cmd/compile: fix rules regression with shifts on PPC64

Some rules for PPC64 were checking for a case
where a shift followed by an 'and' of a mask could
be lowered, depending on the format of the mask. The
function to verify if the mask was valid for this purpose
was not checking if the mask was 0 which we don't want to
allow. This case can happen if previous optimizations
resulted in that mask value.

This fixes isPPC64ValidShiftMask to check for a mask of 0 and return
false.

This also adds a codegen testcase to verify it doesn't try to
match the rules in the future.

Fixes #42610

Change-Id: I565d94e88495f51321ab365d6388c01e791b4dbb
Reviewed-on: https://go-review.googlesource.com/c/go/+/270358
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Paul Murphy <murp@ibm.com>
Reviewed-by: Carlos Eduardo Seo <carlos.seo@linaro.org>
Trust: Lynn Boger <laboger@linux.vnet.ibm.com>

1 files changed, 30 insertions, 0 deletions

diff --git a/test/codegen/issue42610.go b/test/codegen/issue42610.go
new file mode 100644
index 00000000000..c7eeddc53c9
--- /dev/null
+++ b/test/codegen/issue42610.go
@@ -0,0 +1,30 @@
+// asmcheck
+
+// 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.
+
+// Don't allow 0 masks in shift lowering rules on ppc64x.
+// See issue 42610.
+
+package codegen
+
+func f32(a []int32, i uint32) {
+        g := func(p int32) int32 {
+                i = uint32(p) * (uint32(p) & (i & 1))
+                return 1
+        }
+        // ppc64le: -"RLWNIM"
+        // ppc64: -"RLWNIM"
+        a[0] = g(8) >> 1
+}
+
+func f(a []int, i uint) {
+	g := func(p int) int {
+		i = uint(p) * (uint(p) & (i & 1))
+		return 1
+	}
+	// ppc64le: -"RLDIC"
+	// ppc64: -"RLDIC"
+	a[0] = g(8) >> 1
+}