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 <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Paul Murphy <[email protected]>
Reviewed-by: Carlos Eduardo Seo <[email protected]>
Trust: Lynn Boger <[email protected]>

Author: laboger

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

@@ -1427,10 +1427,11 @@ func DecodePPC64RotateMask(sauxint int64) (rotate, mb, me int64, mask uint64) {
 	return
 }
 
-// This verifies that the mask occupies the
-// rightmost bits.
+// This verifies that the mask is a set of
+// consecutive bits including the least
+// significant bit.
 func isPPC64ValidShiftMask(v int64) bool {
-	if ((v + 1) & v) == 0 {
+	if (v != 0) && ((v+1)&v) == 0 {
 		return true
 	}
 	return false

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
+}