1 ------------------------------------------------------------------------------
3 -- GNAT COMPILER COMPONENTS --
9 -- Copyright (C) 2001-2021, Free Software Foundation, Inc. --
11 -- GNAT is free software; you can redistribute it and/or modify it under --
12 -- terms of the GNU General Public License as published by the Free Soft- --
13 -- ware Foundation; either version 3, or (at your option) any later ver- --
14 -- sion. GNAT is distributed in the hope that it will be useful, but WITH- --
15 -- OUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY --
16 -- or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License --
17 -- for more details. You should have received a copy of the GNU General --
18 -- Public License distributed with GNAT; see file COPYING3. If not, go to --
19 -- http://www.gnu.org/licenses for a complete copy of the license. --
21 -- GNAT was originally developed by the GNAT team at New York University. --
22 -- Extensive contributions were provided by Ada Core Technologies Inc. --
24 ------------------------------------------------------------------------------
26 with Atree; use Atree;
27 with Debug; use Debug;
28 with Einfo; use Einfo;
29 with Errout; use Errout;
31 with Sem_Aux; use Sem_Aux;
32 with Sem_Ch13; use Sem_Ch13;
33 with Sem_Eval; use Sem_Eval;
34 with Sem_Util; use Sem_Util;
35 with Sinfo; use Sinfo;
36 with Snames; use Snames;
37 with Ttypes; use Ttypes;
38 with Uintp; use Uintp;
40 package body Layout is
42 ------------------------
43 -- Local Declarations --
44 ------------------------
46 SSU : constant Int := Ttypes.System_Storage_Unit;
47 -- Short hand for System_Storage_Unit
49 -----------------------
50 -- Local Subprograms --
51 -----------------------
53 procedure Compute_Size_Depends_On_Discriminant (E : Entity_Id);
54 -- Given an array type or an array subtype E, compute whether its size
55 -- depends on the value of one or more discriminants and set the flag
56 -- Size_Depends_On_Discriminant accordingly. This need not be called
57 -- in front end layout mode since it does the computation on its own.
59 procedure Set_Composite_Alignment (E : Entity_Id);
60 -- This procedure is called for record types and subtypes, and also for
61 -- atomic array types and subtypes. If no alignment is set, and the size
62 -- is 2 or 4 (or 8 if the word size is 8), then the alignment is set to
65 ----------------------------
66 -- Adjust_Esize_Alignment --
67 ----------------------------
69 procedure Adjust_Esize_Alignment (E : Entity_Id) is
74 -- Nothing to do if size unknown
76 if Unknown_Esize (E) then
80 -- Determine if size is constrained by an attribute definition clause
81 -- which must be obeyed. If so, we cannot increase the size in this
84 -- For a type, the issue is whether an object size clause has been set.
85 -- A normal size clause constrains only the value size (RM_Size)
88 Esize_Set := Has_Object_Size_Clause (E);
90 -- For an object, the issue is whether a size clause is present
93 Esize_Set := Has_Size_Clause (E);
96 -- If size is known it must be a multiple of the storage unit size
98 if Esize (E) mod SSU /= 0 then
100 -- If not, and size specified, then give error
104 ("size for& not a multiple of storage unit size",
108 -- Otherwise bump up size to a storage unit boundary
111 Set_Esize (E, (Esize (E) + SSU - 1) / SSU * SSU);
115 -- Now we have the size set, it must be a multiple of the alignment
116 -- nothing more we can do here if the alignment is unknown here.
118 if Unknown_Alignment (E) then
122 -- At this point both the Esize and Alignment are known, so we need
123 -- to make sure they are consistent.
125 Abits := UI_To_Int (Alignment (E)) * SSU;
127 if Esize (E) mod Abits = 0 then
131 -- Here we have a situation where the Esize is not a multiple of the
132 -- alignment. We must either increase Esize or reduce the alignment to
133 -- correct this situation.
135 -- The case in which we can decrease the alignment is where the
136 -- alignment was not set by an alignment clause, and the type in
137 -- question is a discrete type, where it is definitely safe to reduce
138 -- the alignment. For example:
140 -- t : integer range 1 .. 2;
143 -- In this situation, the initial alignment of t is 4, copied from
144 -- the Integer base type, but it is safe to reduce it to 1 at this
145 -- stage, since we will only be loading a single storage unit.
