RegisterCoalescer: Fix verifier error when merging copy of undef

There's no real read of the register, so the copy introduced a new
live value. Make sure we introduce a replacement implicit_def instead
of just erasing the copy.

Found from llvm-reduce since it tries to set undef on everything.

Author: arsenm

llvm/lib/CodeGen/RegisterCoalescer.cpp

@@ -1641,18 +1641,20 @@ MachineInstr *RegisterCoalescer::eliminateUndefCopy(MachineInstr *CopyMI) {
   SlotIndex RegIndex = Idx.getRegSlot();
   LiveRange::Segment *Seg = DstLI.getSegmentContaining(RegIndex);
   assert(Seg != nullptr && "No segment for defining instruction");
-  if (VNInfo *V = DstLI.getVNInfoAt(Seg->end)) {
-    if (V->isPHIDef()) {
-      CopyMI->setDesc(TII->get(TargetOpcode::IMPLICIT_DEF));
-      for (unsigned i = CopyMI->getNumOperands(); i != 0; --i) {
-        MachineOperand &MO = CopyMI->getOperand(i-1);
-        if (MO.isReg() && MO.isUse())
-          CopyMI->removeOperand(i-1);
-      }
-      LLVM_DEBUG(dbgs() << "\tReplaced copy of <undef> value with an "
-                           "implicit def\n");
-      return CopyMI;
+  VNInfo *V = DstLI.getVNInfoAt(Seg->end);
+
+  // The source interval may also have been on an undef use, in which case the
+  // copy introduced a live value.
+  if (((V && V->isPHIDef()) || (!V && !DstLI.liveAt(Idx)))) {
+    CopyMI->setDesc(TII->get(TargetOpcode::IMPLICIT_DEF));
+    for (unsigned i = CopyMI->getNumOperands(); i != 0; --i) {
+      MachineOperand &MO = CopyMI->getOperand(i-1);
+      if (MO.isReg() && MO.isUse())
+        CopyMI->removeOperand(i-1);
     }
+    LLVM_DEBUG(dbgs() << "\tReplaced copy of <undef> value with an "
+               "implicit def\n");
+    return CopyMI;
   }
 
   // Remove any DstReg segments starting at the instruction.

llvm/test/CodeGen/AMDGPU/coalesce-liveout-undef-copy.mir

@@ -0,0 +1,61 @@
+# 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
+
+# %2 has an undef read in %bb.3, and this IR wouldn't be valid if it
+# was a real read. After merging %2 into %0, we need to replace the
+# copy of undef with an implicit_def since the copy introduced a new
+# value.
+
+---
+name: coalesce_into_undef_copy
+tracksRegLiveness: true
+machineFunctionInfo:
+  scratchRSrcReg:  '$sgpr0_sgpr1_sgpr2_sgpr3'
+  stackPtrOffsetReg: '$sgpr32'
+body:             |
+  ; CHECK-LABEL: name: coalesce_into_undef_copy
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[COPY:%[0-9]+]]:vreg_128_align2 = COPY undef %1:sgpr_128, implicit $exec
+  ; CHECK-NEXT:   S_BRANCH %bb.1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   successors: %bb.2(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   dead %2:vreg_128_align2 = IMPLICIT_DEF
+  ; CHECK-NEXT:   [[COPY]].sub0:vreg_128_align2 = IMPLICIT_DEF
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.2:
+  ; CHECK-NEXT:   successors: %bb.2(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   dead %4:vgpr_32 = V_INDIRECT_REG_READ_GPR_IDX_B32_V4 [[COPY]], undef %5:sgpr_32, 11, implicit-def $m0, implicit $m0, implicit $exec
+  ; CHECK-NEXT:   S_BRANCH %bb.2
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.3:
+  ; CHECK-NEXT:   successors: %bb.1(0x40000000), %bb.4(0x40000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[DEF:%[0-9]+]]:vreg_128_align2 = IMPLICIT_DEF
+  ; CHECK-NEXT:   S_CBRANCH_EXECNZ %bb.1, implicit $exec
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.4:
+  bb.0:
+    %0:vreg_128_align2 = COPY undef %1:sgpr_128, implicit $exec
+    S_BRANCH %bb.1
+
+  bb.1:
+    %2:vreg_128_align2 = IMPLICIT_DEF
+    %3:vreg_128_align2 = COPY killed %0
+    %3.sub0:vreg_128_align2 = IMPLICIT_DEF
+
+  bb.2:
+    dead %5:vgpr_32 = V_INDIRECT_REG_READ_GPR_IDX_B32_V4 %3, undef %6:sgpr_32, 11, implicit-def $m0, implicit $m0, implicit $exec
+    S_BRANCH %bb.2
+
+  bb.3:
+    %0:vreg_128_align2 = COPY undef %2
+    S_CBRANCH_EXECNZ %bb.1, implicit $exec
+
+  bb.4:
+
+...

llvm/test/CodeGen/AMDGPU/undef-subreg-use-after-coalesce.mir

@@ -32,7 +32,8 @@ body:             |
     ; CHECK-LABEL: name: undef_subreg_use_after_full_copy_coalesce_composed
     ; CHECK: undef %0.sub0:vreg_128 = V_MOV_B32_e32 0, implicit $exec
     ; CHECK-NEXT: dead %0.sub1:vreg_128 = V_MOV_B32_e32 0, implicit $exec
-    ; CHECK-NEXT: S_ENDPGM 0, implicit undef %2.sub1:vreg_64
+    ; CHECK-NEXT: [[DEF:%[0-9]+]]:vreg_64 = IMPLICIT_DEF
+    ; CHECK-NEXT: S_ENDPGM 0, implicit [[DEF]].sub1
     undef %0.sub0:vreg_128 = V_MOV_B32_e32 0, implicit $exec
     %0.sub1:vreg_128 = V_MOV_B32_e32 0, implicit $exec
     %1:vreg_128 = COPY killed %0