[ARM][LowOverheadLoops] Check live-out for InsertPt instead of Start

[samtebbs]

Summary

In TryAdjustInsertionPoint an insertion point has already been selected but the live out checks are done on Start instead. Most of the time Start == InserPt, but in other cases where this isn't the case, these live-out checks will fail as false negatives since there will be a mov lr after Start. This patch changes the live-out checks to use InsertPt instead.

[SjoerdMeijer]

Can we add tests for this?

[samparker]

llvm/lib/Target/ARM/ARMLowOverheadLoops.cpp

1088

Start -> InsertPt for these two as well.

[samtebbs]

In D89549#2334729, @SjoerdMeijer wrote:

Can we add tests for this?

Ah yes, I'll add one.

llvm/lib/Target/ARM/ARMLowOverheadLoops.cpp

1088

I missed these, thanks.

[samtebbs]

The code that this change depends on has been reverted so I will delay close this and re-visit it once those changes have been re-worked.

llvm/lib/Target/ARM/ARMLowOverheadLoops.cpp

auto TryAdjustInsertionPoint = [](MachineBasicBlock::iterator &InsertPt,
     MachineBasicBlock *MBB = InsertPt->getParent();
     MachineBasicBlock::iterator FirstNonTerminator =
       MBB->getFirstTerminator();
     unsigned CountReg = Start->getOperand(0).getReg();
 
     // Get the latest possible insertion point and check whether the semantics
     // will be maintained if Start was inserted there.
     if (FirstNonTerminator == MBB->end()) {
-      if (RDA.isReachingDefLiveOut(Start, CountReg) &&
-          RDA.isReachingDefLiveOut(Start, ARM::LR))
+      if (RDA.isReachingDefLiveOut(&*InsertPt, CountReg) &&
+          RDA.isReachingDefLiveOut(&*InsertPt, ARM::LR))
         InsertPt = FirstNonTerminator;
     } else if (RDA.hasSameReachingDef(Start, &*FirstNonTerminator, CountReg) &&
                RDA.hasSameReachingDef(Start, &*FirstNonTerminator, ARM::LR))
       InsertPt = FirstNonTerminator;
   };
 
   if (!FindStartInsertionPoint(Start, Dec, StartInsertPt, StartInsertBB, RDA,
                                ToRemove)) {
     LLVM_DEBUG(dbgs() << "ARM Loops: Unable to find safe insertion point.\n");
     Revert = true;