147 if Is_Discrete_Type (Etype (E)) and then not Has_Alignment_Clause (E)
151 exit when Esize (E) mod Abits = 0;
154 Init_Alignment (E, Abits / SSU);
158 -- Now the only possible approach left is to increase the Esize but we
159 -- can't do that if the size was set by a specific clause.
163 ("size for& is not a multiple of alignment",
166 -- Otherwise we can indeed increase the size to a multiple of alignment
169 Set_Esize (E, ((Esize (E) + (Abits - 1)) / Abits) * Abits);
171 end Adjust_Esize_Alignment;
173 ------------------------------------------
174 -- Compute_Size_Depends_On_Discriminant --
175 ------------------------------------------
177 procedure Compute_Size_Depends_On_Discriminant (E : Entity_Id) is
182 Res : Boolean := False;
185 -- Loop to process array indexes
187 Indx := First_Index (E);
188 while Present (Indx) loop
189 Ityp := Etype (Indx);
191 -- If an index of the array is a generic formal type then there is
192 -- no point in determining a size for the array type.
194 if Is_Generic_Type (Ityp) then
198 Lo := Type_Low_Bound (Ityp);
199 Hi := Type_High_Bound (Ityp);
201 if (Nkind (Lo) = N_Identifier
202 and then Ekind (Entity (Lo)) = E_Discriminant)
204 (Nkind (Hi) = N_Identifier
205 and then Ekind (Entity (Hi)) = E_Discriminant)
214 Set_Size_Depends_On_Discriminant (E);
216 end Compute_Size_Depends_On_Discriminant;
222 procedure Layout_Object (E : Entity_Id) is
223 pragma Unreferenced (E);
225 -- Nothing to do for now, assume backend does the layout
234 procedure Layout_Type (E : Entity_Id) is
235 Desig_Type : Entity_Id;
238 -- For string literal types, kill the size always, because gigi does not
239 -- like or need the size to be set.
241 if Ekind (E) = E_String_Literal_Subtype then
242 Set_Esize (E, Uint_0);
243 Set_RM_Size (E, Uint_0);
247 -- For access types, set size/alignment. This is system address size,
248 -- except for fat pointers (unconstrained array access types), where the
249 -- size is two times the address size, to accommodate the two pointers
250 -- that are required for a fat pointer (data and template). Note that
251 -- E_Access_Protected_Subprogram_Type is not an access type for this
252 -- purpose since it is not a pointer but is equivalent to a record. For
253 -- access subtypes, copy the size from the base type since Gigi
254 -- represents them the same way.
256 if Is_Access_Type (E) then
257 Desig_Type := Underlying_Type (Designated_Type (E));
259 -- If we only have a limited view of the type, see whether the
260 -- non-limited view is available.
262 if From_Limited_With (Designated_Type (E))
263 and then Ekind (Designated_Type (E)) = E_Incomplete_Type
264 and then Present (Non_Limited_View (Designated_Type (E)))
266 Desig_Type := Non_Limited_View (Designated_Type (E));
269 -- If Esize already set (e.g. by a size clause), then nothing further
272 if Known_Esize (E) then
275 -- Access to subprogram is a strange beast, and we let the backend
276 -- figure out what is needed (it may be some kind of fat pointer,
277 -- including the static link for example.
279 elsif Is_Access_Protected_Subprogram_Type (E) then
282 -- For access subtypes, copy the size information from base type
284 elsif Ekind (E) = E_Access_Subtype then
285 Set_Size_Info (E, Base_Type (E));
286 Set_RM_Size (E, RM_Size (Base_Type (E)));
288 -- For other access types, we use either address size, or, if a fat
289 -- pointer is used (pointer-to-unconstrained array case), twice the
290 -- address size to accommodate a fat pointer.
