end;
end if;
- -- If RM_Size is known, set Esize if not known
-
- if Known_RM_Size (E) and then Unknown_Esize (E) then
-
- -- If the alignment is known, we bump the Esize up to the next
- -- alignment boundary if it is not already on one.
-
- if Known_Alignment (E) then
- declare
- A : constant Uint := Alignment_In_Bits (E);
- S : constant SO_Ref := RM_Size (E);
- begin
- Set_Esize (E, (S + A - 1) / A * A);
- end;
- end if;
-
-- If Esize is set, and RM_Size is not, RM_Size is copied from Esize.
-- At least for now this seems reasonable, and is in any case needed
-- for compatibility with old versions of gigi.
- elsif Known_Esize (E) and then Unknown_RM_Size (E) then
+ if Known_Esize (E) and then Unknown_RM_Size (E) then
Set_RM_Size (E, Esize (E));
end if;
end if;
-- Here we calculate the alignment as the largest power of two multiple
- -- of System.Storage_Unit that does not exceed either the actual size of
+ -- of System.Storage_Unit that does not exceed either the object size of
-- the type, or the maximum allowed alignment.
declare
A := 2 * A;
end loop;
- -- Now we think we should set the alignment to A, but we skip this if
- -- an alignment is already set to a value greater than A (happens for
- -- derived types).
+ -- If alignment is currently not set, then we can safetly set it to
+ -- this new calculated value.
+
+ if Unknown_Alignment (E) then
+ Init_Alignment (E, A);
+
+ -- Cases where we have inherited an alignment
+
+ -- For constructed types, always reset the alignment, these are
+ -- Generally invisible to the user anyway, and that way we are
+ -- sure that no constructed types have weird alignments.
+
+ elsif not Comes_From_Source (E) then
+ Init_Alignment (E, A);
- -- However, if the alignment is known and too small it must be
- -- increased, this happens in a case like:
+ -- If this inherited alignment is the same as the one we computed,
+ -- then obviously everything is fine, and we do not need to reset it.
- -- type R is new Character;
- -- for R'Size use 16;
+ elsif Alignment (E) = A then
+ null;
- -- Here the alignment inherited from Character is 1, but it must be
- -- increased to 2 to reflect the increased size.
+ -- Now we come to the difficult cases where we have inherited an
+ -- alignment and size, but overridden the size but not the alignment.
+
+ elsif Has_Size_Clause (E) or else Has_Object_Size_Clause (E) then
+
+ -- This is tricky, it might be thought that we should try to
+ -- inherit the alignment, since that's what the RM implies, but
+ -- that leads to complex rules and oddities. Consider for example:
+
+ -- type R is new Character;
+ -- for R'Size use 16;
+
+ -- It seems quite bogus in this case to inherit an alignment of 1
+ -- from the parent type Character. Furthermore, if that's what the
+ -- programmer really wanted for some odd reason, then they could
+ -- specify the alignment they wanted.
+
+ -- Furthermore we really don't want to inherit the alignment in
+ -- the case of a specified Object_Size for a subtype, since then
+ -- there would be no way of overriding to give a reasonable value
+ -- (we don't have an Object_Subtype attribute). Consider:
+
+ -- subtype R is new Character;
+ -- for R'Object_Size use 16;
+
+ -- If we inherit the alignment of 1, then we have an odd
+ -- inefficient alignment for the subtype, which cannot be fixed.
+
+ -- So we make the decision that if Size (or Object_Size) is given
+ -- (and, in the case of a first subtype, the alignment is not set
+ -- with a specific alignment clause). We reset the alignment to
+ -- the appropriate value for the specified size. This is a nice
+ -- simple rule to implement and document.
+
+ -- There is one slight glitch, which is that a confirming size
+ -- clause can now change the alignment, which, if we really think
+ -- that confirming rep clauses should have no effect, is a no-no.
+
+ -- type R is new Character;
+ -- for R'Alignment use 2;
+ -- type S is new R;
+ -- for S'Size use Character'Size;
+
+ -- Now the alignment of S is 1 instead of 2, as a result of
+ -- applying the above rule to the confirming rep clause for S. Not
+ -- clear this is worth worrying about. If we recorded whether a
+ -- size clause was confirming we could avoid this, but right now
+ -- we have no way of doing that or easily figuring it out, so we
+ -- don't bother.
+
+ -- Historical note. In versions of GNAT prior to Nov 6th, 2010, an
+ -- odd distinction was made between inherited alignments greater
+ -- than the computed alignment (where the larger alignment was
+ -- inherited) and inherited alignments smaller than the computed
+ -- alignment (where the smaller alignment was overridden). This
+ -- was a dubious fix to get around an ACATS problem which seems
+ -- to have disappeared anyway, and in any case, this peculiarity
+ -- was never documented.
- if Unknown_Alignment (E) or else Alignment (E) < A then
Init_Alignment (E, A);
+
+ -- If no Size (or Object_Size) was specified, then we inherited the
+ -- object size, so we should inherit the alignment as well and not
+ -- modify it. This takes care of cases like:
+
+ -- type R is new Integer;
+ -- for R'Alignment use 1;
+ -- subtype S is R;
+
+ -- Here we have R has a default Object_Size of 32, and a specified
+ -- alignment of 1, and it seeems right for S to inherit both values.
+
+ else
+ null;
end if;
end;
end Set_Elem_Alignment;
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