[llvm] [AMDGPU] Add IR LiveReg type-based optimization (PR #66838)

[Matt Arsenault via llvm-commits]

================
@@ -102,14 +182,266 @@ bool AMDGPULateCodeGenPrepare::runOnFunction(Function &F) {
   AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
   UA = &getAnalysis<UniformityInfoWrapperPass>().getUniformityInfo();
 
+  // "Optimize" the virtual regs that cross basic block boundaries. In such
+  // cases, vectors of illegal types will be scalarized and widened, with each
+  // scalar living in its own physical register. The optimization converts the
+  // vectors to equivalent vectors of legal type (which are convereted back
+  // before uses in subsequent blocks), to pack the bits into fewer physical
+  // registers (used in CopyToReg/CopyFromReg pairs).
+  LiveRegOptimizer LRO(Mod);
+
   bool Changed = false;
   for (auto &BB : F)
-    for (Instruction &I : llvm::make_early_inc_range(BB))
+    for (Instruction &I : llvm::make_early_inc_range(BB)) {
       Changed |= visit(I);
+      if (!LRO.shouldReplaceUses(I))
+        continue;
+      Changed |= LRO.replaceUses(I);
+    }
 
+  Changed |= LRO.replacePHIs();
   return Changed;
 }
 
+bool LiveRegOptimizer::replaceUses(Instruction &I) {
+  bool MadeChange = false;
+
+  struct ConvertUseInfo {
+    Instruction *Converted;
+    SmallVector<Instruction *, 4> Users;
+  };
+  DenseMap<BasicBlock *, ConvertUseInfo> InsertedConversionMap;
+
+  ConversionCandidateInfo FromCCI(
+      &I, I.getParent(),
+      static_cast<BasicBlock::iterator>(std::next(I.getIterator())));
+  FromCCI.setNewType(getCompatibleType(FromCCI.getLiveRegDef()));
+  for (auto IUser = I.user_begin(); IUser != I.user_end(); IUser++) {
+
+    if (Instruction *UserInst = dyn_cast<Instruction>(*IUser)) {
+      if (UserInst->getParent() != I.getParent() || isa<PHINode>(UserInst)) {
+        LLVM_DEBUG(dbgs() << *UserInst << "\n\tUses "
+                          << *FromCCI.getOriginalType()
+                          << " from previous block. Needs conversion\n");
+        convertToOptType(FromCCI);
+        if (!FromCCI.hasConverted())
+          continue;
+        // If it is a PHI node, just create and collect the new operand. We can
+        // only replace the PHI node once we have converted all the operands
+        if (auto PhiInst = dyn_cast<PHINode>(UserInst)) {
+          for (unsigned Idx = 0; Idx < PhiInst->getNumIncomingValues(); Idx++) {
+            Value *IncVal = PhiInst->getIncomingValue(Idx);
+            if (&I == dyn_cast<Instruction>(IncVal)) {
+              BasicBlock *IncBlock = PhiInst->getIncomingBlock(Idx);
+              auto PHIOps = find_if(
+                  PHIUpdater,
+                  [&UserInst](
+                      std::pair<Instruction *,
+                                SmallVector<
+                                    std::pair<Instruction *, BasicBlock *>, 4>>
+                          &Entry) { return Entry.first == UserInst; });
+
+              if (PHIOps == PHIUpdater.end())
+                PHIUpdater.push_back(
+                    {UserInst, {{FromCCI.getConverted(), IncBlock}}});
+              else
+                PHIOps->second.push_back({FromCCI.getConverted(), IncBlock});
+
+              break;
+            }
+          }
+          continue;
+        }
+
+        // Do not create multiple conversion sequences if there are multiple
+        // uses in the same block
+        if (InsertedConversionMap.contains(UserInst->getParent())) {
+          InsertedConversionMap[UserInst->getParent()].Users.push_back(
+              UserInst);
+          LLVM_DEBUG(dbgs() << "\tUser already has access to converted def\n");
+          continue;
+        }
+
+        ConversionCandidateInfo ToCCI(
+            FromCCI.getConverted(), I.getType(), UserInst->getParent(),
+            static_cast<BasicBlock::iterator>(
+                UserInst->getParent()->getFirstNonPHIIt()));
+        convertFromOptType(ToCCI);
+        assert(ToCCI.hasConverted());
