cmd/compile: remove 68857 And flowLimit workaround in prove

Change-Id: Id8baeb89e6e11a01d53cd63c665f0b2966f50392
Reviewed-on: https://go-review.googlesource.com/c/go/+/750341
Reviewed-by: Mark Freeman <markfreeman@google.com>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

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

@@ -1884,11 +1884,6 @@ func (ft *factsTable) flowLimit(v *Value) {
 	// TODO: if y.umax and y.umin share a leading bit pattern, y also has that leading bit pattern.
 	// we could compare the patterns of always set bits in a and b and learn more about minimum and maximum.
 	// But I doubt this help any real world code.
-	case OpAnd64, OpAnd32, OpAnd16, OpAnd8:
-		// AND can only make the value smaller.
-		a := ft.limits[v.Args[0].ID]
-		b := ft.limits[v.Args[1].ID]
-		ft.unsignedMax(v, min(a.umax, b.umax))
 	case OpOr64, OpOr32, OpOr16, OpOr8:
 		// OR can only make the value bigger and can't flip bits proved to be zero in both inputs.
 		a := ft.limits[v.Args[0].ID]

test/prove.go

@@ -2823,6 +2823,25 @@ func noopAnd64DoNothingMayClearVarying(x uint64) uint64 {
 	return x
 }
 
+func limitFlowThroughAnd(x, y uint64, ensureAllBranchesCouldHappen func() bool) int {
+	x = min(x, 1337)
+	x &= y
+	if ensureAllBranchesCouldHappen() && x <= 1337 { // ERROR "Proved Leq64U$"
+		return 1
+	}
+	if ensureAllBranchesCouldHappen() && x < 1338 { // ERROR "Proved Less64U$"
+		return 2
+	}
+
+	if ensureAllBranchesCouldHappen() && x <= 1336 {
+		return 3
+	}
+	if ensureAllBranchesCouldHappen() && x < 1337 {
+		return 4
+	}
+	return 0
+}
+
 //go:noinline
 func prove(x int) {
 }