[LiveIntervals] Handle moving up dead partial write

Summary:
In the test case, the machine scheduler moves a dead write to a subreg
up into the middle of a segment of the overall reg's live range, where
the segment had liveness only for other subregs in the reg.
handleMoveUp created an invalid live range, causing an assert a bit
later.

This commit fixes it to handle that situation. The segment is split in
two at the insertion point, and the part after the split, and any
subsequent segments up to the old position, are changed to be defined by
the moved def.

V2: Better test.

Subscribers: MatzeB, nhaehnle, llvm-commits

Differential Revision: https://reviews.llvm.org/D43478

Change-Id: Ibc42445ddca84e79ad1f616401015d22bc63832e
llvm-svn: 326087

1 files changed, 30 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/LiveIntervals.cpp b/llvm/lib/CodeGen/LiveIntervals.cpp
index 79fdba7e062..13fe77e4866 100644
--- a/llvm/lib/CodeGen/LiveIntervals.cpp
+++ b/llvm/lib/CodeGen/LiveIntervals.cpp
@@ -1291,6 +1291,36 @@ private:
           if (OldIdxIn != E && SlotIndex::isEarlierInstr(NewIdx, OldIdxIn->end))
             OldIdxIn->end = NewIdx.getRegSlot();
         }
+      } else if (OldIdxIn != E
+          && SlotIndex::isEarlierInstr(NewIdxOut->start, NewIdx)
+          && SlotIndex::isEarlierInstr(NewIdx, NewIdxOut->end)) {
+        // OldIdxVNI is a dead def that has been moved into the middle of
+        // another value in LR. That can happen when LR is a whole register,
+        // but the dead def is a write to a subreg that is dead at NewIdx.
+        // The dead def may have been moved across other values
+        // in LR, so move OldIdxOut up to NewIdxOut. Slide [NewIdxOut;OldIdxOut)
+        // down one position.
+        //    |- X0/NewIdxOut -| ... |- Xn-1 -| |- Xn/OldIdxOut -| |- next - |
+        // => |- X0/NewIdxOut -| |- X0 -| ... |- Xn-1 -| |- next -|
+        std::copy_backward(NewIdxOut, OldIdxOut, std::next(OldIdxOut));
+        // Modify the segment at NewIdxOut and the following segment to meet at
+        // the point of the dead def, with the following segment getting
+        // OldIdxVNI as its value number.
+        *NewIdxOut = LiveRange::Segment(
+            NewIdxOut->start, NewIdxDef.getRegSlot(), NewIdxOut->valno);
+        *(NewIdxOut + 1) = LiveRange::Segment(
+            NewIdxDef.getRegSlot(), (NewIdxOut + 1)->end, OldIdxVNI);
+        OldIdxVNI->def = NewIdxDef;
+        // Modify subsequent segments to be defined by the moved def OldIdxVNI.
+        for (auto Idx = NewIdxOut + 2; Idx <= OldIdxOut; ++Idx)
+          Idx->valno = OldIdxVNI;
+        // Aggressively remove all dead flags from the former dead definition.
+        // Kill/dead flags shouldn't be used while live intervals exist; they
+        // will be reinserted by VirtRegRewriter.
+        if (MachineInstr *KillMI = LIS.getInstructionFromIndex(NewIdx))
+          for (MIBundleOperands MO(*KillMI); MO.isValid(); ++MO)
+            if (MO->isReg() && !MO->isUse())
+              MO->setIsDead(false);
       } else {
         // OldIdxVNI is a dead def. It may have been moved across other values
         // in LR, so move OldIdxOut up to NewIdxOut. Slide [NewIdxOut;OldIdxOut)