[release-branch.go1.16] cmd/compile: ensure constant shift amounts are in range for arm

Ensure constant shift amounts are in the range [0-31]. When shift amounts
are out of range, bad things happen. Shift amounts out of range occur
when lowering 64-bit shifts (we take an in-range shift s in [0-63] and
calculate s-32 and 32-s, both of which might be out of [0-31]).

The constant shift operations themselves still work, but their shift
amounts get copied unmolested to operations like ORshiftLL which use only
the low 5 bits. That changes an operation like <<100 which unconditionally
produces 0, to <<4, which doesn't.

Fixes #48478

Change-Id: I87363ef2b4ceaf3b2e316426064626efdfbb8ee3
Reviewed-on: https://go-review.googlesource.com/c/go/+/350969
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
(cherry picked from commit eff27e858b771bf5e0b5e7e836827c7d2941e6d4)
Reviewed-on: https://go-review.googlesource.com/c/go/+/351070
Reviewed-by: Austin Clements <austin@google.com>

1 files changed, 21 insertions, 0 deletions

diff --git a/test/fixedbugs/issue48476.go b/test/fixedbugs/issue48476.go
new file mode 100644
index 0000000000..6b77f7c3c6
--- /dev/null
+++ b/test/fixedbugs/issue48476.go
@@ -0,0 +1,21 @@
+// run
+
+// Copyright 2021 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
+
+import "fmt"
+
+//go:noinline
+func f(x uint64) uint64 {
+	s := "\x04"
+	c := s[0]
+	return x << c << 4
+}
+func main() {
+	if want, got := uint64(1<<8), f(1); want != got {
+		panic(fmt.Sprintf("want %x got %x", want, got))
+	}
+}