LiveIntervalAnalysis: fix a crash in repairOldRegInRange

Summary:
See the new test case for one that was (non-deterministically) crashing
on trunk and deterministically hit the assertion that I added in D23302.
Basically, the machine function contains a sequence

     DS_WRITE_B32 %vreg4, %vreg14:sub0, ...
     DS_WRITE_B32 %vreg4, %vreg14:sub0, ...
     %vreg14:sub1<def> = COPY %vreg14:sub0

and SILoadStoreOptimizer::mergeWrite2Pair merges the two DS_WRITE_B32
instructions into one before calling repairIntervalsInRange.

Now repairIntervalsInRange wants to repair %vreg14, in particular, and
ends up trying to repair %vreg14:sub1 as well, but that only becomes
active _after_ the range that is to be repaired, hence the crash due
to LR.find(...) == LR.begin() at the start of repairOldRegInRange.

I believe that just skipping those subrange is fine, but again, not too
familiar with that code.

Reviewers: MatzeB, kparzysz, tstellarAMD

Subscribers: llvm-commits, MatzeB

Differential Revision: https://reviews.llvm.org/D23303

llvm-svn: 278268

1 files changed, 5 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/LiveIntervalAnalysis.cpp b/llvm/lib/CodeGen/LiveIntervalAnalysis.cpp
index 8fd0585f976..26b5cba3294 100644
--- a/llvm/lib/CodeGen/LiveIntervalAnalysis.cpp
+++ b/llvm/lib/CodeGen/LiveIntervalAnalysis.cpp
@@ -1396,6 +1396,11 @@ void LiveIntervals::repairOldRegInRange(const MachineBasicBlock::iterator Begin,
                                         LaneBitmask LaneMask) {
   LiveInterval::iterator LII = LR.find(endIdx);
   SlotIndex lastUseIdx;
+  if (LII == LR.begin()) {
+    // This happens when the function is called for a subregister that only
+    // occurs _after_ the range that is to be repaired.
+    return;
+  }
   if (LII != LR.end() && LII->start < endIdx)
     lastUseIdx = LII->end;
   else