AMDGPU/PromoteAlloca: Simplify how deferred loads work

[nhaehnle]

The second pass of promotion to vector can be quite simple. Reflect that
simplicity in the code for better maintainability.

---

Stack:

• [5/5] AMDGPU/PromoteAlloca: Refactor into analysis / commit phases #170512
• [4/5] AMDGPU/PromoteAlloca: Always use i32 for indexing #170511
• [3/5] AMDGPU/PromoteAlloca: Simplify how deferred loads work #170510 ⬅
• [2/5] AMDGPU/PromoteAlloca: Extract getVectorTypeForAlloca helper #170509
• [1/5] AMDGPU: Generalize and normalize some tests to avoid future churn #170508

⚠️ Part of a stack created by spr. Merging this PR using the GitHub UI may have unexpected results.

[llvmbot]

@llvm/pr-subscribers-backend-amdgpu

Author: Nicolai Hähnle (nhaehnle)

Changes

The second pass of promotion to vector can be quite simple. Reflect that
simplicity in the code for better maintainability.

---

Stack:

• [5/5] #170512
• [4/5] #170511
• [3/5] #170510 ⬅
• [2/5] #170509
• [1/5] #170508

⚠️ Part of a stack created by spr. Merging this PR using the GitHub UI may have unexpected results.

---

Full diff: https://github.com/llvm/llvm-project/pull/170510.diff

1 Files Affected:

• (modified) llvm/lib/Target/AMDGPU/AMDGPUPromoteAlloca.cpp (+34-46)

diff --git a/llvm/lib/Target/AMDGPU/AMDGPUPromoteAlloca.cpp b/llvm/lib/Target/AMDGPU/AMDGPUPromoteAlloca.cpp
index 77db14513254f..73ec607014d31 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPUPromoteAlloca.cpp
+++ b/llvm/lib/Target/AMDGPU/AMDGPUPromoteAlloca.cpp
@@ -502,27 +502,14 @@ static Value *promoteAllocaUserToVector(
     Instruction *Inst, const DataLayout &DL, FixedVectorType *VectorTy,
     unsigned VecStoreSize, unsigned ElementSize,
     DenseMap<MemTransferInst *, MemTransferInfo> &TransferInfo,
-    std::map<GetElementPtrInst *, WeakTrackingVH> &GEPVectorIdx, Value *CurVal,
-    SmallVectorImpl<LoadInst *> &DeferredLoads) {
+    std::map<GetElementPtrInst *, WeakTrackingVH> &GEPVectorIdx,
+    function_ref<Value *()> GetCurVal) {
   // Note: we use InstSimplifyFolder because it can leverage the DataLayout
   // to do more folding, especially in the case of vector splats.
   IRBuilder<InstSimplifyFolder> Builder(Inst->getContext(),
                                         InstSimplifyFolder(DL));
   Builder.SetInsertPoint(Inst);
 
-  const auto GetOrLoadCurrentVectorValue = [&]() -> Value * {
-    if (CurVal)
-      return CurVal;
-
-    // If the current value is not known, insert a dummy load and lower it on
-    // the second pass.
-    LoadInst *Dummy =
-        Builder.CreateLoad(VectorTy, PoisonValue::get(Builder.getPtrTy()),
-                           "promotealloca.dummyload");
-    DeferredLoads.push_back(Dummy);
-    return Dummy;
-  };
-
   const auto CreateTempPtrIntCast = [&Builder, DL](Value *Val,
                                                    Type *PtrTy) -> Value * {
     assert(DL.getTypeStoreSize(Val->getType()) == DL.getTypeStoreSize(PtrTy));
@@ -542,12 +529,7 @@ static Value *promoteAllocaUserToVector(
 
   switch (Inst->getOpcode()) {
   case Instruction::Load: {
-    // Loads can only be lowered if the value is known.
-    if (!CurVal) {
-      DeferredLoads.push_back(cast<LoadInst>(Inst));
-      return nullptr;
-    }
-
+    Value *CurVal = GetCurVal();
     Value *Index = calculateVectorIndex(
         cast<LoadInst>(Inst)->getPointerOperand(), GEPVectorIdx);
 
@@ -637,7 +619,7 @@ static Value *promoteAllocaUserToVector(
 
       Val = Builder.CreateBitOrPointerCast(Val, SubVecTy);
 
