[flang][NFC] Engineering option for optimizing allocatable assignments.

The options is added just for quick experiments with different apps.
If any headroom is found, we will implement proper multiversioning
of hlfir.assign with allocatable LHS.

Author: vzakhari

flang/lib/Optimizer/HLFIR/Transforms/OptimizedBufferization.cpp

@@ -42,6 +42,18 @@ namespace hlfir {
 
 #define DEBUG_TYPE "opt-bufferization"
 
+// An engineering option to allow treating hlfir.assign with realloc
+// attribute as never requiring (re)allocation of the LHS.
+// Setting it to true may result in incorrect code, so it is present
+// just for quicker benchmarking of apps that may benefit
+// from multiversioning such hlfir.assign operations under
+// the dynamic checks of the type/shape/allocation status.
+static llvm::cl::opt<bool> assumeNoLhsReallocation(
+    "flang-assume-no-lhs-reallocation",
+    llvm::cl::desc("Assume that hlfir.assign never (re)allocates the LHS, i.e. "
+                   "that the LHS and RHS are always conformant"),
+    llvm::cl::init(false));
+
 namespace {
 
 /// This transformation should match in place modification of arrays.
@@ -462,16 +474,20 @@ ElementalAssignBufferization::findMatch(hlfir::ElementalOp elemental) {
   // the incoming expression
   match.array = match.assign.getLhs();
   mlir::Type arrayType = mlir::dyn_cast<fir::SequenceType>(
-      fir::unwrapPassByRefType(match.array.getType()));
-  if (!arrayType)
+      hlfir::getFortranElementOrSequenceType(match.array.getType()));
+  if (!arrayType) {
+    LLVM_DEBUG(llvm::dbgs() << "AssignOp's result is not an array\n");
     return std::nullopt;
+  }
 
   // require that the array elements are trivial
   // TODO: this is just to make the pass easier to think about. Not an inherent
   // limitation
   mlir::Type eleTy = hlfir::getFortranElementType(arrayType);
-  if (!fir::isa_trivial(eleTy))
+  if (!fir::isa_trivial(eleTy)) {
+    LLVM_DEBUG(llvm::dbgs() << "AssignOp's data type is not trivial\n");
     return std::nullopt;
+  }
 
   // The array must have the same shape as the elemental.
   //
@@ -485,8 +501,10 @@ ElementalAssignBufferization::findMatch(hlfir::ElementalOp elemental) {
   // there is no reallocation of the lhs due to the assignment.
   // We can probably try generating multiple versions of the code
   // with checking for the shape match, length parameters match, etc.
-  if (match.assign.getRealloc())
+  if (match.assign.isAllocatableAssignment() && !assumeNoLhsReallocation) {
+    LLVM_DEBUG(llvm::dbgs() << "AssignOp may involve (re)allocation of LHS\n");
     return std::nullopt;
+  }
 
   // the transformation wants to apply the elemental in a do-loop at the
   // hlfir.assign, check there are no effects which make this unsafe
@@ -606,6 +624,8 @@ llvm::LogicalResult ElementalAssignBufferization::matchAndRewrite(
 
   // create the loop at the assignment
   builder.setInsertionPoint(match->assign);
+  hlfir::Entity arrayObj = hlfir::derefPointersAndAllocatables(
+      loc, builder, hlfir::Entity{match->array});
 
   // Generate a loop nest looping around the hlfir.elemental shape and clone
   // hlfir.elemental region inside the inner loop
@@ -619,8 +639,8 @@ llvm::LogicalResult ElementalAssignBufferization::matchAndRewrite(
   rewriter.eraseOp(yield);
 
   // Assign the element value to the array element for this iteration.
-  auto arrayElement = hlfir::getElementAt(
-      loc, builder, hlfir::Entity{match->array}, loopNest.oneBasedIndices);
+  auto arrayElement =
+      hlfir::getElementAt(loc, builder, arrayObj, loopNest.oneBasedIndices);
   builder.create<hlfir::AssignOp>(
       loc, elementValue, arrayElement, /*realloc=*/false,
       /*keep_lhs_length_if_realloc=*/false, match->assign.getTemporaryLhs());