[CodeGen] Return false from LiveIntervals::runOnMachineFunction

[foad]

Summary

This is an analysis pass so it does not modify the MachineFunction.

[serge-sans-paille]

Looks good. I've got the feeling that we could do something at the analysis level to make sure we never return true for an analysis.

[foad]

I've got the feeling that we could do something at the analysis level to make sure we never return true for an analysis.

Maybe, but I thought the legacy pass manager generally did not distinguish analysis passes from non-analysis passes.

I think we could also use hashing on MachineFunctions to check for MachineFunctionPasses that modify the MF and then return false.

[myhsu]

In D119941#3326565, @foad wrote:

I've got the feeling that we could do something at the analysis level to make sure we never return true for an analysis.

Maybe, but I thought the legacy pass manager generally did not distinguish analysis passes from non-analysis passes.

maybe we can use PassInfo::isAnalysis()?

I think we could also use hashing on MachineFunctions to check for MachineFunctionPasses that modify the MF and then return false.

I vaguely remember a similar idea was also proposed for the new PassManager, couldn't find it on the mailing list (archive) though

llvm/lib/CodeGen/LiveIntervals.cpp

bool LiveIntervals::runOnMachineFunction(MachineFunction &fn) {
 
   if (EnablePrecomputePhysRegs) {
     // For stress testing, precompute live ranges of all physical register
     // units, including reserved registers.
     for (unsigned i = 0, e = TRI->getNumRegUnits(); i != e; ++i)
       getRegUnit(i);
   }
   LLVM_DEBUG(dump());
-  return true;
+  return false;
 }
 
 void LiveIntervals::print(raw_ostream &OS, const Module* ) const {
   OS << "********** INTERVALS **********\n";
 
   // Dump the regunits.
   for (unsigned Unit = 0, UnitE = RegUnitRanges.size(); Unit != UnitE; ++Unit)
     if (LiveRange *LR = RegUnitRanges[Unit])