[llvm] [DirectX] Support ConstExpr GEPs (PR #150082)

[Deric C. via llvm-commits]

================
@@ -301,41 +301,53 @@ bool DataScalarizerVisitor::visitExtractElementInst(ExtractElementInst &EEI) {
 }
 
 bool DataScalarizerVisitor::visitGetElementPtrInst(GetElementPtrInst &GEPI) {
-  Value *PtrOperand = GEPI.getPointerOperand();
-  Type *OrigGEPType = GEPI.getSourceElementType();
-  Type *NewGEPType = OrigGEPType;
+  GEPOperator *GOp = cast<GEPOperator>(&GEPI);
+  Value *PtrOperand = GOp->getPointerOperand();
+  Type *NewGEPType = GOp->getSourceElementType();
   bool NeedsTransform = false;
 
+  // Unwrap GEP ConstantExprs to find the base operand and element type
+  while (auto *CE = dyn_cast<ConstantExpr>(PtrOperand)) {
+    if (auto *GEPCE = dyn_cast<GEPOperator>(CE)) {
+      GOp = GEPCE;
+      PtrOperand = GEPCE->getPointerOperand();
+      NewGEPType = GEPCE->getSourceElementType();
+    } else
+      break;
+  }
+
   if (GlobalVariable *NewGlobal = lookupReplacementGlobal(PtrOperand)) {
     NewGEPType = NewGlobal->getValueType();
     PtrOperand = NewGlobal;
     NeedsTransform = true;
   } else if (AllocaInst *Alloca = dyn_cast<AllocaInst>(PtrOperand)) {
     Type *AllocatedType = Alloca->getAllocatedType();
-    // Only transform if the allocated type is an array
-    if (AllocatedType != OrigGEPType && isa<ArrayType>(AllocatedType)) {
+    if (isa<ArrayType>(AllocatedType) &&
+        AllocatedType != GOp->getResultElementType()) {
       NewGEPType = AllocatedType;
       NeedsTransform = true;
     }
   }
 
-  // Scalar geps should remain scalars geps. The dxil-flatten-arrays pass will
-  // convert these scalar geps into flattened array geps
-  if (!isa<ArrayType>(OrigGEPType))
-    NewGEPType = OrigGEPType;
-
-  // Note: We bail if this isn't a gep touched via alloca or global
-  // transformations
   if (!NeedsTransform)
     return false;
 
-  IRBuilder<> Builder(&GEPI);
-  SmallVector<Value *, MaxVecSize> Indices(GEPI.indices());
+  // Keep scalar GEPs scalar; dxil-flatten-arrays will do flattening later
+  if (!isa<ArrayType>(GOp->getSourceElementType()))
+    NewGEPType = GOp->getSourceElementType();
 
+  IRBuilder<> Builder(&GEPI);
+  SmallVector<Value *, MaxVecSize> Indices(GOp->indices());
   Value *NewGEP = Builder.CreateGEP(NewGEPType, PtrOperand, Indices,
-                                    GEPI.getName(), GEPI.getNoWrapFlags());
-  GEPI.replaceAllUsesWith(NewGEP);
-  GEPI.eraseFromParent();
+                                    GOp->getName(), GOp->getNoWrapFlags());
+
+  GOp->replaceAllUsesWith(NewGEP);
+
+  if (auto *CE = dyn_cast<ConstantExpr>(GOp))
+    CE->destroyConstant();
+  else if (auto *OldGEPI = dyn_cast<GetElementPtrInst>(GOp))
+    OldGEPI->eraseFromParent(); // This will always be true in visit* context
----------------
Icohedron wrote:

nit: The way I read this comment it sounds like GOp is always a GetElementPtrInst on every visitGetElementPtrInst. Which is incorrect.

This line is executed if and only if the GEP visited by visitGetElementPtrInst does not have a ConstantExpr GEP pointer operand. This is because in this case GOp is in fact the visited GEP instruction (GEPI) and not a ConstantExpr GEP operand of GEPI.

https://github.com/llvm/llvm-project/pull/150082