292 elsif Present (Desig_Type)
293 and then Is_Array_Type (Desig_Type)
294 and then not Is_Constrained (Desig_Type)
295 and then not Has_Completion_In_Body (Desig_Type)
297 -- Debug Flag -gnatd6 says make all pointers to unconstrained thin
299 and then not Debug_Flag_6
301 Init_Size (E, 2 * System_Address_Size);
303 -- Check for bad convention set
305 if Warn_On_Export_Import
307 (Convention (E) = Convention_C
309 Convention (E) = Convention_CPP)
312 ("?x?this access type does not correspond to C pointer", E);
315 -- If the designated type is a limited view it is unanalyzed. We can
316 -- examine the declaration itself to determine whether it will need a
319 elsif Present (Desig_Type)
320 and then Present (Parent (Desig_Type))
321 and then Nkind (Parent (Desig_Type)) = N_Full_Type_Declaration
322 and then Nkind (Type_Definition (Parent (Desig_Type))) =
323 N_Unconstrained_Array_Definition
324 and then not Debug_Flag_6
326 Init_Size (E, 2 * System_Address_Size);
328 -- If unnesting subprograms, subprogram access types contain the
329 -- address of both the subprogram and an activation record. But if we
330 -- set that, we'll get a warning on different unchecked conversion
331 -- sizes in the RTS. So leave unset in that case.
333 elsif Unnest_Subprogram_Mode
334 and then Is_Access_Subprogram_Type (E)
338 -- Normal case of thin pointer
341 Init_Size (E, System_Address_Size);
344 Set_Elem_Alignment (E);
346 -- Scalar types: set size and alignment
348 elsif Is_Scalar_Type (E) then
350 -- For discrete types, the RM_Size and Esize must be set already,
351 -- since this is part of the earlier processing and the front end is
352 -- always required to lay out the sizes of such types (since they are
353 -- available as static attributes). All we do is to check that this
354 -- rule is indeed obeyed.
356 if Is_Discrete_Type (E) then
358 -- If the RM_Size is not set, then here is where we set it
360 -- Note: an RM_Size of zero looks like not set here, but this
361 -- is a rare case, and we can simply reset it without any harm.
363 if not Known_RM_Size (E) then
364 Set_Discrete_RM_Size (E);
367 -- If Esize for a discrete type is not set then set it
369 if not Known_Esize (E) then
375 -- If size is big enough, set it and exit
377 if S >= RM_Size (E) then
381 -- If the RM_Size is greater than System_Max_Integer_Size
382 -- (happens only when strange values are specified by the
383 -- user), then Esize is simply a copy of RM_Size, it will
384 -- be further refined later on.
386 elsif S = System_Max_Integer_Size then
387 Set_Esize (E, RM_Size (E));
390 -- Otherwise double possible size and keep trying
399 -- For non-discrete scalar types, if the RM_Size is not set, then set
400 -- it now to a copy of the Esize if the Esize is set.
403 if Known_Esize (E) and then Unknown_RM_Size (E) then
404 Set_RM_Size (E, Esize (E));
408 Set_Elem_Alignment (E);
410 -- Non-elementary (composite) types
413 -- For packed arrays, take size and alignment values from the packed
414 -- array type if a packed array type has been created and the fields
415 -- are not currently set.
418 and then Present (Packed_Array_Impl_Type (E))
421 PAT : constant Entity_Id := Packed_Array_Impl_Type (E);
424 if Unknown_Esize (E) then
425 Set_Esize (E, Esize (PAT));
428 if Unknown_RM_Size (E) then
429 Set_RM_Size (E, RM_Size (PAT));
432 if Unknown_Alignment (E) then
433 Set_Alignment (E, Alignment (PAT));
438 -- For array base types, set the component size if object size of the
439 -- component type is known and is a small power of 2 (8, 16, 32, 64
440 -- or 128), since this is what will always be used, except if a very
441 -- large alignment was specified and so Adjust_Esize_For_Alignment
442 -- gave up because, in this case, the object size is not a multiple
443 -- of the alignment and, therefore, cannot be the component size.
445 if Ekind (E) = E_Array_Type and then Unknown_Component_Size (E) then
447 CT : constant Entity_Id := Component_Type (E);
450 -- For some reason, access types can cause trouble, So let's
451 -- just do this for scalar types.
454 and then Is_Scalar_Type (CT)
455 and then Known_Static_Esize (CT)
456 and then not (Known_Alignment (CT)
457 and then Alignment_In_Bits (CT) >
458 System_Max_Integer_Size)
461 S : constant Uint := Esize (CT);
463 if Addressable (S) then
464 Set_Component_Size (E, S);
471 -- For non-packed arrays set the alignment of the array to the
472 -- alignment of the component type if it is unknown. Skip this
473 -- in full access case since a larger alignment may be needed.
