[AMDGPU] Fix DPP combiner check for exec modification

Summary:
r363675 changed the exec modification helper function, now called
execMayBeModifiedBeforeUse, so that if no UseMI is specified it checks
all instructions in the basic block, even beyond the last use. That
meant that the DPP combiner no longer worked in any basic block that
ended with a control flow instruction, and in particular it didn't work
on code sequences generated by the atomic optimizer.

Fix it by reinstating the old behaviour but in a new helper function
execMayBeModifiedBeforeAnyUse, and limiting the number of instructions
scanned.

Reviewers: arsenm, vpykhtin

Subscribers: kzhuravl, nemanjai, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, hiraditya, kbarton, MaskRay, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64393

llvm-svn: 365910

4 files changed, 63 insertions, 29 deletions

diff --git a/llvm/lib/Target/AMDGPU/GCNDPPCombine.cpp b/llvm/lib/Target/AMDGPU/GCNDPPCombine.cpp
index 7348b5b56c8..e1845e2e8e8 100644
--- a/llvm/lib/Target/AMDGPU/GCNDPPCombine.cpp
+++ b/llvm/lib/Target/AMDGPU/GCNDPPCombine.cpp
@@ -344,7 +344,7 @@ bool GCNDPPCombine::combineDPPMov(MachineInstr &MovMI) const {
   auto *DstOpnd = TII->getNamedOperand(MovMI, AMDGPU::OpName::vdst);
   assert(DstOpnd && DstOpnd->isReg());
   auto DPPMovReg = DstOpnd->getReg();
-  if (execMayBeModifiedBeforeUse(*MRI, DPPMovReg, MovMI)) {
+  if (execMayBeModifiedBeforeAnyUse(*MRI, DPPMovReg, MovMI)) {
     LLVM_DEBUG(dbgs() << "  failed: EXEC mask should remain the same"
                          " for all uses\n");
     return false;
diff --git a/llvm/lib/Target/AMDGPU/SIFoldOperands.cpp b/llvm/lib/Target/AMDGPU/SIFoldOperands.cpp
index bcc3478c67b..74d77d32801 100644
--- a/llvm/lib/Target/AMDGPU/SIFoldOperands.cpp
+++ b/llvm/lib/Target/AMDGPU/SIFoldOperands.cpp
@@ -650,7 +650,7 @@ void SIFoldOperands::foldOperand(
         if (execMayBeModifiedBeforeUse(*MRI,
                                        UseMI->getOperand(UseOpIdx).getReg(),
                                        *OpToFold.getParent(),
-                                       UseMI))
+                                       *UseMI))
           return;
 
         UseMI->setDesc(TII->get(AMDGPU::S_MOV_B32));
@@ -669,7 +669,7 @@ void SIFoldOperands::foldOperand(
         if (execMayBeModifiedBeforeUse(*MRI,
                                        UseMI->getOperand(UseOpIdx).getReg(),
                                        *OpToFold.getParent(),
-                                       UseMI))
+                                       *UseMI))
           return;
 
         // %vgpr = COPY %sgpr0
diff --git a/llvm/lib/Target/AMDGPU/SIInstrInfo.cpp b/llvm/lib/Target/AMDGPU/SIInstrInfo.cpp
index d855f3f0e42..34741850f82 100644
--- a/llvm/lib/Target/AMDGPU/SIInstrInfo.cpp
+++ b/llvm/lib/Target/AMDGPU/SIInstrInfo.cpp
@@ -6269,47 +6269,75 @@ MachineInstr *llvm::getVRegSubRegDef(const TargetInstrInfo::RegSubRegPair &P,
 }
 
 bool llvm::execMayBeModifiedBeforeUse(const MachineRegisterInfo &MRI,
-                                      unsigned VReg,
+                                      Register VReg,
                                       const MachineInstr &DefMI,
-                                      const MachineInstr *UseMI) {
+                                      const MachineInstr &UseMI) {
   assert(MRI.isSSA() && "Must be run on SSA");
 
   auto *TRI = MRI.getTargetRegisterInfo();
   auto *DefBB = DefMI.getParent();
 
