[dev.ssa] cmd/compile: use string contents instead of offset from string header

This generates more efficient code.

Before:

	0x003a 00058 (rr.go:7)	LEAQ	go.string.hdr."="(SB), BX
	0x0041 00065 (rr.go:7)	LEAQ	16(BX), BP
	0x0045 00069 (rr.go:7)	MOVQ	BP, 16(SP)

After:

	0x003a 00058 (rr.go:7)	LEAQ	go.string."="(SB), BX
	0x0041 00065 (rr.go:7)	MOVQ	BX, 16(SP)

It also matches the existing backend
and is more robust to other changes,
such as CL 11698, which I believe broke
the current code.

This CL fixes the encoding/base64 tests, as run with:

	GOGC=off GOSSAPKG=base64 go test -a encoding/base64

Change-Id: I3c475bed1dd3335cc14e13309e11d23f0ed32c17
Reviewed-on: https://go-review.googlesource.com/12654Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -1922,12 +1922,12 @@ type ssaExport struct {
 	mustImplement bool
 }
 
-// StringSym returns a symbol (a *Sym wrapped in an interface) which
-// is a global string constant containing s.
-func (*ssaExport) StringSym(s string) interface{} {
+// StringData returns a symbol (a *Sym wrapped in an interface) which
+// is the data component of a global string constant containing s.
+func (*ssaExport) StringData(s string) interface{} {
 	// TODO: is idealstring correct?  It might not matter...
-	hdr, _ := stringsym(s)
-	return &ssa.ExternSymbol{Typ: idealstring, Sym: hdr}
+	_, data := stringsym(s)
+	return &ssa.ExternSymbol{Typ: idealstring, Sym: data}
 }
 
 // Log logs a message from the compiler.

src/cmd/compile/internal/ssa/config.go

@@ -17,10 +17,8 @@ type Config struct {
 }
 
 type Frontend interface {
-	// StringSym returns a symbol pointing to the given string.
-	// Strings are laid out in read-only memory with one word of pointer,
-	// one word of length, then the contents of the string.
-	StringSym(string) interface{} // returns *gc.Sym
+	// StringData returns a symbol pointing to the given string's contents.
+	StringData(string) interface{} // returns *gc.Sym
 
 	// Log logs a message from the compiler.
 	Logf(string, ...interface{})

src/cmd/compile/internal/ssa/export_test.go

@@ -15,7 +15,7 @@ type DummyFrontend struct {
 	t testing.TB
 }
 
-func (DummyFrontend) StringSym(s string) interface{} {
+func (DummyFrontend) StringData(s string) interface{} {
 	return nil
 }
 

src/cmd/compile/internal/ssa/gen/generic.rules

@@ -45,7 +45,7 @@
 (Store dst (Load <t> src mem) mem) && t.Size() > 8 -> (Move [t.Size()] dst src mem)
 
 // string ops
-(Const <t> {s}) && t.IsString() -> (StringMake (OffPtr <TypeBytePtr> [2*config.PtrSize] (Addr <TypeBytePtr> {config.fe.StringSym(s.(string))} (SB <config.Uintptr>))) (Const <config.Uintptr> [int64(len(s.(string)))])) // TODO: ptr
+(Const <t> {s}) && t.IsString() -> (StringMake (Addr <TypeBytePtr> {config.fe.StringData(s.(string))} (SB <config.Uintptr>)) (Const <config.Uintptr> [int64(len(s.(string)))]))
 (Load <t> ptr mem) && t.IsString() -> (StringMake (Load <TypeBytePtr> ptr mem) (Load <config.Uintptr> (OffPtr <TypeBytePtr> [config.PtrSize] ptr) mem))
 (StringPtr (StringMake ptr _)) -> ptr
 (StringLen (StringMake _ len)) -> len

src/cmd/compile/internal/ssa/rewritegeneric.go

@@ -102,36 +102,32 @@ func rewriteValuegeneric(v *Value, config *Config) bool {
 	case OpConst:
 		// match: (Const <t> {s})
 		// cond: t.IsString()
-		// result: (StringMake (OffPtr <TypeBytePtr> [2*config.PtrSize] (Addr <TypeBytePtr> {config.fe.StringSym(s.(string))} (SB <config.Uintptr>))) (Const <config.Uintptr> [int64(len(s.(string)))]))
+		// result: (StringMake (Addr <TypeBytePtr> {config.fe.StringData(s.(string))} (SB <config.Uintptr>)) (Const <config.Uintptr> [int64(len(s.(string)))]))
 		{
 			t := v.Type
 			s := v.Aux
 			if !(t.IsString()) {
-				goto endedcb8bd24122d6a47bdc9b752460c344
+				goto enda6f250a3c775ae5a239ece8074b46cea
 			}
 			v.Op = OpStringMake
 			v.AuxInt = 0
 			v.Aux = nil
 			v.resetArgs()
-			v0 := v.Block.NewValue0(v.Line, OpOffPtr, TypeInvalid)
+			v0 := v.Block.NewValue0(v.Line, OpAddr, TypeInvalid)
 			v0.Type = TypeBytePtr
-			v0.AuxInt = 2 * config.PtrSize
-			v1 := v.Block.NewValue0(v.Line, OpAddr, TypeInvalid)
-			v1.Type = TypeBytePtr
-			v1.Aux = config.fe.StringSym(s.(string))
-			v2 := v.Block.NewValue0(v.Line, OpSB, TypeInvalid)
-			v2.Type = config.Uintptr
-			v1.AddArg(v2)
+			v0.Aux = config.fe.StringData(s.(string))
+			v1 := v.Block.NewValue0(v.Line, OpSB, TypeInvalid)
+			v1.Type = config.Uintptr
 			v0.AddArg(v1)
 			v.AddArg(v0)
-			v3 := v.Block.NewValue0(v.Line, OpConst, TypeInvalid)
-			v3.Type = config.Uintptr
-			v3.AuxInt = int64(len(s.(string)))
-			v.AddArg(v3)
+			v2 := v.Block.NewValue0(v.Line, OpConst, TypeInvalid)
+			v2.Type = config.Uintptr
+			v2.AuxInt = int64(len(s.(string)))
+			v.AddArg(v2)
 			return true
 		}
-		goto endedcb8bd24122d6a47bdc9b752460c344
-	endedcb8bd24122d6a47bdc9b752460c344:
+		goto enda6f250a3c775ae5a239ece8074b46cea
+	enda6f250a3c775ae5a239ece8074b46cea:
 		;
 	case OpIsInBounds:
 		// match: (IsInBounds (Const [c]) (Const [d]))