476 and then not Is_Packed (E)
477 and then Unknown_Alignment (E)
478 and then Known_Alignment (Component_Type (E))
479 and then Known_Static_Component_Size (E)
480 and then Known_Static_Esize (Component_Type (E))
481 and then Component_Size (E) = Esize (Component_Type (E))
482 and then not Is_Full_Access (E)
484 Set_Alignment (E, Alignment (Component_Type (E)));
488 -- Even if the backend performs the layout, we still do a little in
491 -- Processing for record types
493 if Is_Record_Type (E) then
495 -- Special remaining processing for record types with a known
496 -- size of 16, 32, or 64 bits whose alignment is not yet set.
497 -- For these types, we set a corresponding alignment matching
498 -- the size if possible, or as large as possible if not.
500 if Convention (E) = Convention_Ada and then not Debug_Flag_Q then
501 Set_Composite_Alignment (E);
504 -- Processing for array types
506 elsif Is_Array_Type (E) then
508 -- For arrays that are required to be full access, we do the same
509 -- processing as described above for short records, since we really
510 -- need to have the alignment set for the whole array.
512 if Is_Full_Access (E) and then not Debug_Flag_Q then
513 Set_Composite_Alignment (E);
516 -- For unpacked array types, set an alignment of 1 if we know
517 -- that the component alignment is not greater than 1. The reason
518 -- we do this is to avoid unnecessary copying of slices of such
519 -- arrays when passed to subprogram parameters (see special test
520 -- in Exp_Ch6.Expand_Actuals).
522 if not Is_Packed (E) and then Unknown_Alignment (E) then
523 if Known_Static_Component_Size (E)
524 and then Component_Size (E) = 1
526 Set_Alignment (E, Uint_1);
530 -- We need to know whether the size depends on the value of one
531 -- or more discriminants to select the return mechanism. Skip if
532 -- errors are present, to prevent cascaded messages.
534 if Serious_Errors_Detected = 0 then
535 Compute_Size_Depends_On_Discriminant (E);
539 -- Final step is to check that Esize and RM_Size are compatible
541 if Known_Static_Esize (E) and then Known_Static_RM_Size (E) then
542 if Esize (E) < RM_Size (E) then
544 -- Esize is less than RM_Size. That's not good. First we test
545 -- whether this was set deliberately with an Object_Size clause
546 -- and if so, object to the clause.
548 if Has_Object_Size_Clause (E) then
549 Error_Msg_Uint_1 := RM_Size (E);
551 ("object size is too small, minimum allowed is ^",
552 Expression (Get_Attribute_Definition_Clause
553 (E, Attribute_Object_Size)));
556 -- Adjust Esize up to RM_Size value
559 Size : constant Uint := RM_Size (E);
562 Set_Esize (E, RM_Size (E));
564 -- For scalar types, increase Object_Size to power of 2, but
565 -- not less than a storage unit in any case (i.e., normally
566 -- this means it will be storage-unit addressable).
568 if Is_Scalar_Type (E) then
571 elsif Size <= 16 then
573 elsif Size <= 32 then
576 Set_Esize (E, (Size + 63) / 64 * 64);
579 -- Finally, make sure that alignment is consistent with
580 -- the newly assigned size.
582 while Alignment (E) * SSU < Esize (E)
583 and then Alignment (E) < Maximum_Alignment
585 Set_Alignment (E, 2 * Alignment (E));
593 -----------------------------
594 -- Set_Composite_Alignment --
595 -----------------------------
597 procedure Set_Composite_Alignment (E : Entity_Id) is
602 -- If alignment is already set, then nothing to do
604 if Known_Alignment (E) then
608 -- Alignment is not known, see if we can set it, taking into account
609 -- the setting of the Optimize_Alignment mode.
611 -- If Optimize_Alignment is set to Space, then we try to give packed
612 -- records an aligmment of 1, unless there is some reason we can't.
