Since it may take some time before an official update to the ARM AAPCS document can be made, I'm publishing a proposed change here for advanced notice. Alan will follow up with some GCC patches shortly to implement these changes.
The proposed changes should deal with types that have been either under-aligned (packed) or over-aligned by language extensions or language defined alignment modifiers. They work by assuming that the values passed to a procedure are *copies* of values and that these copies can safely have alignments that differ from both the source of the copy and also from the target use inside the called procedure (in the latter case a second copy to suitably aligned memory might be necessary). Since the ABI has not previously defined rules for parameter passing of values with alignment modifiers it is possible that existing implementations will not be 100% compatible with all these rules. Modifying the compiler to conform may result in a silent code-generation change. (There should be no change for types that are naturally aligned). We believe this should be very rare and because the ABI has not previously sanctioned such types they are unlikely to appear at shared library boundaries. It may help if compilers could emit a warning should they detect that a parameter may cause such change in behaviour. R. Definitions used in this description: 'Alignment-Adjusted' An Alignment-Adjusted type is a type to which a language alignment modifier has been applied. 'Member Alignment' The Member Alignment of an element of an aggregate type is the alignment of that member /after/ the application of any language alignment modifiers to that member. 'Natural Alignment' The Natural Alignment of an aggregate type is the maximum of each of the Member Alignments of the 'top-level' members of the aggregate type (ie before any alignment adjustment of the entire aggregate is applied). The Natural Alignment of all fundamental data types is that specified in Table 1 of the AAPCS. For the purposes of passing alignment-adjusted types as parameters, the following rules are proposed as additions to Stage B of Section 5.5 of the AAPCS: * Values of alignment-adjusted types passed as parameters are /copies/ of the actual values used in the call list. * The copies have alignments defined as follows: - For fundamental types, the alignment is the natural alignment of that type (after any promotions). - For aggregate types the copy of the aggregate type has 4-byte alignment if its natural alignment is <= 4 and 8-byte alignment if its natural alignment is >=8. * The alignment of the copy is used for applying the argument marshalling rules. ------ Similar changes will be published for AArch64, except that the lower and upper bounds for the natural alignment checks are are changed from (4,8) to (8, 16) bytes.
