[LiveIntervals] Fix verification of early-clobbered segments

[foad]

Summary

Enable verification of live intervals immediately after computing them
(when -early-live-intervals is used) and fix a problem that that
provokes: currently the verifier insists that a segment that ends at an
early-clobber slot must be followed by another segment starting at the
same slot. But before TwoAddressInstruction runs, the equivalent
condition is: a segment that ends at an early-clobber slot must have its
last use tied to an early-clobber def. That condition is harder to check
here, so for now just disable this check until tied operands have been
rewritten.

[foad]

I assume it is correct, that we want to tweak the rules enforced by the verifier before and after tied ops are rewritten? The alternative is that we actually modify the end points of the live segments, so that they only end at an early-clobber slot after tied ops have been rewritten, but that seems needlessly invasive.

[MatzeB]

Yes, before the TwoAddressInstructionass the code is in SSA form which means the def must use a different register than the use of the tied op. This makes sense to me.

llvm/lib/CodeGen/MachineVerifier.cpp

if (S.end.isDead()) {
     // That means there must be a dead def.
     if (!SlotIndex::isSameInstr(S.start, S.end)) {
       report("Live segment ending at dead slot spans instructions", EndMBB);
       report_context(LR, Reg, LaneMask);
       report_context(S);
     }
   }
 
-  // A live segment can only end at an early-clobber slot if it is being
-  // redefined by an early-clobber def.
-  if (S.end.isEarlyClobber()) {
+  // After tied operands are rewritten, a live segment can only end at an
+  // early-clobber slot if it is being redefined by an early-clobber def.
+  // TODO: Before tied operands are rewritten, a live segment can only end at an
+  // early-clobber slot if the last use is tied to an early-clobber def.
+  if (MF->getProperties().hasProperty(
+          MachineFunctionProperties::Property::TiedOpsRewritten) &&
+      S.end.isEarlyClobber()) {
     if (I+1 == LR.end() || (I+1)->start != S.end) {
       report("Live segment ending at early clobber slot must be "
              "redefined by an EC def in the same instruction", EndMBB);
       report_context(LR, Reg, LaneMask);
       report_context(S);
     }
   }
 

llvm/lib/CodeGen/TargetPassConfig.cpp

void TargetPassConfig::addOptimizedRegAlloc() {
   addPass(&LiveVariablesID);
 
   // Edge splitting is smarter with machine loop info.
   addPass(&MachineLoopInfoID);
   addPass(&PHIEliminationID, false);
 
   // Eventually, we want to run LiveIntervals before PHI elimination.
   if (EarlyLiveIntervals)
-    addPass(&LiveIntervalsID, false);
+    addPass(&LiveIntervalsID);
 
   addPass(&TwoAddressInstructionPassID, false);
   addPass(&RegisterCoalescerID);
 
   // The machine scheduler may accidentally create disconnected components
   // when moving subregister definitions around, avoid this by splitting them to
   // separate vregs before. Splitting can also improve reg. allocation quality.
   addPass(&RenameIndependentSubregsID);