CalcSpillWeights: Hack to prevent x87 nonsense

This gross hack forces `hweight` into memory, preventing hidden precision from making `1 > 1` occasionally equal `true`. <rdar://problem/14292693> llvm-svn: 206765

Author: dexonsmith

llvm/lib/CodeGen/CalcSpillWeights.cpp

@@ -149,7 +149,11 @@ VirtRegAuxInfo::calculateSpillWeightAndHint(LiveInterval &li) {
  unsigned hint = copyHint(mi, li.reg, tri, mri);
  if (!hint)
  continue;
- float hweight = Hint[hint] += weight;
+ // Force hweight onto the stack so that x86 doesn't add hidden precision,
+ // making the comparison incorrectly pass (i.e., 1 > 1 == true??).
+ //
+ // FIXME: we probably shouldn't use floats at all.
+ volatile float hweight = Hint[hint] += weight;
  if (TargetRegisterInfo::isPhysicalRegister(hint)) {
  if (hweight > bestPhys && mri.isAllocatable(hint))
  bestPhys = hweight, hintPhys = hint;