diff options
Diffstat (limited to 'gcc/ada/doc')
| -rw-r--r-- | gcc/ada/doc/gnat_rm/representation_clauses_and_pragmas.rst | 42 |
1 files changed, 22 insertions, 20 deletions
diff --git a/gcc/ada/doc/gnat_rm/representation_clauses_and_pragmas.rst b/gcc/ada/doc/gnat_rm/representation_clauses_and_pragmas.rst index 82dc97c..efcdc80 100644 --- a/gcc/ada/doc/gnat_rm/representation_clauses_and_pragmas.rst +++ b/gcc/ada/doc/gnat_rm/representation_clauses_and_pragmas.rst @@ -30,9 +30,11 @@ Alignment Clauses .. index:: Alignment Clause -GNAT requires that all alignment clauses specify a power of 2, and all -default alignments are always a power of 2. The default alignment -values are as follows: +GNAT requires that all alignment clauses specify 0 or a power of 2, and +all default alignments are always a power of 2. Specifying 0 is the +same as specifying 1. + +The default alignment values are as follows: * *Elementary Types*. @@ -610,23 +612,23 @@ alignment of the type (this is true for all types). In some cases the end record; -On a typical 32-bit architecture, the X component will occupy four bytes -and the Y component will occupy one byte, for a total of 5 bytes. As a -result ``R'Value_Size`` will be 40 (bits) since this is the minimum size -required to store a value of this type. For example, it is permissible -to have a component of type R in an array whose component size is -specified to be 40 bits. - -However, ``R'Object_Size`` will be 64 (bits). The difference is due to -the alignment requirement for objects of the record type. The X -component will require four-byte alignment because that is what type -Integer requires, whereas the Y component, a Character, will only -require 1-byte alignment. Since the alignment required for X is the -greatest of all the components' alignments, that is the alignment -required for the enclosing record type, i.e., 4 bytes or 32 bits. As -indicated above, the actual object size must be rounded up so that it is -a multiple of the alignment value. Therefore, 40 bits rounded up to the -next multiple of 32 yields 64 bits. +On a typical 32-bit architecture, the X component will occupy four bytes +and the Y component will occupy one byte, for a total of 5 bytes. As a +result ``R'Value_Size`` will be 40 (bits) since this is the minimum size +required to store a value of this type. For example, it is permissible +to have a component of type R in an array whose component size is +specified to be 40 bits. + +However, ``R'Object_Size`` will be 64 (bits). The difference is due to +the alignment requirement for objects of the record type. The X +component will require four-byte alignment because that is what type +Integer requires, whereas the Y component, a Character, will only +require 1-byte alignment. Since the alignment required for X is the +greatest of all the components' alignments, that is the alignment +required for the enclosing record type, i.e., 4 bytes or 32 bits. As +indicated above, the actual object size must be rounded up so that it is +a multiple of the alignment value. Therefore, 40 bits rounded up to the +next multiple of 32 yields 64 bits. For all other types, the ``Object_Size`` and ``Value_Size`` are the same (and equivalent to the RM attribute ``Size``). |