614 if Optimize_Alignment_Space (E)
615 and then Is_Record_Type (E)
616 and then Is_Packed (E)
618 -- No effect for record with full access components
620 if Is_Full_Access (E) then
621 Error_Msg_N ("Optimize_Alignment has no effect for &??", E);
623 if Is_Atomic (E) then
625 ("\pragma ignored for atomic record??", E);
628 ("\pragma ignored for bolatile full access record??", E);
634 -- No effect if independent components
636 if Has_Independent_Components (E) then
637 Error_Msg_N ("Optimize_Alignment has no effect for &??", E);
639 ("\pragma ignored for record with independent components??", E);
643 -- No effect if a component is full access or of a by-reference type
649 Ent := First_Component_Or_Discriminant (E);
650 while Present (Ent) loop
651 if Is_By_Reference_Type (Etype (Ent))
652 or else Is_Full_Access (Etype (Ent))
653 or else Is_Full_Access (Ent)
655 Error_Msg_N ("Optimize_Alignment has no effect for &??", E);
657 if Is_Atomic (Etype (Ent)) or else Is_Atomic (Ent) then
659 ("\pragma is ignored if atomic "
660 & "components present??", E);
663 ("\pragma is ignored if volatile full access "
664 & "components present??", E);
669 Next_Component_Or_Discriminant (Ent);
674 -- Optimize_Alignment has no effect on variable length record
676 if not Size_Known_At_Compile_Time (E) then
677 Error_Msg_N ("Optimize_Alignment has no effect for &??", E);
678 Error_Msg_N ("\pragma is ignored for variable length record??", E);
682 -- All tests passed, we can set alignment to 1
686 -- Not a record, or not packed
689 -- The only other cases we worry about here are where the size is
690 -- statically known at compile time.
692 if Known_Static_Esize (E) then
694 elsif Unknown_Esize (E) and then Known_Static_RM_Size (E) then
700 -- Size is known, alignment is not set
702 -- Reset alignment to match size if the known size is exactly 2, 4,
703 -- or 8 storage units.
705 if Siz = 2 * SSU then
707 elsif Siz = 4 * SSU then
709 elsif Siz = 8 * SSU then
712 -- If Optimize_Alignment is set to Space, then make sure the
713 -- alignment matches the size, for example, if the size is 17
714 -- bytes then we want an alignment of 1 for the type.
716 elsif Optimize_Alignment_Space (E) then
717 if Siz mod (8 * SSU) = 0 then
719 elsif Siz mod (4 * SSU) = 0 then
721 elsif Siz mod (2 * SSU) = 0 then
727 -- If Optimize_Alignment is set to Time, then we reset for odd
728 -- "in between sizes", for example a 17 bit record is given an
731 elsif Optimize_Alignment_Time (E)
733 and then Siz <= 8 * SSU
735 if Siz <= 2 * SSU then
737 elsif Siz <= 4 * SSU then
739 else -- Siz <= 8 * SSU then
743 -- No special alignment fiddling needed
750 -- Here we have Set Align to the proposed improved value. Make sure the
751 -- value set does not exceed Maximum_Alignment for the target.
753 if Align > Maximum_Alignment then
754 Align := Maximum_Alignment;
757 -- Further processing for record types only to reduce the alignment
758 -- set by the above processing in some specific cases. We do not
759 -- do this for full access records, since we need max alignment there,
761 if Is_Record_Type (E) and then not Is_Full_Access (E) then
763 -- For records, there is generally no point in setting alignment
764 -- higher than word size since we cannot do better than move by
765 -- words in any case. Omit this if we are optimizing for time,
766 -- since conceivably we may be able to do better.
768 if Align > System_Word_Size / SSU
769 and then not Optimize_Alignment_Time (E)
771 Align := System_Word_Size / SSU;
774 -- Check components. If any component requires a higher alignment,
775 -- then we set that higher alignment in any case. Don't do this if we
776 -- have Optimize_Alignment set to Space. Note that covers the case of
777 -- packed records, where we already set alignment to 1.
779 if not Optimize_Alignment_Space (E) then
784 Comp := First_Component (E);
785 while Present (Comp) loop
786 if Known_Alignment (Etype (Comp)) then
788 Calign : constant Uint := Alignment (Etype (Comp));
791 -- The cases to process are when the alignment of the
792 -- component type is larger than the alignment we have
793 -- so far, and either there is no component clause for
794 -- the component, or the length set by the component
795 -- clause matches the length of the component type.
