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[Ada] Adjust documentation of 'Bit and 'Descriptor_Size attributes

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2020-06-17  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* doc/gnat_rm/implementation_defined_attributes.rst (Bit): Sharpen
	the comparison with System.Storage_Unit.
	(Descriptor_Size): Clear confusion about alignment and padding.
	* gnat_rm.texi: Regenerate.
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Eric Botcazou 2020-04-13 11:10:43 +02:00 committed by Pierre-Marie de Rodat
parent d27ec3f60a
commit 741826eff5
2 changed files with 19 additions and 15 deletions
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16
gcc/ada/doc/gnat_rm/implementation_defined_attributes.rst
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@ -93,8 +93,8 @@ Attribute Bit
``obj'Bit``, where ``obj`` is any object, yields the bit
offset within the storage unit (byte) that contains the first bit of
storage allocated for the object. The value of this attribute is of the
type *universal_integer*, and is always a non-negative number not
exceeding the value of ``System.Storage_Unit``.
type *universal_integer* and is always a nonnegative number smaller
than ``System.Storage_Unit``.
For an object that is a variable or a constant allocated in a register,
the value is zero. (The use of this attribute does not force the
@ -241,14 +241,16 @@ the first element of the array.
.. code-block:: ada
type Unconstr_Array is array (Positive range <>) of Boolean;
type Unconstr_Array is array (Short_Short_Integer range <>) of Positive;
Put_Line ("Descriptor size = " & Unconstr_Array'Descriptor_Size'Img);
The attribute takes into account any additional padding due to type alignment.
In the example above, the descriptor contains two values of type
``Positive`` representing the low and high bound. Since ``Positive`` has
a size of 31 bits and an alignment of 4, the descriptor size is ``2 * Positive'Size + 2`` or 64 bits.
The attribute takes into account any padding due to the alignment of the
component type. In the example above, the descriptor contains two values
of type ``Short_Short_Integer`` representing the low and high bound. But,
since ``Positive`` has an alignment of 4, the size of the descriptor is
``2 * Short_Short_Integer'Size`` rounded up to the next multiple of 32,
which yields a size of 32 bits, i.e. including 16 bits of padding.
Attribute Elaborated
====================

18
gcc/ada/gnat_rm.texi
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@ -21,7 +21,7 @@
@copying
@quotation
GNAT Reference Manual , May 13, 2020
GNAT Reference Manual , May 14, 2020
AdaCore
@ -10167,8 +10167,8 @@ supported by the target for the given type.
@code{obj'Bit}, where @code{obj} is any object, yields the bit
offset within the storage unit (byte) that contains the first bit of
storage allocated for the object. The value of this attribute is of the
type @emph{universal_integer}, and is always a non-negative number not
exceeding the value of @code{System.Storage_Unit}.
type @emph{universal_integer} and is always a nonnegative number smaller
than @code{System.Storage_Unit}.
For an object that is a variable or a constant allocated in a register,
the value is zero. (The use of this attribute does not force the
@ -10337,14 +10337,16 @@ array descriptor contains bounds information and is located immediately before
the first element of the array.
@example
type Unconstr_Array is array (Positive range <>) of Boolean;
type Unconstr_Array is array (Short_Short_Integer range <>) of Positive;
Put_Line ("Descriptor size = " & Unconstr_Array'Descriptor_Size'Img);
@end example
The attribute takes into account any additional padding due to type alignment.
In the example above, the descriptor contains two values of type
@code{Positive} representing the low and high bound. Since @code{Positive} has
a size of 31 bits and an alignment of 4, the descriptor size is @code{2 * Positive'Size + 2} or 64 bits.
The attribute takes into account any padding due to the alignment of the
component type. In the example above, the descriptor contains two values
of type @code{Short_Short_Integer} representing the low and high bound. But,
since @code{Positive} has an alignment of 4, the size of the descriptor is
@code{2 * Short_Short_Integer'Size} rounded up to the next multiple of 32,
which yields a size of 32 bits, i.e. including 16 bits of padding.
@node Attribute Elaborated,Attribute Elab_Body,Attribute Descriptor_Size,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-elaborated}@anchor{174}