Darwin ignores the alignment/packed attributes on this structure, forcing the structure to have 8-byte alignment and 16-byte size: --- struct Test { double D __attribute__((packed,aligned(4))); short X; }; --- Ian Lance Taylor did a great analysis of the history of this, tracking it back to a patch by David Edelsohn in 1996. The analysis is here: http://gcc.gnu.org/ml/gcc-help/2005-07/msg00280.html -Chris
I still don't believe this is a bug. As the alignment of whole struct is still 8 as double is first, even if the alignment of that double is 4.
The ABI specifies the alignment of the entire record is doubleword if the first field is an FP double, regardless of the alignment of the type itself.
Okay, in that case, how does one change the alignment of the structure? The only way I'm aware of to do this is with attribute packed, but that will also modify the inter-field padding of the structure. For example, in this case: struct Test { double D __attribute__((packed,aligned(4))); short X; int Y; short Z; } __attribute__((packed,aligned(4))); Not only is the alignment of the structure reduced to 4 bytes, the structure is also packed, resulting in the 'int' being unaligned and the size of the structure being 16 bytes (instead of 20). "it sure would be nice if you could use 'attribute aligned' to reduce alignment without attribute packed" -Chris
The whole point of attribute ((packed)) is to change the ABI. If it can't change the ABI, it is meaningless. So I don't find the argument based on the ABI to be particularly convincing.
Subject: Re: Darwin alignment ignores "attribute packed" for first 'double' element of a struct If Chris and Apple want to change the behavior for Darwin, be my guest. David
reopen
Subject: Bug 23071 Author: mrs Date: Mon Mar 22 23:13:10 2010 New Revision: 157654 URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=157654 Log: PR target/23071 * config/rs6000/rs6000.c (darwin_rs6000_special_round_type_align): Don't overly align based upon packed packed fields. Added: trunk/gcc/testsuite/gcc.target/powerpc/darwin-abi-12.c Modified: trunk/gcc/ChangeLog trunk/gcc/config/rs6000/rs6000.c
The previous behavior was fairly broken, adding packed, _increased_ the alignment. A user that adds packing, never wants more alignment: struct Test { double D __attribute__((packed,aligned(4))); short X; } x; struct { char x; struct Test t; } b = { 1, { 2, 3}}; compared to: struct Test { double D /* __attribute__((packed,aligned(4))) */; short X; } x; struct { char x; struct Test t; } b = { 1, { 2, 3}};
This has now been fixed.