[release-branch.go1.15] cmd/compile: sign extend consant folding properly

MOVLconst must have a properly sign-extended auxint constant.
The bit operations in these rules don't enforce that invariant.

The easiest fix is just to turn on properly typed auxint fields
(which is what fixed this issue at tip).

Fixes #42753

Change-Id: I264245fad45067a6ade65326f7fe681feb5f3739
Reviewed-on: https://go-review.googlesource.com/c/go/+/272028
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/test/fixedbugs/issue42753.go b/test/fixedbugs/issue42753.go
new file mode 100644
index 0000000000..a998d1d3b3
--- /dev/null
+++ b/test/fixedbugs/issue42753.go
@@ -0,0 +1,13 @@
+// compile -d=ssa/check/on
+
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+func f() uint32 {
+	s := "\x01"
+	x := -int32(s[0])
+	return uint32(x) & 0x7fffffff
+}