RegisterCoalescer: Shrink main range after shrinking subranges

[arsenm]

Summary

If the subregister uses were dead, this would leave the main range
segment pointing to a deleted instruction.

Not sure if this should try to avoid shrinking if we know we don't
have dead components.

[arsenm]

Fix missing test update

[arsenm]

ping

[arsenm]

a4834ad06880740ee289681ea8b5844caceaa1c6

llvm/lib/CodeGen/RegisterCoalescer.cpp

bool RegisterCoalescer::joinCopy(MachineInstr *CopyMI, bool &Again) {
   if (ShrinkMask.any()) {
     LiveInterval &LI = LIS->getInterval(CP.getDstReg());
     for (LiveInterval::SubRange &S : LI.subranges()) {
       if ((S.LaneMask & ShrinkMask).none())
         continue;
       LLVM_DEBUG(dbgs() << "Shrink LaneUses (Lane " << PrintLaneMask(S.LaneMask)
                         << ")\n");
       LIS->shrinkToUses(S, LI.reg());
+      ShrinkMainRange = true;
     }
     LI.removeEmptySubRanges();
   }
 
   // CP.getSrcReg()'s live interval has been merged into CP.getDstReg's live
   // interval. Since CP.getSrcReg() is in ToBeUpdated set and its live interval
   // is not up-to-date, need to update the merged live interval here.
   if (ToBeUpdated.count(CP.getSrcReg()))

llvm/test/CodeGen/AMDGPU/coalesce-into-dead-subreg-copies.mir

+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx900 -verify-coalescing -run-pass=simple-register-coalescing -o - %s | FileCheck %s
+
+# Check that there's no  "Live segment doesn't end at a valid
+# instruction" failure after coalescing %0 into %2, which is
+# ultimately a pair of dead copies.
+
+---
+name: coalesce_into_dead_subreg_copy
+tracksRegLiveness: true
+machineFunctionInfo:
+  isEntryFunction: true
+  scratchRSrcReg:  '$sgpr0_sgpr1_sgpr2_sgpr3'
+  stackPtrOffsetReg: '$sgpr32'
+  occupancy:       8
+body:             |
+  ; CHECK-LABEL: name: coalesce_into_dead_subreg_copy
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   dead %2:sreg_64_xexec = S_LOAD_DWORDX2_IMM undef %1:sgpr_64, 24, 0 :: (dereferenceable invariant load (s64), addrspace 4)
+  ; CHECK-NEXT:   S_BRANCH %bb.1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  bb.0:
+    %0:sreg_64_xexec = S_LOAD_DWORDX2_IMM undef %1:sgpr_64, 24, 0 :: (dereferenceable invariant load (s64), addrspace 4)
+    undef %2.sub0:sreg_64 = COPY %0.sub0:sreg_64_xexec
+    %2.sub1:sreg_64 = COPY killed %0.sub1:sreg_64_xexec
+    S_BRANCH %bb.1
+
+  bb.1:
+
+...