cmd/compile: teach prove about bitwise OR operation

For now, only apply the rule if either of arguments are constants. That
would catch a lot of real user code, without slowing down the compiler
with code generated for string comparison (experience in CL 410336).

Updates #57959
Fixes #45928

Change-Id: Ie2e830d6d0d71cda3947818b22c2775bd94f7971
Reviewed-on: https://go-review.googlesource.com/c/go/+/483359
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -867,6 +867,14 @@ func prove(f *Func) {
 					logicVars = make(map[*Block][]*Value)
 				}
 				logicVars[b] = append(logicVars[b], v)
+			case OpOr64, OpOr32, OpOr16, OpOr8:
+				// TODO: investigate how to always add facts without much slowdown, see issue #57959.
+				if v.Args[0].isGenericIntConst() {
+					ft.update(b, v, v.Args[0], unsigned, gt|eq)
+				}
+				if v.Args[1].isGenericIntConst() {
+					ft.update(b, v, v.Args[1], unsigned, gt|eq)
+				}
 			case OpDiv64u, OpDiv32u, OpDiv16u, OpDiv8u,
 				OpRsh8Ux64, OpRsh8Ux32, OpRsh8Ux16, OpRsh8Ux8,
 				OpRsh16Ux64, OpRsh16Ux32, OpRsh16Ux16, OpRsh16Ux8,

test/prove.go

@@ -1111,6 +1111,11 @@ func issue51622(b []byte) int {
 	return 0
 }
 
+func issue45928(x int) {
+	combinedFrac := x / (x | (1 << 31)) // ERROR "Proved Neq64$"
+	useInt(combinedFrac)
+}
+
 //go:noinline
 func useInt(a int) {
 }