[Ada] Improve array packing for small record component

[Eric Botcazou]

This change ensures that the packing of array types subject to pragma Pack
and whose component type is a record with a size in the range 33 .. 63 bits
is optimal, in the sense that the 'Component_Size is equal to the 'Size of
the component type.

The following package P must yield the specified output with -gnatR1:

package P is

  type R is record
    I : Integer;
    B : Boolean;
  end record;
  pragma Pack (R);

  type A1 is array (1 .. 8) of R;
  pragma Pack (A1);

  type A2 is array (1 .. 8) of R;
  for A2'Component_Size use 33;

end P;

Representation information for unit P (spec)
--------------------------------------------

for R'Object_Size use 40;
for R'Value_Size use 33;
for R'Alignment use 1;
for R use record
   I at 0 range  0 .. 31;
   B at 4 range  0 ..  0;
end record;

for A1'Size use 264;
for A1'Alignment use 1;
for A1'Component_Size use 33;

for A2'Size use 264;
for A2'Alignment use 1;
for A2'Component_Size use 33;


Tested on x86_64-suse-linux, applied on the mainline.


2016-05-16  Eric Botcazou  <ebotcazou@adacore.com>

	* exp_util.adb (Remove_Side_Effects): Also make a constant if we need
	to capture the value for a small not by-reference record type.
	* freeze.ads (Check_Compile_Time_Size): Adjust comment.
	* freeze.adb (Set_Small_Size): Likewise.  Accept a size in the range
	of 33 .. 64 bits.
	(Check_Compile_Time_Size): Merge scalar and access type cases. Change
	variable name in array type case.  For the computation of the packed
	size, deal with record components and remove redundant test.
	(Freeze_Array_Type): Also adjust packing status when the size of the
	component type is in the range 33 .. 64 bits.
	* doc/gnat_rm/representation_clauses_and_pragmas.rst: Turn primitive
	into elementary type throughout.  Minor tweaks.
	(Alignment Clauses): Document actual alignment of packed array types.
	(Pragma Pack for Arrays): List only the 3 main cases and adjust.  Add
	"simple" to the record case.  Document effect on non packable types.
	(Pragma Pack for Records): Likewise.  Add record case and adjust.


-- 
Eric Botcazou
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