtoslicerune (? *[32]rune, ? string) (? []rune)\n"
 	"func @\"\".stringiter (? string, ? int) (? int)\n"
 	"func @\"\".stringiter2 (? string, ? int) (@\"\".retk·1 int, @\"\".retv·2 rune)\n"
 	"func @\"\".slicecopy (@\"\".to·2 any, @\"\".fr·3 any, @\"\".wid·4 uintptr) (? int)\n"

src/cmd/gc/esc.c

@@ -693,12 +693,10 @@ esc(EscState *e, Node *n, Node *up)
 	case OMAKEMAP:
 	case OMAKESLICE:
 	case ONEW:
-		n->escloopdepth = e->loopdepth;
-		n->esc = EscNone;  // until proven otherwise
-		e->noesc = list(e->noesc, n);
-		break;
-
+	case OARRAYRUNESTR:
 	case OARRAYBYTESTR:
+	case OSTRARRAYRUNE:
+	case OSTRARRAYBYTE:
 	case ORUNESTR:
 		n->escloopdepth = e->loopdepth;
 		n->esc = EscNone;  // until proven otherwise
@@ -824,7 +822,10 @@ escassign(EscState *e, Node *dst, Node *src)
 	case OMAKECHAN:
 	case OMAKEMAP:
 	case OMAKESLICE:
+	case OARRAYRUNESTR:
 	case OARRAYBYTESTR:
+	case OSTRARRAYRUNE:
+	case OSTRARRAYBYTE:
 	case OADDSTR:
 	case ONEW:
 	case OCLOSURE:
@@ -1249,7 +1250,10 @@ escwalk(EscState *e, int level, Node *dst, Node *src)
 	case OMAKECHAN:
 	case OMAKEMAP:
 	case OMAKESLICE:
+	case OARRAYRUNESTR:
 	case OARRAYBYTESTR:
+	case OSTRARRAYRUNE:
+	case OSTRARRAYBYTE:
 	case OADDSTR:
 	case OMAPLIT:
 	case ONEW:

src/cmd/gc/runtime.go

@@ -50,10 +50,10 @@ func eqstring(string, string) bool
 func intstring(*[4]byte, int64) string
 func slicebytetostring(*[32]byte, []byte) string
 func slicebytetostringtmp([]byte) string
-func slicerunetostring([]rune) string
-func stringtoslicebyte(string) []byte
+func slicerunetostring(*[32]byte, []rune) string
+func stringtoslicebyte(*[32]byte, string) []byte
 func stringtoslicebytetmp(string) []byte
-func stringtoslicerune(string) []rune
+func stringtoslicerune(*[32]rune, string) []rune
 func stringiter(string, int) int
 func stringiter2(string, int) (retk int, retv rune)
 func slicecopy(to any, fr any, wid uintptr) int

src/cmd/gc/walk.c

@@ -1398,13 +1398,25 @@ walkexpr(Node **np, NodeList **init)
 		goto ret;
 
 	case OARRAYRUNESTR:
-		// slicerunetostring([]rune) string;
-		n = mkcall("slicerunetostring", n->type, init, n->left);
+		// slicerunetostring(*[32]byte, []rune) string;
+		a = nodnil();
+		if(n->esc == EscNone) {
+			// Create temporary buffer for string on stack.
+			t = aindex(nodintconst(tmpstringbufsize), types[TUINT8]);
+			a = nod(OADDR, temp(t), N);
+		}
+		n = mkcall("slicerunetostring", n->type, init, a, n->left);
 		goto ret;
 
 	case OSTRARRAYBYTE:
-		// stringtoslicebyte(string) []byte;
-		n = mkcall("stringtoslicebyte", n->type, init, conv(n->left, types[TSTRING]));
+		// stringtoslicebyte(*32[byte], string) []byte;
+		a = nodnil();
+		if(n->esc == EscNone) {
+			// Create temporary buffer for slice on stack.
+			t = aindex(nodintconst(tmpstringbufsize), types[TUINT8]);
+			a = nod(OADDR, temp(t), N);
+		}
+		n = mkcall("stringtoslicebyte", n->type, init, a, conv(n->left, types[TSTRING]));
 		goto ret;
 
 	case OSTRARRAYBYTETMP:
@@ -1413,8 +1425,14 @@ walkexpr(Node **np, NodeList **init)
 		goto ret;
 
 	case OSTRARRAYRUNE:
-		// stringtoslicerune(string) []rune
-		n = mkcall("stringtoslicerune", n->type, init, n->left);
+		// stringtoslicerune(*[32]rune, string) []rune
+		a = nodnil();
+		if(n->esc == EscNone) {
+			// Create temporary buffer for slice on stack.
+			t = aindex(nodintconst(tmpstringbufsize), types[TINT32]);
+			a = nod(OADDR, temp(t), N);
+		}
+		n = mkcall("stringtoslicerune", n->type, init, a, n->left);
 		goto ret;
 
 	case OCMPIFACE:

src/runtime/string.go

@@ -129,8 +129,13 @@ func slicebytetostringtmp(b []byte) string {
 	return *(*string)(unsafe.Pointer(&b))
 }
 