+        InsertedConversionMap[UserInst->getParent()] = {ToCCI.getConverted(),
+                                                        {UserInst}};
+      }
+    }
+  }
+
+  // Replace uses of with in a separate loop that is not dependent upon the
+  // state of the uses
+  for (auto &Entry : InsertedConversionMap) {
+    for (auto &UserInst : Entry.second.Users) {
+      LLVM_DEBUG(dbgs() << *UserInst
+                        << "\n\tNow uses: " << *Entry.second.Converted << "\n");
+      UserInst->replaceUsesOfWith(&I, Entry.second.Converted);
+      MadeChange = true;
+    }
+  }
+  return MadeChange;
+}
+
+bool LiveRegOptimizer::replacePHIs() {
+  bool MadeChange = false;
+  for (auto Ele : PHIUpdater) {
+    auto ThePHINode = cast<PHINode>(Ele.first);
+    auto NewPHINodeOps = Ele.second;
+    LLVM_DEBUG(dbgs() << "Attempting to replace: " << *ThePHINode << "\n");
+    // If we have conveted all the required operands, then do the replacement
+    if (ThePHINode->getNumIncomingValues() == NewPHINodeOps.size()) {
+      IRBuilder<> Builder(Ele.first);
+      auto NPHI = Builder.CreatePHI(NewPHINodeOps[0].first->getType(),
+                                    NewPHINodeOps.size());
+      for (auto IncVals : NewPHINodeOps) {
+        NPHI->addIncoming(IncVals.first, IncVals.second);
+        LLVM_DEBUG(dbgs() << "  Using: " << *IncVals.first
+                          << "  For: " << IncVals.second->getName() << "\n");
+      }
+      LLVM_DEBUG(dbgs() << "Sucessfully replaced with " << *NPHI << "\n");
+      ConversionCandidateInfo ToCCI(
+          NPHI, ThePHINode->getType(), ThePHINode->getParent(),
+          static_cast<BasicBlock::iterator>(
+              ThePHINode->getParent()->getFirstNonPHIIt()));
+      convertFromOptType(ToCCI);
+      assert(ToCCI.hasConverted());
+      Ele.first->replaceAllUsesWith(ToCCI.getConverted());
+      // The old PHI is no longer used
+      ThePHINode->eraseFromParent();
+      MadeChange = true;
+    }
+  }
+  return MadeChange;
+}
+
+Type *LiveRegOptimizer::getCompatibleType(Instruction *InstToConvert) {
+  Type *OriginalType = InstToConvert->getType();
+  assert(OriginalType->getScalarSizeInBits() <=
+         ConvertToScalar->getScalarSizeInBits());
+  VectorType *VTy = dyn_cast<VectorType>(OriginalType);
+  if (!VTy)
+    return ConvertToScalar;
+
+  unsigned OriginalSize = VTy->getPrimitiveSizeInBits();
+  unsigned ConvertScalarSize = ConvertToScalar->getScalarSizeInBits();
+  unsigned ConvertEltCount =
+      (OriginalSize + ConvertScalarSize - 1) / ConvertScalarSize;
+
+  if (OriginalSize <= ConvertScalarSize)
+    return IntegerType::get(Mod->getContext(), ConvertScalarSize);
+
+  return VectorType::get(Type::getIntNTy(Mod->getContext(), ConvertScalarSize),
+                         llvm::ElementCount::getFixed(ConvertEltCount));
+}
+
+void LiveRegOptimizer::convertToOptType(ConversionCandidateInfo &LR) {
+  if (LR.hasConverted()) {
+    LLVM_DEBUG(dbgs() << "\tAlready has converted def\n");
+    return;
+  }
+
+  VectorType *VTy = cast<VectorType>(LR.getOriginalType());
+  Type *NewTy = LR.getNewType();
+
+  unsigned OriginalSize = VTy->getPrimitiveSizeInBits();
+  unsigned NewSize = NewTy->getPrimitiveSizeInBits();
+
+  auto &Builder = LR.getConvertBuilder();
+  Value *V = static_cast<Value *>(LR.getLiveRegDef());
+  // If there is a bitsize match, we can fit the old vector into a new vector of
+  // desired type
+  if (OriginalSize == NewSize) {
+    LR.setConverted(dyn_cast<Instruction>(Builder.CreateBitCast(V, NewTy)));
+    LLVM_DEBUG(dbgs() << "\tConverted def to " << *LR.getConverted()->getType()
+                      << "\n");
+    return;
+  }
+
+  // If there is a bitsize mismatch, we must use a wider vector
+  assert(NewSize > OriginalSize);
+  ElementCount ExpandedVecElementCount =
+      llvm::ElementCount::getFixed(NewSize / VTy->getScalarSizeInBits());
----------------
arsenm wrote:

Don't need llvm:: 

https://github.com/llvm/llvm-project/pull/66838