799 (Unknown_Esize (Comp)
800 or else (Known_Static_Esize (Comp)
802 Esize (Comp) = Calign * SSU))
804 Align := UI_To_Int (Calign);
809 Next_Component (Comp);
815 -- Set chosen alignment, and increase Esize if necessary to match the
818 Set_Alignment (E, UI_From_Int (Align));
820 if Known_Static_Esize (E)
821 and then Esize (E) < Align * SSU
823 Set_Esize (E, UI_From_Int (Align * SSU));
825 end Set_Composite_Alignment;
827 --------------------------
828 -- Set_Discrete_RM_Size --
829 --------------------------
831 procedure Set_Discrete_RM_Size (Def_Id : Entity_Id) is
832 FST : constant Entity_Id := First_Subtype (Def_Id);
835 -- All discrete types except for the base types in standard are
836 -- constrained, so indicate this by setting Is_Constrained.
838 Set_Is_Constrained (Def_Id);
840 -- Set generic types to have an unknown size, since the representation
841 -- of a generic type is irrelevant, in view of the fact that they have
842 -- nothing to do with code.
844 if Is_Generic_Type (Root_Type (FST)) then
845 Set_RM_Size (Def_Id, Uint_0);
847 -- If the subtype statically matches the first subtype, then it is
848 -- required to have exactly the same layout. This is required by
849 -- aliasing considerations.
851 elsif Def_Id /= FST and then
852 Subtypes_Statically_Match (Def_Id, FST)
854 Set_RM_Size (Def_Id, RM_Size (FST));
855 Set_Size_Info (Def_Id, FST);
857 -- In all other cases the RM_Size is set to the minimum size. Note that
858 -- this routine is never called for subtypes for which the RM_Size is
859 -- set explicitly by an attribute clause.
862 Set_RM_Size (Def_Id, UI_From_Int (Minimum_Size (Def_Id)));
864 end Set_Discrete_RM_Size;
866 ------------------------
867 -- Set_Elem_Alignment --
868 ------------------------
870 procedure Set_Elem_Alignment (E : Entity_Id; Align : Nat := 0) is
872 -- Do not set alignment for packed array types, this is handled in the
875 if Is_Packed_Array_Impl_Type (E) then
878 -- If there is an alignment clause, then we respect it
880 elsif Has_Alignment_Clause (E) then
883 -- If the size is not set, then don't attempt to set the alignment. This
884 -- happens in the backend layout case for access-to-subprogram types.
886 elsif not Known_Static_Esize (E) then
889 -- For access types, do not set the alignment if the size is less than
890 -- the allowed minimum size. This avoids cascaded error messages.
892 elsif Is_Access_Type (E) and then Esize (E) < System_Address_Size then
896 -- We attempt to set the alignment in all the other cases
904 -- The given Esize may be larger that int'last because of a previous
905 -- error, and the call to UI_To_Int will fail, so use default.
907 if Esize (E) / SSU > Ttypes.Maximum_Alignment then
908 S := Ttypes.Maximum_Alignment;
910 -- If this is an access type and the target doesn't have strict
911 -- alignment, then cap the alignment to that of a regular access
912 -- type. This will avoid giving fat pointers twice the usual
913 -- alignment for no practical benefit since the misalignment doesn't
916 elsif Is_Access_Type (E)
917 and then not Target_Strict_Alignment
919 S := System_Address_Size / SSU;
922 S := UI_To_Int (Esize (E)) / SSU;
925 -- If the default alignment of "double" floating-point types is
926 -- specifically capped, enforce the cap.
928 if Ttypes.Target_Double_Float_Alignment > 0
930 and then Is_Floating_Point_Type (E)
932 M := Ttypes.Target_Double_Float_Alignment;
934 -- If the default alignment of "double" or larger scalar types is
935 -- specifically capped, enforce the cap.
937 elsif Ttypes.Target_Double_Scalar_Alignment > 0
939 and then Is_Scalar_Type (E)
941 M := Ttypes.Target_Double_Scalar_Alignment;
943 -- Otherwise enforce the overall alignment cap
946 M := Ttypes.Maximum_Alignment;
949 -- We calculate the alignment as the largest power-of-two multiple
950 -- of System.Storage_Unit that does not exceed the object size of
951 -- the type and the maximum allowed alignment, if none was specified.
952 -- Otherwise we only cap it to the maximum allowed alignment.
956 while 2 * A <= S and then 2 * A <= M loop
960 A := Nat'Min (Align, M);
963 -- If alignment is currently not set, then we can safely set it to
964 -- this new calculated value.