-func stringtoslicebyte(s string) []byte {
-	b := rawbyteslice(len(s))
+func stringtoslicebyte(buf *tmpBuf, s string) []byte {
+	var b []byte
+	if buf != nil && len(s) <= len(buf) {
+		b = buf[:len(s)]
+	} else {
+		b = rawbyteslice(len(s))
+	}
 	copy(b, s)
 	return b
 }
@@ -147,7 +152,7 @@ func stringtoslicebytetmp(s string) []byte {
 	return *(*[]byte)(unsafe.Pointer(&ret))
 }
 
-func stringtoslicerune(s string) []rune {
+func stringtoslicerune(buf *[tmpStringBufSize]rune, s string) []rune {
 	// two passes.
 	// unlike slicerunetostring, no race because strings are immutable.
 	n := 0
@@ -157,7 +162,12 @@ func stringtoslicerune(s string) []rune {
 		s = s[k:]
 		n++
 	}
-	a := rawruneslice(n)
+	var a []rune
+	if buf != nil && n <= len(buf) {
+		a = buf[:n]
+	} else {
+		a = rawruneslice(n)
+	}
 	n = 0
 	for len(t) > 0 {
 		r, k := charntorune(t)
@@ -168,7 +178,7 @@ func stringtoslicerune(s string) []rune {
 	return a
 }
 
-func slicerunetostring(a []rune) string {
+func slicerunetostring(buf *tmpBuf, a []rune) string {
 	if raceenabled && len(a) > 0 {
 		racereadrangepc(unsafe.Pointer(&a[0]),
 			uintptr(len(a))*unsafe.Sizeof(a[0]),
@@ -180,7 +190,7 @@ func slicerunetostring(a []rune) string {
 	for _, r := range a {
 		size1 += runetochar(dum[:], r)
 	}
-	s, b := rawstring(size1 + 3)
+	s, b := rawstringtmp(buf, size1+3)
 	size2 := 0
 	for _, r := range a {
 		// check for race
@@ -309,11 +319,6 @@ func gobytes(p *byte, n int) []byte {
 	return x
 }
 
-func gostringsize(n int) string {
-	s, _ := rawstring(n)
-	return s
-}
-
 func gostring(p *byte) string {
 	l := findnull(p)
 	if l == 0 {

test/escape2.go

@@ -212,7 +212,7 @@ func foo21() func() int {
 func foo21a() func() int {
 	x := 42             // ERROR "moved to heap: x"
 	return func() int { // ERROR "func literal escapes to heap"
-		x++  // ERROR "&x escapes to heap"
+		x++ // ERROR "&x escapes to heap"
 		return x
 	}
 }
@@ -1560,14 +1560,14 @@ func ptrlitNoescape() {
 
 func ptrlitNoEscape2() {
 	// Literal does not escape, but element does.
-	i := 0 // ERROR "moved to heap: i"
+	i := 0        // ERROR "moved to heap: i"
 	x := &Lit{&i} // ERROR "&Lit literal does not escape" "&i escapes to heap"
 	sink = *x
 }
 
