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>

diff --git a/src/cmd/compile/internal/ssa/rewrite.go b/src/cmd/compile/internal/ssa/rewrite.go
index 39aa63d94750b8dbe827b7216d0ebafed83f1037..24efd38fb7c44957b5bf3f8c26a08781e7efa48d 100644 (file)
--- a/src/cmd/compile/internal/ssa/rewrite.go
+++ b/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

diff --git a/test/codegen/issue42610.go b/test/codegen/issue42610.go
new file mode 100644 (file)
index 0000000..c7eeddc
--- /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
+}