[llvm] [DirectX] Simplify and correct the flattening of GEPs in DXILFlattenArrays (PR #146173)

[Finn Plummer via llvm-commits]

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@@ -225,131 +212,145 @@ bool DXILFlattenArraysVisitor::visitAllocaInst(AllocaInst &AI) {
   return true;
 }
 
-void DXILFlattenArraysVisitor::collectIndicesAndDimsFromGEP(
-    GetElementPtrInst &GEP, SmallVectorImpl<Value *> &Indices,
-    SmallVectorImpl<uint64_t> &Dims, bool &AllIndicesAreConstInt) {
-
-  Type *CurrentType = GEP.getSourceElementType();
+bool DXILFlattenArraysVisitor::visitGetElementPtrInst(GetElementPtrInst &GEP) {
+  // Do not visit GEPs more than once
+  if (GEPChainInfoMap.contains(cast<GEPOperator>(&GEP)))
+    return false;
 
-  // Note index 0 is the ptr index.
-  for (Value *Index : llvm::drop_begin(GEP.indices(), 1)) {
-    Indices.push_back(Index);
-    AllIndicesAreConstInt &= isa<ConstantInt>(Index);
+  Value *PtrOperand = GEP.getPointerOperand();
 
-    if (auto *ArrayTy = dyn_cast<ArrayType>(CurrentType)) {
-      Dims.push_back(ArrayTy->getNumElements());
-      CurrentType = ArrayTy->getElementType();
-    } else {
-      assert(false && "Expected array type in GEP chain");
-    }
+  // Replace a GEP ConstantExpr pointer operand with a GEP instruction so that
+  // it can be visited
+  if (auto *PtrOpGEPCE = dyn_cast<ConstantExpr>(PtrOperand);
+      PtrOpGEPCE && PtrOpGEPCE->getOpcode() == Instruction::GetElementPtr) {
+    GetElementPtrInst *OldGEPI =
+        cast<GetElementPtrInst>(PtrOpGEPCE->getAsInstruction());
+    OldGEPI->insertBefore(GEP.getIterator());
+
+    IRBuilder<> Builder(&GEP);
+    SmallVector<Value *> Indices(GEP.indices());
+    Value *NewGEP =
+        Builder.CreateGEP(GEP.getSourceElementType(), OldGEPI, Indices,
+                          GEP.getName(), GEP.getNoWrapFlags());
+    assert(isa<GetElementPtrInst>(NewGEP) &&
+           "Expected newly-created GEP to be an instruction");
+    GetElementPtrInst *NewGEPI = cast<GetElementPtrInst>(NewGEP);
+
+    GEP.replaceAllUsesWith(NewGEPI);
+    GEP.eraseFromParent();
+    visitGetElementPtrInst(*OldGEPI);
+    visitGetElementPtrInst(*NewGEPI);
+    return true;
   }
-}
 
-void DXILFlattenArraysVisitor::recursivelyCollectGEPs(
-    GetElementPtrInst &CurrGEP, ArrayType *FlattenedArrayType,
-    Value *PtrOperand, unsigned &GEPChainUseCount, SmallVector<Value *> Indices,
-    SmallVector<uint64_t> Dims, bool AllIndicesAreConstInt) {
-  // Check if this GEP is already in the map to avoid circular references
-  if (GEPChainMap.count(&CurrGEP) > 0)
-    return;
-
-  // Collect indices and dimensions from the current GEP
-  collectIndicesAndDimsFromGEP(CurrGEP, Indices, Dims, AllIndicesAreConstInt);
-  bool IsMultiDimArr = isMultiDimensionalArray(CurrGEP.getSourceElementType());
-  if (!IsMultiDimArr) {
-    assert(GEPChainUseCount < FlattenedArrayType->getNumElements());
-    GEPChainMap.insert(
-        {&CurrGEP,
-         {std::move(FlattenedArrayType), PtrOperand, std::move(Indices),
-          std::move(Dims), AllIndicesAreConstInt}});
-    return;
-  }
-  bool GepUses = false;
-  for (auto *User : CurrGEP.users()) {
-    if (GetElementPtrInst *NestedGEP = dyn_cast<GetElementPtrInst>(User)) {
-      recursivelyCollectGEPs(*NestedGEP, FlattenedArrayType, PtrOperand,
-                             ++GEPChainUseCount, Indices, Dims,
-                             AllIndicesAreConstInt);
-      GepUses = true;
+  // Construct GEPInfo for this GEP
+  GEPInfo Info;
+
+  // Obtain the variable and constant byte offsets computed by this GEP
+  const DataLayout &DL = GEP.getDataLayout();
+  unsigned BitWidth = DL.getIndexTypeSizeInBits(GEP.getType());
+  Info.ConstantOffset = {BitWidth, 0};
+  [[maybe_unused]] bool Success = GEP.collectOffset(
----------------
inbelic wrote:

Does `collectOffset` have a side-effect that we want? Or is this just for the assert?

nit: if it is just for the assert we could probably just but the expression directly there

https://github.com/llvm/llvm-project/pull/146173