cmd/compile: add Neq support to known bits

When known bits analyze an Neq if it finds any bit that is known
in both sides but different, it also knows the neq will always be true.

Uniqued by LOC this adds 7 known bits hits when building the std.

Updates #78633

Change-Id: I3ec907e2def9d365e6360b6c81dde47920c62bf4
Reviewed-on: https://go-review.googlesource.com/c/go/+/766040
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

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

@@ -101,6 +101,17 @@ func (kb *knownBitsState) fold(v *Value) (value, known int64) {
 			return boolToAuxInt(x == y), -1
 		}
 		return 0, -1 << 1
+	case OpNeq64, OpNeq32, OpNeq16, OpNeq8, OpNeqB:
+		x, xk := kb.fold(v.Args[0])
+		y, yk := kb.fold(v.Args[1])
+		differentBits := x ^ y
+		if differentBits&xk&yk != 0 {
+			return 1, -1
+		}
+		if xk == -1 && yk == -1 {
+			return boolToAuxInt(x != y), -1
+		}
+		return 0, -1 << 1
 	case OpZeroExt8to16, OpZeroExt8to32, OpZeroExt8to64, OpZeroExt16to32, OpZeroExt16to64, OpZeroExt32to64:
 		x, k := kb.fold(v.Args[0])
 		srcSize := v.Args[0].Type.Size() * 8

test/known_bits.go

@@ -168,6 +168,26 @@ func unknownBitsEq(x, y uint64) bool {
 	return x == y
 }
 
+func knownBitsNeqTrue(x, y uint64) bool {
+	x |= 1
+	y &^= 1
+	return x != y // ERROR "known value of v[0-9]+ \(Neq64\): true$"
+}
+
+func knownBitsNeqFalse(x uint64, cond bool) bool {
+	x |= (1<<32 - 1) << 32
+	if cond {
+		x |= 42
+	}
+	x |= 1<<32 - 1
+	return x != 1<<64-1 // ERROR "known value of v[0-9]+ \(Neq64\): false$"
+}
+
+func unknownBitsNeq(x, y uint64) bool {
+	x |= 1
+	return x != y
+}
+
 func knownBitsZeroExtPassThrough(x uint8) uint64 {
 	x |= 6
 	return uint64(x) & 6 // ERROR "known value of v[0-9]+ \(And64\): 6$"