[PATCH] D26261: AMDGPU: Preserve vcc undef flags when inverting branch

[Matt Arsenault via llvm-commits]

arsenm created this revision.
arsenm added a subscriber: llvm-commits.
Herald added a reviewer: tstellarAMD.
Herald added subscribers: tony-tye, yaxunl, nhaehnle, wdng, kzhuravl.

If the branch was on a read-undef of vcc, passes that used
analyzeBranch to invert the branch condition wouldn't preserve
the undef flag resulting in a verifier error.

      

Fixes verifier failures in a future commit.

Also fix verifier error when inserting copy for vccz corruption bug.


https://reviews.llvm.org/D26261

Files:
  lib/Target/AMDGPU/SIInsertWaits.cpp
  lib/Target/AMDGPU/SIInstrInfo.cpp


Index: lib/Target/AMDGPU/SIInstrInfo.cpp
===================================================================
--- lib/Target/AMDGPU/SIInstrInfo.cpp
+++ lib/Target/AMDGPU/SIInstrInfo.cpp
@@ -1268,6 +1268,7 @@
 
   MachineBasicBlock *CondBB = I->getOperand(0).getMBB();
   Cond.push_back(MachineOperand::CreateImm(Pred));
+  Cond.push_back(I->getOperand(1)); // Save the branch register.
 
   ++I;
 
@@ -1370,30 +1371,42 @@
     = getBranchOpcode(static_cast<BranchPredicate>(Cond[0].getImm()));
 
   if (!FBB) {
-    BuildMI(&MBB, DL, get(Opcode))
+    Cond[1].isUndef();
+    MachineInstr *CondBr =
+      BuildMI(&MBB, DL, get(Opcode))
       .addMBB(TBB);
 
+    // Copy the flags onto the implicit condition register operand.
+    MachineOperand &CondReg = CondBr->getOperand(1);
+    CondReg.setIsUndef(Cond[1].isUndef());
+    CondReg.setIsKill(Cond[1].isKill());
+
     if (BytesAdded)
       *BytesAdded = 4;
     return 1;
   }
 
   assert(TBB && FBB);
 
-  BuildMI(&MBB, DL, get(Opcode))
+  MachineInstr *CondBr =
+    BuildMI(&MBB, DL, get(Opcode))
     .addMBB(TBB);
   BuildMI(&MBB, DL, get(AMDGPU::S_BRANCH))
     .addMBB(FBB);
 
+  MachineOperand &CondReg = CondBr->getOperand(1);
+  CondReg.setIsUndef(Cond[1].isUndef());
+  CondReg.setIsKill(Cond[1].isKill());
+
   if (BytesAdded)
       *BytesAdded = 8;
 
   return 2;
 }
 
 bool SIInstrInfo::reverseBranchCondition(
   SmallVectorImpl<MachineOperand> &Cond) const {
-  assert(Cond.size() == 1);
+  assert(Cond.size() == 2);
   Cond[0].setImm(-Cond[0].getImm());
   return false;
 }
Index: lib/Target/AMDGPU/SIInsertWaits.cpp
===================================================================
--- lib/Target/AMDGPU/SIInsertWaits.cpp
+++ lib/Target/AMDGPU/SIInsertWaits.cpp
@@ -178,8 +178,10 @@
 
 const Counters SIInsertWaits::ZeroCounts = { { 0, 0, 0 } };
 
-static bool readsVCCZ(unsigned Opcode) {
-  return Opcode == AMDGPU::S_CBRANCH_VCCNZ || Opcode == AMDGPU::S_CBRANCH_VCCZ;
+static bool readsVCCZ(const MachineInstr &MI) {
+  unsigned Opc = MI.getOpcode();
+    return (Opc == AMDGPU::S_CBRANCH_VCCNZ || Opc == AMDGPU::S_CBRANCH_VCCZ) &&
+           !MI.getOperand(1).isUndef();
 }
 
 bool SIInsertWaits::hasOutstandingLGKM() const {
@@ -574,7 +576,7 @@
         }
 
         // Check if we need to apply the bug work-around
-        if (readsVCCZ(I->getOpcode()) && VCCZCorrupt) {
+        if (VCCZCorrupt && readsVCCZ(*I)) {
           DEBUG(dbgs() << "Inserting vccz bug work-around before: " << *I << '\n');
 
           // Wait on everything, not just LGKM.  vccz reads usually come from
@@ -589,7 +591,7 @@
           // vcc and then writing it back to the register.
           BuildMI(MBB, I, I->getDebugLoc(), TII->get(AMDGPU::S_MOV_B64),
                   AMDGPU::VCC)
-                  .addReg(AMDGPU::VCC);
+            .addReg(AMDGPU::VCC);
         }
       }
 


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