From 56f506811f622031c1c7e0403bc4db5e89039f03 Mon Sep 17 00:00:00 2001
From: Bjorn Pettersson <bjorn.a.pettersson@ericsson.com>
Date: Tue, 21 Aug 2018 07:49:05 +0000
Subject: [PATCH] [RegisterCoalescer] Do not assert when trying to remat dead
 values

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.

Reviewers: kparzysz, wmi, tpr

Reviewed By: kparzysz

Subscribers: MatzeB, qcolombet, tpr, llvm-commits

Differential Revision: https://reviews.llvm.org/D50842

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@340255 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/CodeGen/RegisterCoalescer.cpp        |  3 ++-
 test/CodeGen/X86/coalesce-dead-lanes.mir | 19 +++++++++++++++++++
 2 files changed, 21 insertions(+), 1 deletion(-)
 create mode 100644 test/CodeGen/X86/coalesce-dead-lanes.mir

diff --git a/lib/CodeGen/RegisterCoalescer.cpp b/lib/CodeGen/RegisterCoalescer.cpp
index a9cec3f2a01..933d5daf168 100644
--- a/lib/CodeGen/RegisterCoalescer.cpp
+++ b/lib/CodeGen/RegisterCoalescer.cpp
@@ -1159,7 +1159,8 @@ bool RegisterCoalescer::reMaterializeTrivialDef(const CoalescerPair &CP,
   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);
diff --git a/test/CodeGen/X86/coalesce-dead-lanes.mir b/test/CodeGen/X86/coalesce-dead-lanes.mir
new file mode 100644
index 00000000000..a1bd9df9e67
--- /dev/null
+++ b/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
-- 
2.11.0