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

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.

diff --git a/gcc/ada/doc/gnat_rm/implementation_defined_attributes.rst b/gcc/ada/doc/gnat_rm/implementation_defined_attributes.rst
index e285ebc..cfde81e 100644 (file)
--- a/gcc/ada/doc/gnat_rm/implementation_defined_attributes.rst
+++ b/gcc/ada/doc/gnat_rm/implementation_defined_attributes.rst
@@ -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
 ====================

diff --git a/gcc/ada/gnat_rm.texi b/gcc/ada/gnat_rm.texi
index 8aa446e..8dfbed6 100644 (file)
--- a/gcc/ada/gnat_rm.texi
+++ b/gcc/ada/gnat_rm.texi
@@ -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}