[LLVMdev] LiveIntervals invalidates LiveVariables?

[Tzu-Chien Chiu]

I though LiveVariables may be invalidated by LiveIntervals, but it's
declared not:

void LiveIntervals::getAnalysisUsage(AnalysisUsage &AU) const
{
  AU.addPreserved<LiveVariables>();
  AU.addRequired<LiveVariables>();
  ...

LiveInterval may coalesce virtual registers and remove identity moves
instructions:

bool LiveIntervals::runOnMachineFunction(MachineFunction &fn) {
  ...
  // perform a final pass over the instructions and compute spill
  // weights, coalesce virtual registers and remove identity moves

but the data structure LiveVariables::VirtRegInfo is _not_ updated.
That is, VarInfo::DefInstr may point to an invalid (being coalesced)
instruction.

-- 
Tzu-Chien Chiu,
3D Graphics Hardware Architect
<URL:http://www.csie.nctu.edu.tw/~jwchiu>