cmd/compile: optimize s==s for strings

s==s is always true for strings. This comes up in NaN testing in
generic code, where we want x==x to compile completely away except for
float types.

Fixes #60777

Change-Id: I3ce054b5121354de2f9751b010fb409f148cb637
Reviewed-on: https://go-review.googlesource.com/c/go/+/503795
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>

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

@@ -2121,6 +2121,11 @@
   && isSameCall(callAux, "runtime.memequal")
   => (MakeResult (ConstBool <typ.Bool> [true]) mem)
 
+(Static(Call|LECall) {callAux} p q _ mem)
+  && isSameCall(callAux, "runtime.memequal")
+  && isSamePtr(p, q)
+  => (MakeResult (ConstBool <typ.Bool> [true]) mem)
+
 // Turn known-size calls to memclrNoHeapPointers into a Zero.
 // Note that we are using types.Types[types.TUINT8] instead of sptr.Type.Elem() - see issue 55122 and CL 431496 for more details.
 (SelectN [0] call:(StaticCall {sym} sptr (Const(64|32) [c]) mem))

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

@@ -397,6 +397,8 @@ func rewriteValuegeneric(v *Value) bool {
 		return rewriteValuegeneric_OpSlicemask(v)
 	case OpSqrt:
 		return rewriteValuegeneric_OpSqrt(v)
+	case OpStaticCall:
+		return rewriteValuegeneric_OpStaticCall(v)
 	case OpStaticLECall:
 		return rewriteValuegeneric_OpStaticLECall(v)
 	case OpStore:
@@ -28219,6 +28221,31 @@ func rewriteValuegeneric_OpSqrt(v *Value) bool {
 	}
 	return false
 }
+func rewriteValuegeneric_OpStaticCall(v *Value) bool {
+	b := v.Block
+	typ := &b.Func.Config.Types
+	// match: (StaticCall {callAux} p q _ mem)
+	// cond: isSameCall(callAux, "runtime.memequal") && isSamePtr(p, q)
+	// result: (MakeResult (ConstBool <typ.Bool> [true]) mem)
+	for {
+		if len(v.Args) != 4 {
+			break
+		}
+		callAux := auxToCall(v.Aux)
+		mem := v.Args[3]
+		p := v.Args[0]
+		q := v.Args[1]
+		if !(isSameCall(callAux, "runtime.memequal") && isSamePtr(p, q)) {
+			break
+		}
+		v.reset(OpMakeResult)
+		v0 := b.NewValue0(v.Pos, OpConstBool, typ.Bool)
+		v0.AuxInt = boolToAuxInt(true)
+		v.AddArg2(v0, mem)
+		return true
+	}
+	return false
+}
 func rewriteValuegeneric_OpStaticLECall(v *Value) bool {
 	b := v.Block
 	config := b.Func.Config
@@ -28506,6 +28533,26 @@ func rewriteValuegeneric_OpStaticLECall(v *Value) bool {
 		v.AddArg2(v0, mem)
 		return true
 	}
+	// match: (StaticLECall {callAux} p q _ mem)
+	// cond: isSameCall(callAux, "runtime.memequal") && isSamePtr(p, q)
+	// result: (MakeResult (ConstBool <typ.Bool> [true]) mem)
+	for {
+		if len(v.Args) != 4 {
+			break
+		}
+		callAux := auxToCall(v.Aux)
+		mem := v.Args[3]
+		p := v.Args[0]
+		q := v.Args[1]
+		if !(isSameCall(callAux, "runtime.memequal") && isSamePtr(p, q)) {
+			break
+		}
+		v.reset(OpMakeResult)
+		v0 := b.NewValue0(v.Pos, OpConstBool, typ.Bool)
+		v0.AuxInt = boolToAuxInt(true)
+		v.AddArg2(v0, mem)
+		return true
+	}
 	// match: (StaticLECall {callAux} _ (Const64 [0]) (Const64 [0]) mem)
 	// cond: isSameCall(callAux, "runtime.makeslice")
 	// result: (MakeResult (Addr <v.Type.FieldType(0)> {ir.Syms.Zerobase} (SB)) mem)

test/codegen/strings.go

@@ -67,4 +67,14 @@ func ConstantLoad() {
 	bsink = []byte("0123456789ab")
 }
 
+// self-equality is always true. See issue 60777.
+func EqualSelf(s string) bool {
+	// amd64:`MOVL\t\$1, AX`,-`.*memequal.*`
+	return s == s
+}
+func NotEqualSelf(s string) bool {
+	// amd64:`XORL\tAX, AX`,-`.*memequal.*`
+	return s != s
+}
+
 var bsink []byte