966 if Unknown_Alignment (E) then
967 Init_Alignment (E, A);
969 -- Cases where we have inherited an alignment
971 -- For constructed types, always reset the alignment, these are
972 -- generally invisible to the user anyway, and that way we are
973 -- sure that no constructed types have weird alignments.
975 elsif not Comes_From_Source (E) then
976 Init_Alignment (E, A);
978 -- If this inherited alignment is the same as the one we computed,
979 -- then obviously everything is fine, and we do not need to reset it.
981 elsif Alignment (E) = A then
985 -- Now we come to the difficult cases of subtypes for which we
986 -- have inherited an alignment different from the computed one.
987 -- We resort to the presence of alignment and size clauses to
988 -- guide our choices. Note that they can generally be present
989 -- only on the first subtype (except for Object_Size) and that
990 -- we need to look at the Rep_Item chain to correctly handle
994 FST : constant Entity_Id := First_Subtype (E);
996 function Has_Attribute_Clause
998 Id : Attribute_Id) return Boolean;
999 -- Wrapper around Get_Attribute_Definition_Clause which tests
1000 -- for the presence of the specified attribute clause.
1002 --------------------------
1003 -- Has_Attribute_Clause --
1004 --------------------------
1006 function Has_Attribute_Clause
1008 Id : Attribute_Id) return Boolean is
1010 return Present (Get_Attribute_Definition_Clause (E, Id));
1011 end Has_Attribute_Clause;
1014 -- If the alignment comes from a clause, then we respect it.
1015 -- Consider for example:
1017 -- type R is new Character;
1018 -- for R'Alignment use 1;
1019 -- for R'Size use 16;
1022 -- Here R has a specified size of 16 and a specified alignment
1023 -- of 1, and it seems right for S to inherit both values.
1025 if Has_Attribute_Clause (FST, Attribute_Alignment) then
1028 -- Now we come to the cases where we have inherited alignment
1029 -- and size, and overridden the size but not the alignment.
1031 elsif Has_Attribute_Clause (FST, Attribute_Size)
1032 or else Has_Attribute_Clause (FST, Attribute_Object_Size)
1033 or else Has_Attribute_Clause (E, Attribute_Object_Size)
1035 -- This is tricky, it might be thought that we should try to
1036 -- inherit the alignment, since that's what the RM implies,
1037 -- but that leads to complex rules and oddities. Consider
1040 -- type R is new Character;
1041 -- for R'Size use 16;
1043 -- It seems quite bogus in this case to inherit an alignment
1044 -- of 1 from the parent type Character. Furthermore, if that
1045 -- is what the programmer really wanted for some odd reason,
1046 -- then he could specify the alignment directly.
1048 -- Moreover we really don't want to inherit the alignment in
1049 -- the case of a specified Object_Size for a subtype, since
1050 -- there would be no way of overriding to give a reasonable
1051 -- value (as we don't have an Object_Alignment attribute).
1052 -- Consider for example:
1054 -- subtype R is Character;
1055 -- for R'Object_Size use 16;
1057 -- If we inherit the alignment of 1, then it will be very
1058 -- inefficient for the subtype and this cannot be fixed.
1060 -- So we make the decision that if Size (or Object_Size) is
1061 -- given and the alignment is not specified with a clause,
1062 -- we reset the alignment to the appropriate value for the
1063 -- specified size. This is a nice simple rule to implement
1066 -- There is a theoretical glitch, which is that a confirming
1067 -- size clause could now change the alignment, which, if we
1068 -- really think that confirming rep clauses should have no
1069 -- effect, could be seen as a no-no. However that's already
1070 -- implemented by Alignment_Check_For_Size_Change so we do
1071 -- not change the philosophy here.
1073 -- Historical note: in versions prior to Nov 6th, 2011, an
1074 -- odd distinction was made between inherited alignments
1075 -- larger than the computed alignment (where the larger
1076 -- alignment was inherited) and inherited alignments smaller
1077 -- than the computed alignment (where the smaller alignment
1078 -- was overridden). This was a dubious fix to get around an
1079 -- ACATS problem which seems to have disappeared anyway, and
1080 -- in any case, this peculiarity was never documented.
1082 Init_Alignment (E, A);
1084 -- If no Size (or Object_Size) was specified, then we have
1085 -- inherited the object size, so we should also inherit the
1086 -- alignment and not modify it.
1094 end Set_Elem_Alignment;