-  if (UseMI) {
+  // Don't bother searching between blocks, although it is possible this block
+  // doesn't modify exec.
+  if (UseMI.getParent() != DefBB)
+    return true;
+
+  const int MaxInstScan = 20;
+  int NumInst = 0;
+
+  // Stop scan at the use.
+  auto E = UseMI.getIterator();
+  for (auto I = std::next(DefMI.getIterator()); I != E; ++I) {
+    if (I->isDebugInstr())
+      continue;
+
+    if (++NumInst > MaxInstScan)
+      return true;
+
+    if (I->modifiesRegister(AMDGPU::EXEC, TRI))
+      return true;
+  }
+
+  return false;
+}
+
+bool llvm::execMayBeModifiedBeforeAnyUse(const MachineRegisterInfo &MRI,
+                                         Register VReg,
+                                         const MachineInstr &DefMI) {
+  assert(MRI.isSSA() && "Must be run on SSA");
+
+  auto *TRI = MRI.getTargetRegisterInfo();
+  auto *DefBB = DefMI.getParent();
+
+  const int MaxUseInstScan = 10;
+  int NumUseInst = 0;
+
+  for (auto &UseInst : MRI.use_nodbg_instructions(VReg)) {
     // Don't bother searching between blocks, although it is possible this block
     // doesn't modify exec.
-    if (UseMI->getParent() != DefBB)
+    if (UseInst.getParent() != DefBB)
       return true;
-  } else {
-    int NumUse = 0;
-    const int MaxUseScan = 10;
-
-    for (auto &UseInst : MRI.use_nodbg_instructions(VReg)) {
-      if (UseInst.getParent() != DefBB)
-        return true;
 
-      if (NumUse++ > MaxUseScan)
-        return true;
-    }
+    if (++NumUseInst > MaxUseInstScan)
+      return true;
   }
 
   const int MaxInstScan = 20;
-  int NumScan = 0;
+  int NumInst = 0;
 
-  // Stop scan at the use if known.
-  auto E = UseMI ? UseMI->getIterator() : DefBB->end();
-  for (auto I = std::next(DefMI.getIterator()); I != E; ++I) {
+  // Stop scan when we have seen all the uses.
+  for (auto I = std::next(DefMI.getIterator()); ; ++I) {
     if (I->isDebugInstr())
       continue;
 
-    if (NumScan++ > MaxInstScan)
+    if (++NumInst > MaxInstScan)
       return true;
 
+    if (I->readsRegister(VReg))
+      if (--NumUseInst == 0)
+        return false;
+
     if (I->modifiesRegister(AMDGPU::EXEC, TRI))
       return true;
   }
-
-  return false;
 }
diff --git a/llvm/lib/Target/AMDGPU/SIInstrInfo.h b/llvm/lib/Target/AMDGPU/SIInstrInfo.h
index b8275b62272..1f3c659f9d9 100644
--- a/llvm/lib/Target/AMDGPU/SIInstrInfo.h
+++ b/llvm/lib/Target/AMDGPU/SIInstrInfo.h
@@ -1016,13 +1016,19 @@ MachineInstr *getVRegSubRegDef(const TargetInstrInfo::RegSubRegPair &P,
                                MachineRegisterInfo &MRI);
 
 /// \brief Return false if EXEC is not changed between the def of \p VReg at \p
-/// DefMI and uses. If \p UseMI is not specified, this checks all uses of \p
-/// VReg. Should be run on SSA. Currently does not attempt to track between
-/// blocks.
+/// DefMI and the use at \p UseMI. Should be run on SSA. Currently does not
+/// attempt to track between blocks.
 bool execMayBeModifiedBeforeUse(const MachineRegisterInfo &MRI,
-                                unsigned VReg,
+                                Register VReg,
                                 const MachineInstr &DefMI,
-                                const MachineInstr *UseMI = nullptr);
+                                const MachineInstr &UseMI);
+
+/// \brief Return false if EXEC is not changed between the def of \p VReg at \p
+/// DefMI and all its uses. Should be run on SSA. Currently does not attempt to
+/// track between blocks.
+bool execMayBeModifiedBeforeAnyUse(const MachineRegisterInfo &MRI,
+                                   Register VReg,
+                                   const MachineInstr &DefMI);
 
 namespace AMDGPU {