[RegisterCoalescer] Do not assert when trying to remat dead values

[bjope]

Summary

RegisterCoalescer::reMaterializeTrivialDef used to assert that
the input register was live in. But as shown by the new
coalesce-dead-lanes.mir test case that seems to be a valid
scenario. We now return false instead of the assert, simply
avoiding to remat the dead def.

Normally a COPY of an undef value is eliminated by
eliminateUndefCopy(). Although we only do that when the
destination isn't a physical register. So the situation
above should be limited to the case when we copy an undef
value to a physical register.

[bjope]

Just adding some of the latest commiters to RegisterCoalescer.cpp as reviewers. I hope someone is able to review this.

[tpr]

Hi Bjorn. Looks ok to me, although I don't feel competent to approve as I am new to this area. Hopefully Krzysztof will be able to review it for you.

[kparzysz]

We should convert the copy to an IMPLICIT_DEF, but this patch is good as is.

llvm/trunk/lib/CodeGen/RegisterCoalescer.cpp

bool RegisterCoalescer::reMaterializeTrivialDef(const CoalescerPair &CP,
   unsigned DstReg = CP.isFlipped() ? CP.getSrcReg() : CP.getDstReg();
   unsigned DstIdx = CP.isFlipped() ? CP.getSrcIdx() : CP.getDstIdx();
   if (TargetRegisterInfo::isPhysicalRegister(SrcReg))
     return false;
 
   LiveInterval &SrcInt = LIS->getInterval(SrcReg);
   SlotIndex CopyIdx = LIS->getInstructionIndex(*CopyMI);
   VNInfo *ValNo = SrcInt.Query(CopyIdx).valueIn();
-  assert(ValNo && "CopyMI input register not live");
+  if (!ValNo)
+    return false;
   if (ValNo->isPHIDef() || ValNo->isUnused())
     return false;
   MachineInstr *DefMI = LIS->getInstructionFromIndex(ValNo->def);
   if (!DefMI)
     return false;
   if (DefMI->isCopyLike()) {
     IsDefCopy = true;
     return false;

llvm/trunk/test/CodeGen/X86/coalesce-dead-lanes.mir

+# RUN: llc -run-pass simple-register-coalescing -O0 -mtriple x86_64-pc-linux-gnu -o - %s | FileCheck %s
+
+---
+name:            foo
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    undef %18.sub_8bit_hi:gr16_abcd = COPY undef $al
+    %1:gr16_abcd = COPY killed %18
+    dead %8:gr8 = COPY undef %1.sub_8bit_hi
+    $al = COPY undef %8
+...
+
+# This used to hit an assertion:
+#   lib/CodeGen/RegisterCoalescer.cpp:1110: RegisterCoalescer::reMaterializeTrivialDef(...): Assertion `ValNo && "CopyMI input register not live"' failed.
+#
+# CHECK_LABEL: name:            foo
+# CHECK:       bb.0:
+# CHECK-NEXT:    $al = COPY undef %2:gr8