[clang] [CIR] Upstream ArraySubscriptExpr for fixed size array (PR #134536)

[Amr Hesham via cfe-commits]

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@@ -390,6 +391,148 @@ LValue CIRGenFunction::emitUnaryOpLValue(const UnaryOperator *e) {
   llvm_unreachable("Unknown unary operator kind!");
 }
 
+/// If the specified expr is a simple decay from an array to pointer,
+/// return the array subexpression.
+/// FIXME: this could be abstracted into a common AST helper.
+static const Expr *getSimpleArrayDecayOperand(const Expr *e) {
+  // If this isn't just an array->pointer decay, bail out.
+  const auto *castExpr = dyn_cast<CastExpr>(e);
+  if (!castExpr || castExpr->getCastKind() != CK_ArrayToPointerDecay)
+    return nullptr;
+
+  // If this is a decay from variable width array, bail out.
+  const Expr *subExpr = castExpr->getSubExpr();
+  if (subExpr->getType()->isVariableArrayType())
+    return nullptr;
+
+  return subExpr;
+}
+
+static mlir::IntegerAttr getConstantIndexOrNull(mlir::Value idx) {
+  // TODO(cir): should we consider using MLIRs IndexType instead of IntegerAttr?
+  if (auto constantOp = dyn_cast<cir::ConstantOp>(idx.getDefiningOp()))
+    return mlir::dyn_cast<mlir::IntegerAttr>(constantOp.getValue());
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AmrDeveloper wrote:

I updated the function signature and cast, I will understand this part more in the incubator because that means it was only depend on that condition `(!CGF.IsInPreservedAIRegion && !isPreserveAIArrayBase(CGF, Base))` not the index itself

https://github.com/llvm/llvm-project/pull/134536