[AMDGPU] Preserve deadness of vcc when shrinking instructions

This doesn't have any effect on codegen now, but it might do in the
future if we shrink instructions before post-RA scheduling, which is
sensitive to live vs dead defs.

Differential Revision: https://reviews.llvm.org/D112305

Author: jayfoad

llvm/lib/Target/AMDGPU/SIShrinkInstructions.cpp

@@ -810,6 +810,10 @@ bool SIShrinkInstructions::runOnMachineFunction(MachineFunction &MF) {
       // Copy extra operands not present in the instruction definition.
       copyExtraImplicitOps(*Inst32, MF, MI);
 
+      // Copy deadness from the old explicit vcc def to the new implicit def.
+      if (SDst && SDst->isDead())
+        Inst32->findRegisterDefOperand(VCCReg)->setIsDead();
+
       MI.eraseFromParent();
       foldImmediates(*Inst32, TII, MRI);
 

llvm/test/CodeGen/AMDGPU/shrink-carry.mir

@@ -1,7 +1,7 @@
 # RUN: llc -march=amdgcn -mcpu=gfx900 -verify-machineinstrs -start-before si-shrink-instructions -stop-before si-late-branch-lowering -o - %s | FileCheck -check-prefix=GCN %s
 
 # GCN-LABEL: name: subbrev{{$}}
-# GCN:       V_SUBBREV_U32_e32 0, undef $vgpr0, implicit-def $vcc, implicit killed $vcc, implicit $exec
+# GCN:       V_SUBBREV_U32_e32 0, undef $vgpr0, implicit-def dead $vcc, implicit killed $vcc, implicit $exec
 
 ---
 name:            subbrev
@@ -26,7 +26,7 @@ body:             |
 ...
 
 # GCN-LABEL: name: subb{{$}}
-# GCN:       V_SUBBREV_U32_e32 0, undef $vgpr0, implicit-def $vcc, implicit killed $vcc, implicit $exec
+# GCN:       V_SUBBREV_U32_e32 0, undef $vgpr0, implicit-def dead $vcc, implicit killed $vcc, implicit $exec
 
 ---
 name:            subb
@@ -51,7 +51,7 @@ body:             |
 ...
 
 # GCN-LABEL: name: addc{{$}}
-# GCN:       V_ADDC_U32_e32 0, undef $vgpr0, implicit-def $vcc, implicit killed $vcc, implicit $exec
+# GCN:       V_ADDC_U32_e32 0, undef $vgpr0, implicit-def dead $vcc, implicit killed $vcc, implicit $exec
 
 ---
 name:            addc
@@ -76,7 +76,7 @@ body:             |
 ...
 
 # GCN-LABEL: name: addc2{{$}}
-# GCN:       V_ADDC_U32_e32 0, undef $vgpr0, implicit-def $vcc, implicit killed $vcc, implicit $exec
+# GCN:       V_ADDC_U32_e32 0, undef $vgpr0, implicit-def dead $vcc, implicit killed $vcc, implicit $exec
 
 ---
 name:            addc2