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 <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>

1 files changed, 20 insertions, 0 deletions

diff --git a/test/prove.go b/test/prove.go
index 28b950ce41..16f7b7de96 100644
--- a/test/prove.go
+++ b/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) {
 }