Generic: Make isMask_N and isShiftedMask_N consistent over 0

Previously, isMask_N returned false for 0 but isShiftedMask_N returned true.
Almost all uses are for pattern matching bitfield operations in the backends,
and expect false (this was discovered because of AArch64's copy of this logic).

Unfortunately, I couldn't put together a small non-fragile test for this. The
nature of the bitfield operations means that this edge case is only really
triggered for nodes like "(and N, 0)", which the DAG combiner is usually very
good at folding away before they get to this stage.

rdar://20501377

llvm-svn: 234659

1 files changed, 10 insertions, 10 deletions

diff --git a/llvm/include/llvm/Support/MathExtras.h b/llvm/include/llvm/Support/MathExtras.h
index 388d82c..7edc2ac 100644
--- a/llvm/include/llvm/Support/MathExtras.h
+++ b/llvm/include/llvm/Support/MathExtras.h
@@ -318,31 +318,31 @@ inline bool isIntN(unsigned N, int64_t x) {
   return N >= 64 || (-(INT64_C(1)<<(N-1)) <= x && x < (INT64_C(1)<<(N-1)));
 }
 
-/// isMask_32 - This function returns true if the argument is a sequence of ones
-/// starting at the least significant bit with the remainder zero (32 bit
-/// version).   Ex. isMask_32(0x0000FFFFU) == true.
+/// isMask_32 - This function returns true if the argument is a non-empty
+/// sequence of ones starting at the least significant bit with the remainder
+/// zero (32 bit version).  Ex. isMask_32(0x0000FFFFU) == true.
 inline bool isMask_32(uint32_t Value) {
   return Value && ((Value + 1) & Value) == 0;
 }
 
-/// isMask_64 - This function returns true if the argument is a sequence of ones
-/// starting at the least significant bit with the remainder zero (64 bit
-/// version).
+/// isMask_64 - This function returns true if the argument is a non-empty
+/// sequence of ones starting at the least significant bit with the remainder
+/// zero (64 bit version).
 inline bool isMask_64(uint64_t Value) {
   return Value && ((Value + 1) & Value) == 0;
 }
 
 /// isShiftedMask_32 - This function returns true if the argument contains a
-/// sequence of ones with the remainder zero (32 bit version.)
+/// non-empty sequence of ones with the remainder zero (32 bit version.)
 /// Ex. isShiftedMask_32(0x0000FF00U) == true.
 inline bool isShiftedMask_32(uint32_t Value) {
-  return isMask_32((Value - 1) | Value);
+  return Value && isMask_32((Value - 1) | Value);
 }
 
 /// isShiftedMask_64 - This function returns true if the argument contains a
-/// sequence of ones with the remainder zero (64 bit version.)
+/// non-empty sequence of ones with the remainder zero (64 bit version.)
 inline bool isShiftedMask_64(uint64_t Value) {
-  return isMask_64((Value - 1) | Value);
+  return Value && isMask_64((Value - 1) | Value);
 }
 
 /// isPowerOf2_32 - This function returns true if the argument is a power of