 func ptrlitEscape() {
 	// Both literal and element escape.
-	i := 0 // ERROR "moved to heap: i"
+	i := 0        // ERROR "moved to heap: i"
 	x := &Lit{&i} // ERROR "&Lit literal escapes to heap" "&i escapes to heap"
 	sink = x
 }
@@ -1619,7 +1619,7 @@ type StructWithString struct {
 // to just x, and thus &i looks escaping.
 func fieldFlowTracking() {
 	var x StructWithString
-	i := 0 // ERROR "moved to heap: i"
+	i := 0   // ERROR "moved to heap: i"
 	x.p = &i // ERROR "&i escapes to heap"
 	sink = x.s
 }
@@ -1703,3 +1703,51 @@ func intstring2() {
 	s := string(x) // ERROR "string\(x\) escapes to heap" "moved to heap: s"
 	sink = &s      // ERROR "&s escapes to heap"
 }
+
+func stringtoslicebyte0() {
+	s := "foo"
+	x := []byte(s) // ERROR "\(\[\]byte\)\(s\) does not escape"
+	_ = x
+}
+
+func stringtoslicebyte1() []byte {
+	s := "foo"
+	return []byte(s) // ERROR "\(\[\]byte\)\(s\) escapes to heap"
+}
+
+func stringtoslicebyte2() {
+	s := "foo"
+	sink = []byte(s) // ERROR "\(\[\]byte\)\(s\) escapes to heap"
+}
+
+func stringtoslicerune0() {
+	s := "foo"
+	x := []rune(s) // ERROR "\(\[\]rune\)\(s\) does not escape"
+	_ = x
+}
+
+func stringtoslicerune1() []rune {
+	s := "foo"
+	return []rune(s) // ERROR "\(\[\]rune\)\(s\) escapes to heap"
+}
+
+func stringtoslicerune2() {
+	s := "foo"
+	sink = []rune(s) // ERROR "\(\[\]rune\)\(s\) escapes to heap"
+}
+
+func slicerunetostring0() {
+	r := []rune{1, 2, 3} // ERROR "\[\]rune literal does not escape"
+	s := string(r)       // ERROR "string\(r\) does not escape"
+	_ = s
+}
+
+func slicerunetostring1() string {
+	r := []rune{1, 2, 3} // ERROR "\[\]rune literal does not escape"
+	return string(r)     // ERROR "string\(r\) escapes to heap"
+}
+
+func slicerunetostring2() {
+	r := []rune{1, 2, 3} // ERROR "\[\]rune literal does not escape"
+	sink = string(r)     // ERROR "string\(r\) escapes to heap"
+}

test/escape2n.go

@@ -212,7 +212,7 @@ func foo21() func() int {
 func foo21a() func() int {
 	x := 42             // ERROR "moved to heap: x"
 	return func() int { // ERROR "func literal escapes to heap"
-		x++  // ERROR "&x escapes to heap"
+		x++ // ERROR "&x escapes to heap"
 		return x
 	}
 }
@@ -1560,14 +1560,14 @@ func ptrlitNoescape() {
 
 func ptrlitNoEscape2() {
 	// Literal does not escape, but element does.
-	i := 0 // ERROR "moved to heap: i"
+	i := 0        // ERROR "moved to heap: i"
 	x := &Lit{&i} // ERROR "&Lit literal does not escape" "&i escapes to heap"
 	sink = *x
 }
 
 func ptrlitEscape() {
 	// Both literal and element escape.
-	i := 0 // ERROR "moved to heap: i"
+	i := 0        // ERROR "moved to heap: i"
 	x := &Lit{&i} // ERROR "&Lit literal escapes to heap" "&i escapes to heap"
 	sink = x
 }
@@ -1619,7 +1619,7 @@ type StructWithString struct {
 // to just x, and thus &i looks escaping.
 func fieldFlowTracking() {
 	var x StructWithString
-	i := 0 // ERROR "moved to heap: i"
+	i := 0   // ERROR "moved to heap: i"
 	x.p = &i // ERROR "&i escapes to heap"
 	sink = x.s
 }
@@ -1703,3 +1703,51 @@ func intstring2() {
 	s := string(x) // ERROR "string\(x\) escapes to heap" "moved to heap: s"
 	sink = &s      // ERROR "&s escapes to heap"
 }
+
+func stringtoslicebyte0() {
+	s := "foo"
+	x := []byte(s) // ERROR "\(\[\]byte\)\(s\) does not escape"
+	_ = x
+}
+
+func stringtoslicebyte1() []byte {
+	s := "foo"
+	return []byte(s) // ERROR "\(\[\]byte\)\(s\) escapes to heap"
+}
+
+func stringtoslicebyte2() {
+	s := "foo"
+	sink = []byte(s) // ERROR "\(\[\]byte\)\(s\) escapes to heap"
+}
+
+func stringtoslicerune0() {
+	s := "foo"
+	x := []rune(s) // ERROR "\(\[\]rune\)\(s\) does not escape"
+	_ = x
+}
+
+func stringtoslicerune1() []rune {
+	s := "foo"
+	return []rune(s) // ERROR "\(\[\]rune\)\(s\) escapes to heap"
+}
+
+func stringtoslicerune2() {
+	s := "foo"
+	sink = []rune(s) // ERROR "\(\[\]rune\)\(s\) escapes to heap"
+}
+
+func slicerunetostring0() {
+	r := []rune{1, 2, 3} // ERROR "\[\]rune literal does not escape"
+	s := string(r)       // ERROR "string\(r\) does not escape"
+	_ = s
+}
+
+func slicerunetostring1() string {
+	r := []rune{1, 2, 3} // ERROR "\[\]rune literal does not escape"
+	return string(r)     // ERROR "string\(r\) escapes to heap"
+}
+
+func slicerunetostring2() {
+	r := []rune{1, 2, 3} // ERROR "\[\]rune literal does not escape"
+	sink = string(r)     // ERROR "string\(r\) escapes to heap"
+}