[flang][hlfir] deref pointers before lowering assignment to hlfir.assign

There is little point not to dereference pointers LHS and RHS before
before emitting an hlfir.assign when lowering an assignment.
This pushes complexity and descriptor read side effects that are better
expressed in a load before the assignment.

Differential Revision: https://reviews.llvm.org/D143372

Author: jeanPerier

flang/lib/Lower/Bridge.cpp

@@ -2731,12 +2731,15 @@ class FirConverter : public Fortran::lower::AbstractConverter {
                 Fortran::lower::StatementContext stmtCtx;
                 if (Fortran::lower::isWholeAllocatable(assign.lhs))
                   TODO(loc, "HLFIR assignment to whole allocatable");
-                hlfir::EntityWithAttributes rhs =
-                    Fortran::lower::convertExprToHLFIR(loc, *this, assign.rhs,
-                                                       localSymbols, stmtCtx);
-                hlfir::EntityWithAttributes lhs =
-                    Fortran::lower::convertExprToHLFIR(loc, *this, assign.lhs,
-                                                       localSymbols, stmtCtx);
+                hlfir::Entity rhs = Fortran::lower::convertExprToHLFIR(
+                    loc, *this, assign.rhs, localSymbols, stmtCtx);
+                // Dereference pointers and allocatables RHS: the target is
+                // being assigned from.
+                rhs = hlfir::derefPointersAndAllocatables(loc, builder, rhs);
+                hlfir::Entity lhs = Fortran::lower::convertExprToHLFIR(
+                    loc, *this, assign.lhs, localSymbols, stmtCtx);
+                // Dereference pointers LHS: the target is being assigned to.
+                lhs = hlfir::derefPointersAndAllocatables(loc, builder, lhs);
                 builder.create<hlfir::AssignOp>(loc, rhs, lhs);
               },
               // [2] User defined assignment. If the context is a scalar

flang/lib/Optimizer/Builder/HLFIRTools.cpp

@@ -570,9 +570,9 @@ hlfir::Entity hlfir::derefPointersAndAllocatables(mlir::Location loc,
   if (entity.isMutableBox()) {
     hlfir::Entity boxLoad{builder.create<fir::LoadOp>(loc, entity)};
     if (entity.isScalar()) {
-      mlir::Type elementType = boxLoad.getFortranElementType();
-      if (fir::isa_trivial(elementType))
+      if (!entity.isPolymorphic() && !entity.hasLengthParameters())
         return hlfir::Entity{builder.create<fir::BoxAddrOp>(loc, boxLoad)};
+      mlir::Type elementType = boxLoad.getFortranElementType();
       if (auto charType = elementType.dyn_cast<fir::CharacterType>()) {
         mlir::Value base = builder.create<fir::BoxAddrOp>(loc, boxLoad);
         if (charType.hasConstantLen())
@@ -585,7 +585,10 @@ hlfir::Entity hlfir::derefPointersAndAllocatables(mlir::Location loc,
                 .getResult()};
       }
     }
-    // Keep the entity boxed for now.
+    // Otherwise, the entity is either an array, a polymorphic entity, or a
+    // derived type with length parameters. All these entities require a fir.box
+    // or fir.class to hold bounds, dynamic type or length parameter
+    // information. Keep them boxed.
     return boxLoad;
   }
   return entity;

flang/test/Lower/HLFIR/allocatable-and-pointer-subparts.f90

@@ -26,7 +26,8 @@ subroutine test_pointer_component_followed_by_component_ref(x)
 ! CHECK:  %[[VAL_1:.*]]:2 = hlfir.declare %{{.*}} {{.*}}Ex
 ! CHECK:  %[[VAL_2:.*]] = hlfir.designate %[[VAL_1]]#0{"p"}   {fortran_attrs = #fir.var_attrs<pointer>} : (!fir.ref<!fir.type<_QMmTt2{p:!fir.box<!fir.ptr<!fir.type<_QMmTt1{x:f32}>>>}>>) -> !fir.ref<!fir.box<!fir.ptr<!fir.type<_QMmTt1{x:f32}>>>>
 ! CHECK:  %[[VAL_3:.*]] = fir.load %[[VAL_2]] : !fir.ref<!fir.box<!fir.ptr<!fir.type<_QMmTt1{x:f32}>>>>
-! CHECK:  %[[VAL_4:.*]] = hlfir.designate %[[VAL_3]]{"x"}   : (!fir.box<!fir.ptr<!fir.type<_QMmTt1{x:f32}>>>) -> !fir.ref<f32>
+! CHECK:  %[[VAL_4:.*]] = fir.box_addr %[[VAL_3:.*]] : (!fir.box<!fir.ptr<!fir.type<_QMmTt1{x:f32}>>>) -> !fir.ptr<!fir.type<_QMmTt1{x:f32}>>
+! CHECK:  hlfir.designate %[[VAL_4]]{"x"}   : (!fir.ptr<!fir.type<_QMmTt1{x:f32}>>) -> !fir.ref<f32>
 
 subroutine test_symbol_followed_by_ref(x)
   character(:), allocatable :: x(:)

flang/test/Lower/HLFIR/assignment-intrinsics.f90

@@ -135,6 +135,17 @@ subroutine array_character(x, y)
 ! CHECK:  %[[VAL_11:.*]]:2 = hlfir.declare {{.*}}  {uniq_name = "_QFarray_characterEy"} : (!fir.ref<!fir.array<10x!fir.char<1,?>>>, !fir.shape<1>, index) -> (!fir.box<!fir.array<10x!fir.char<1,?>>>, !fir.ref<!fir.array<10x!fir.char<1,?>>>)
 ! CHECK:  hlfir.assign %[[VAL_11]]#0 to %[[VAL_6]]#0 : !fir.box<!fir.array<10x!fir.char<1,?>>>, !fir.box<!fir.array<10x!fir.char<1,?>>>
 
+subroutine array_pointer(x, y)
+  real, pointer :: x(:), y(:)
+  x = y
+end subroutine
+! CHECK-LABEL: func.func @_QParray_pointer(
+! CHECK:  %[[VAL_1:.*]]:2 = hlfir.declare %{{.*}}Ex
+! CHECK:  %[[VAL_2:.*]]:2 = hlfir.declare %{{.*}}Ey
+! CHECK:  %[[VAL_3:.*]] = fir.load %[[VAL_2]]#0 : !fir.ref<!fir.box<!fir.ptr<!fir.array<?xf32>>>>
+! CHECK:  %[[VAL_4:.*]] = fir.load %[[VAL_1]]#0 : !fir.ref<!fir.box<!fir.ptr<!fir.array<?xf32>>>>
+! CHECK:  hlfir.assign %[[VAL_3]] to %[[VAL_4]] : !fir.box<!fir.ptr<!fir.array<?xf32>>>, !fir.box<!fir.ptr<!fir.array<?xf32>>>
+
 ! -----------------------------------------------------------------------------
 !     Test assignments with array LHS and scalar RHS
 ! -----------------------------------------------------------------------------