-      Value *CurVec = GetOrLoadCurrentVectorValue();
+      Value *CurVec = GetCurVal();
       for (unsigned K = 0, NumElts = std::min(NumWrittenElts, NumVecElts);
            K < NumElts; ++K) {
         Value *CurIdx =
@@ -650,8 +632,7 @@ static Value *promoteAllocaUserToVector(
 
     if (Val->getType() != VecEltTy)
       Val = Builder.CreateBitOrPointerCast(Val, VecEltTy);
-    return Builder.CreateInsertElement(GetOrLoadCurrentVectorValue(), Val,
-                                       Index);
+    return Builder.CreateInsertElement(GetCurVal(), Val, Index);
   }
   case Instruction::Call: {
     if (auto *MTI = dyn_cast<MemTransferInst>(Inst)) {
@@ -673,7 +654,7 @@ static Value *promoteAllocaUserToVector(
         }
       }
 
-      return Builder.CreateShuffleVector(GetOrLoadCurrentVectorValue(), Mask);
+      return Builder.CreateShuffleVector(GetCurVal(), Mask);
     }
 
     if (auto *MSI = dyn_cast<MemSetInst>(Inst)) {
@@ -1038,37 +1019,44 @@ bool AMDGPUPromoteAllocaImpl::tryPromoteAllocaToVector(AllocaInst &Alloca) {
 
   Updater.AddAvailableValue(EntryBB, AllocaInitValue);
 
-  // First handle the initial worklist.
-  SmallVector<LoadInst *, 4> DeferredLoads;
+  // First handle the initial worklist, in basic block order.
+  //
+  // Insert a placeholder whenever we need the vector value at the top of a
+  // basic block.
+  SmallVector<Instruction *> Placeholders;
   forEachWorkListItem(WorkList, [&](Instruction *I) {
     BasicBlock *BB = I->getParent();
-    // On the first pass, we only take values that are trivially known, i.e.
-    // where AddAvailableValue was already called in this block.
-    Value *Result = promoteAllocaUserToVector(
-        I, *DL, VectorTy, VecStoreSize, ElementSize, TransferInfo, GEPVectorIdx,
-        Updater.FindValueForBlock(BB), DeferredLoads);
+    auto GetCurVal = [&]() -> Value * {
+      if (Value *CurVal = Updater.FindValueForBlock(BB))
+        return CurVal;
+
+      // If the current value in the basic block is not yet known, insert a
+      // placeholder that we will replace later.
+      IRBuilder<> Builder(I);
+      auto *Placeholder = cast<Instruction>(Builder.CreateFreeze(
+          PoisonValue::get(VectorTy), "promotealloca.placeholder"));
+      Placeholders.push_back(Placeholder);
+      Updater.AddAvailableValue(BB, Placeholder);
+      return Placeholder;
+    };
+
+    Value *Result =
+        promoteAllocaUserToVector(I, *DL, VectorTy, VecStoreSize, ElementSize,
+                                  TransferInfo, GEPVectorIdx, GetCurVal);
     if (Result)
       Updater.AddAvailableValue(BB, Result);
   });
 
-  // Then handle deferred loads.
-  forEachWorkListItem(DeferredLoads, [&](Instruction *I) {
-    SmallVector<LoadInst *, 0> NewDLs;
-    BasicBlock *BB = I->getParent();
-    // On the second pass, we use GetValueInMiddleOfBlock to guarantee we always
-    // get a value, inserting PHIs as needed.
-    Value *Result = promoteAllocaUserToVector(
-        I, *DL, VectorTy, VecStoreSize, ElementSize, TransferInfo, GEPVectorIdx,
-        Updater.GetValueInMiddleOfBlock(I->getParent()), NewDLs);
-    if (Result)
-      Updater.AddAvailableValue(BB, Result);
-    assert(NewDLs.empty() && "No more deferred loads should be queued!");
-  });
+  // Now fixup the placeholders.
+  for (Instruction *Placeholder : Placeholders) {
+    Placeholder->replaceAllUsesWith(
+        Updater.GetValueInMiddleOfBlock(Placeholder->getParent()));
+    Placeholder->eraseFromParent();
+  }
 
   // Delete all instructions. On the first pass, new dummy loads may have been
   // added so we need to collect them too.
   DenseSet<Instruction *> InstsToDelete(WorkList.begin(), WorkList.end());
-  InstsToDelete.insert_range(DeferredLoads);
   for (Instruction *I : InstsToDelete) {
     assert(I->use_empty());
     I->eraseFromParent();

[zGoldthorpe]

LGTM