[PATCH] D50842: [RegisterCoalescer] Do not assert when trying to remat dead values

[Bjorn Pettersson via Phabricator via llvm-commits]

bjope created this revision.
Herald added subscribers: tpr, qcolombet, MatzeB.

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.


Repository:
  rL LLVM

https://reviews.llvm.org/D50842

Files:
  lib/CodeGen/RegisterCoalescer.cpp
  test/CodeGen/X86/coalesce-dead-lanes.mir


Index: test/CodeGen/X86/coalesce-dead-lanes.mir
===================================================================
--- /dev/null
+++ test/CodeGen/X86/coalesce-dead-lanes.mir
@@ -0,0 +1,19 @@
+# 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
Index: lib/CodeGen/RegisterCoalescer.cpp
===================================================================
--- lib/CodeGen/RegisterCoalescer.cpp
+++ lib/CodeGen/RegisterCoalescer.cpp
@@ -1138,7 +1138,8 @@
   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);


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