[PATCH] D112305: [AMDGPU] Preserve deadness of vcc when shrinking instructions

[Jay Foad via Phabricator via llvm-commits]

foad created this revision.
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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.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D112305

Files:
  llvm/lib/Target/AMDGPU/SIShrinkInstructions.cpp
  llvm/test/CodeGen/AMDGPU/shrink-carry.mir


Index: llvm/test/CodeGen/AMDGPU/shrink-carry.mir
===================================================================
--- llvm/test/CodeGen/AMDGPU/shrink-carry.mir
+++ 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 @@
 ...
 
 # 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 @@
 ...
 
 # 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 @@
 ...
 
 # 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
Index: llvm/lib/Target/AMDGPU/SIShrinkInstructions.cpp
===================================================================
--- llvm/lib/Target/AMDGPU/SIShrinkInstructions.cpp
+++ llvm/lib/Target/AMDGPU/SIShrinkInstructions.cpp
@@ -810,6 +810,10 @@
       // 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);
 


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