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, 4 insertions, 3 deletions

diff --git a/src/cmd/compile/internal/ssa/rewrite.go b/src/cmd/compile/internal/ssa/rewrite.go
index 39aa63d947..24efd38fb7 100644
--- 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