cmd/compile: compute Negation's limits from argument's limits

Change-Id: I2e4d74a86faa95321e847a061e06c3efff7f20df
Reviewed-on: https://go-review.googlesource.com/c/go/+/605775
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: David Chase <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: Jorropo

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

@@ -1787,6 +1787,10 @@ func (ft *factsTable) flowLimit(v *Value) bool {
 		a := ft.limits[v.Args[0].ID]
 		b := ft.limits[v.Args[1].ID]
 		return ft.newLimit(v, a.sub(b, 8))
+	case OpNeg64, OpNeg32, OpNeg16, OpNeg8:
+		a := ft.limits[v.Args[0].ID]
+		bitsize := uint(v.Type.Size()) * 8
+		return ft.newLimit(v, a.com(bitsize).add(limit{min: 1, max: 1, umin: 1, umax: 1}, bitsize))
 	case OpMul64:
 		a := ft.limits[v.Args[0].ID]
 		b := ft.limits[v.Args[1].ID]

test/prove.go

@@ -1627,6 +1627,49 @@ func com64(a uint64, ensureAllBranchesCouldHappen func() bool) uint64 {
 	return z
 }
 
+func neg64(a uint64, ensureAllBranchesCouldHappen func() bool) uint64 {
+	var lo, hi uint64 = 0xff, 0xfff
+	a &= hi
+	a |= lo
+
+	z := -a
+
+	if ensureAllBranchesCouldHappen() && z > -lo { // ERROR "Disproved Less64U$"
+		return 42
+	}
+	if ensureAllBranchesCouldHappen() && z <= -lo { // ERROR "Proved Leq64U$"
+		return 1337
+	}
+	if ensureAllBranchesCouldHappen() && z < -hi { // ERROR "Disproved Less64U$"
+		return 42
+	}
+	if ensureAllBranchesCouldHappen() && z >= -hi { // ERROR "Proved Leq64U$"
+		return 1337
+	}
+	return z
+}
+func neg64mightOverflowDuringNeg(a uint64, ensureAllBranchesCouldHappen func() bool) uint64 {
+	var lo, hi uint64 = 0, 0xfff
+	a &= hi
+	a |= lo
+
+	z := -a
+
+	if ensureAllBranchesCouldHappen() && z > -lo {
+		return 42
+	}
+	if ensureAllBranchesCouldHappen() && z <= -lo {
+		return 1337
+	}
+	if ensureAllBranchesCouldHappen() && z < -hi {
+		return 42
+	}
+	if ensureAllBranchesCouldHappen() && z >= -hi {
+		return 1337
+	}
+	return z
+}
+
 //go:noinline
 func useInt(a int) {
 }