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>

Author: randall77

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

@@ -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