[llvm] r240520 - SystemZ: Rephrase this allOnes calculation to avoid UB

[Justin Bogner]

Author: bogner
Date: Wed Jun 24 00:59:19 2015
New Revision: 240520

URL: http://llvm.org/viewvc/llvm-project?rev=240520&view=rev
Log:
SystemZ: Rephrase this allOnes calculation to avoid UB

This allOnes function hits undefined behaviour if Count is greater
than 64, but we can avoid that and simplify the calculation by just
saturating if such a value is passed in.

This comes up under ubsan becauseRxSBGOperands is sometimes created
with values that are 128 bits wide. Somebody more familiar with this
code should probably look into whether that's expected, as a 64 bit
mask may or may not be appropriate for such types.

Modified:
    llvm/trunk/lib/Target/SystemZ/SystemZISelDAGToDAG.cpp

Modified: llvm/trunk/lib/Target/SystemZ/SystemZISelDAGToDAG.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/SystemZ/SystemZISelDAGToDAG.cpp?rev=240520&r1=240519&r2=240520&view=diff
==============================================================================
--- llvm/trunk/lib/Target/SystemZ/SystemZISelDAGToDAG.cpp (original)
+++ llvm/trunk/lib/Target/SystemZ/SystemZISelDAGToDAG.cpp Wed Jun 24 00:59:19 2015
@@ -96,7 +96,9 @@ struct SystemZAddressingMode {
 
 // Return a mask with Count low bits set.
 static uint64_t allOnes(unsigned int Count) {
-  return Count == 0 ? 0 : (uint64_t(1) << (Count - 1) << 1) - 1;
+  if (Count > 63)
+    return UINT64_MAX;
+  return (uint64_t(1) << Count) - 1;
 }
 
 // Represents operands 2 to 5 of the ROTATE AND ... SELECTED BITS operation