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

Change-Id: I419faa781db085b98ea25008ca127d0317fb34e1
Reviewed-on: https://go-review.googlesource.com/c/go/+/605695
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]>
Reviewed-by: Carlos Amedee <[email protected]>

Author: Jorropo

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

@@ -1643,6 +1643,11 @@ func (ft *factsTable) flowLimit(v *Value) bool {
 	case OpSignExt8to64, OpSignExt8to32, OpSignExt8to16, OpSignExt16to64, OpSignExt16to32, OpSignExt32to64:
 		a := ft.limits[v.Args[0].ID]
 		return ft.signedMinMax(v, a.min, a.max)
+	case OpTrunc64to8, OpTrunc64to16, OpTrunc64to32, OpTrunc32to8, OpTrunc32to16, OpTrunc16to8:
+		a := ft.limits[v.Args[0].ID]
+		if a.umax <= 1<<(uint64(v.Type.Size())*8)-1 {
+			return ft.unsignedMinMax(v, a.umin, a.umax)
+		}
 
 	// math/bits
 	case OpCtz64:

test/prove.go

@@ -1586,6 +1586,26 @@ func div64s(a, b int64, ensureAllBranchesCouldHappen func() bool) int64 {
 	return z
 }
 
+func trunc64to16(a uint64, ensureAllBranchesCouldHappen func() bool) uint16 {
+	a &= 0xfff
+	a |= 0xff
+
+	z := uint16(a)
+	if ensureAllBranchesCouldHappen() && z > 0xfff { // ERROR "Disproved Less16U$"
+		return 42
+	}
+	if ensureAllBranchesCouldHappen() && z <= 0xfff { // ERROR "Proved Leq16U$"
+		return 1337
+	}
+	if ensureAllBranchesCouldHappen() && z < 0xff { // ERROR "Disproved Less16U$"
+		return 42
+	}
+	if ensureAllBranchesCouldHappen() && z >= 0xff { // ERROR "Proved Leq16U$"
+		return 1337
+	}
+	return z
+}
+
 //go:noinline
 func useInt(a int) {
 }