RegisterCoalescer: Forcibly leave SSA to avoid MIR test errors

[arsenm]

Summary

Not sure how to produce a test that demonstrates the problem
today. The coalescer would have to introduce a verifier caught SSA
violation, like multiple defs of a virtual register. I'm not sure what
would do that now, but an upcoming patch will.

[MatzeB]

LGTM

[MatzeB]

FWIW: There is a getClearedProperties() API that you can implement for a pass to get things cleared too. Though I remember that I personally always found that API a bit artificial as you can also just clear the bits within the pass as you do here. So no complaints about this patch from my side :)

[arsenm]

In D156271#4533548, @MatzeB wrote:

FWIW: There is a getClearedProperties() API that you can implement for a pass to get things cleared too. Though I remember that I personally always found that API a bit artificial as you can also just clear the bits within the pass as you do here. So no complaints about this patch from my side :)

Right I tried that first, but that’s a post-pass adjustment. I could try moving it before

[MatzeB]

Right I tried that first, but that’s a post-pass adjustment. I could try moving it before

I remember proposing to just not have the getClearedProperties API in code-review but people didn't like that. Either way I'm fine with the change as-is.

[arsenm]

32a23aecf8002e181eb1022b8733ef8666b3241f

llvm/lib/CodeGen/RegisterCoalescer.cpp

bool RegisterCoalescer::runOnMachineFunction(MachineFunction &fn) {
   LIS = &getAnalysis<LiveIntervals>();
   AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
   Loops = &getAnalysis<MachineLoopInfo>();
   if (EnableGlobalCopies == cl::BOU_UNSET)
     JoinGlobalCopies = STI.enableJoinGlobalCopies();
   else
     JoinGlobalCopies = (EnableGlobalCopies == cl::BOU_TRUE);
 
+  // FIXME: MachineFunctionProperties cannot express the required pre-property
+  // no-SSA. When running a MIR testcase without any virtual register defs, the
+  // MIR parser assumes SSA. MachineFunctionPass::getClearedProperties is called
+  // after the pass is run, so the properties at this point say it's an SSA
+  // function.  Forcibly clear it here so -verify-coalescing doesn't complain
+  // after multiple virtual register defs are introduced.
+  MRI->leaveSSA();
+
   // If there are PHIs tracked by debug-info, they will need updating during
   // coalescing. Build an index of those PHIs to ease updating.
   SlotIndexes *Slots = LIS->getSlotIndexes();
   for (const auto &DebugPHI : MF->DebugPHIPositions) {
     MachineBasicBlock *MBB = DebugPHI.second.MBB;
     Register Reg = DebugPHI.second.Reg;
     unsigned SubReg = DebugPHI.second.SubReg;
     SlotIndex SI = Slots->getMBBStartIdx(MBB);