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Diffstat (limited to 'gcc/ada/gnat_rm.texi')
| -rw-r--r-- | gcc/ada/gnat_rm.texi | 27 |
1 files changed, 27 insertions, 0 deletions
diff --git a/gcc/ada/gnat_rm.texi b/gcc/ada/gnat_rm.texi index 0a89386af57..257ee1f170a 100644 --- a/gcc/ada/gnat_rm.texi +++ b/gcc/ada/gnat_rm.texi @@ -6852,6 +6852,9 @@ This means that if a @code{Scalar_Storage_Order} attribute definition clause is not confirming, then the type's @code{Bit_Order} shall be specified explicitly and set to the same value. +For a record extension, the derived type shall have the same scalar storage +order as the parent type. + If a component of @var{S} has itself a record or array type, then it shall also have a @code{Scalar_Storage_Order} attribute definition clause. In addition, if the component does not start on a byte boundary, then the scalar storage @@ -11543,6 +11546,30 @@ The default alignment for the type @code{V} is 4, as a result of the Integer field in the record, but it is permissible, as shown, to override the default alignment of the record with a smaller value. +@cindex Alignment, subtypes +Note that according to the Ada standard, an alignment clause applies only +to the first named subtype. If additional subtypes are declared, then the +compiler is allowed to choose any alignment it likes, and there is no way +to control this choice. Consider: + +@smallexample @c ada + type R is range 1 .. 10_000; + for R'Alignment use 1; + subtype RS is R range 1 .. 1000; +@end smallexample + +@noindent +The alignment clause specifies an alignment of 1 for the first named subtype +@code{R} but this does not necessarily apply to @code{RS}. When writing +portable Ada code, you should avoid writing code that explicitly or +implicitly relies on the alignment of such subtypes. + +For the GNAT compiler, if an explicit alignment clause is given, this +value is also used for any subsequent subtypes. So for GNAT, in the +above example, you can count on the alignment of @code{RS} being 1. But this +assumption is non-portable, and other compilers may choose different +alignments for the subtype @code{RS}. + @node Size Clauses @section Size Clauses @cindex Size Clause |