RegisterCoalescer: Report impossible for erasable implicit defs

[arsenm]

Summary

When mapValues was called on one half, this was reporting CR_Erase.
For the second value, it would see the value live out of the block
and report CR_Impossible. Check if it's live out when clearing
ErasableImplicitDef to avoid reporting inconsistent results.

I'm not very confident this is the right way to fix this. Changing
ErasableImplicitDef in the middle of this seems strange to me.

[arsenm]

I have another case that fails from the same fundamental problem of reporting OK for one side, and then aborting on the range check on the other side, so this might not be the right place to put this check

[arsenm]

D54551 solves this more completely

lib/CodeGen/RegisterCoalescer.cpp

JoinVals::analyzeValue(unsigned ValNo, JoinVals &Other) {
   // to erase the IMPLICIT_DEF instruction.
   if (OtherV.ErasableImplicitDef && DefMI &&
       DefMI->getParent() != Indexes->getMBBFromIndex(V.OtherVNI->def)) {
     LLVM_DEBUG(dbgs() << "IMPLICIT_DEF defined at " << V.OtherVNI->def
                       << " extends into "
                       << printMBBReference(*DefMI->getParent())
                       << ", keeping it.\n");
     OtherV.ErasableImplicitDef = false;
+
+    MachineBasicBlock *MBB = Indexes->getMBBFromIndex(V.OtherVNI->def);
+    if (OtherLRQ.endPoint() >= Indexes->getMBBEndIdx(MBB))
+      return CR_Impossible;
   }
 
   // Allow overlapping PHI values. Any real interference would show up in a
   // predecessor, the PHI itself can't introduce any conflicts.
   if (VNI->isPHIDef())
     return CR_Replace;
 
   // Check for simple erasable conflicts.

test/CodeGen/AMDGPU/pytorch-cannot-join-subrange.mir

+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple=amdgcn-amd-amdhsa -verify-coalescing -run-pass=simple-register-coalescing -verify-machineinstrs -o - %s | FileCheck %s
+
+---
+name: couldnt_join_subrange_live_out_implicit_def
+tracksRegLiveness: true
+body:             |
+  ; CHECK-LABEL: name: couldnt_join_subrange_live_out_implicit_def
+  ; CHECK: bb.0:
+  ; CHECK:   successors: %bb.1(0x80000000)
+  ; CHECK: bb.1:
+  ; CHECK:   successors: %bb.2(0x80000000)
+  ; CHECK:   undef %1.sub0:sreg_64_xexec = IMPLICIT_DEF
+  ; CHECK:   S_BRANCH %bb.2
+  ; CHECK: bb.2:
+  ; CHECK:   S_ENDPGM implicit %1
+  bb.0:
+    successors: %bb.1
+
+    undef %0.sub0:sreg_64_xexec = IMPLICIT_DEF
+
+  bb.1:
+    successors: %bb.2
+
+    %1:sreg_64 = COPY %0:sreg_64_xexec
+    %0.sub1:sreg_64_xexec = COPY %0.sub0:sreg_64_xexec
+    S_BRANCH %bb.2
+
+  bb.2:
+    dead %2:sreg_32_xm0 = COPY %0.sub0:sreg_64_xexec
+    S_ENDPGM implicit killed %1
+
+...