RE: Alignment of large structures in GCC

["Nicholas Miller" <Nick at advanced-flight-systems dot com>]

Add the packed attribute "__attribute__ ((packed))" to where you are
instantiating these structures, if not the structures themselves. This
should override the aligning that is automatically done on it.

> -----Original Message-----
> From: gcc-help-owner@gcc.gnu.org [mailto:gcc-help-owner@gcc.gnu.org] On
> Behalf Of Alexey Neyman
> Sent: Sunday, July 13, 2008 2:28 PM
> To: gcc-help@gcc.gnu.org
> Subject: Alignment of large structures in GCC
> 
> Hi,
> 
> I ran into the following problem using gcc: I am using some structures,
> which are put into a dedicated section. The linker concatenates these
> sections from all files; I have a linker script which assigns symbols
> to the start and end of this section. When I need to traverse all these
> structures, I then use the following loop:
> 
>   struct somename *p;
>   for (p = &__start_section; p < &__end_section; p++) {
>     ...
> 
> All worked well when the size of the structure was below 32 bytes. When
> I added an additional field, GCC suddenly started aligning each
> structure to 32 bytes - so the structures in this section are padded to
> 32-byte boundary. As the size of the structure is 36 bytes, though, the
> loop above breaks on the 2nd element: it tries to access it at
> &__start_section + 36, while the structure is actually at
> &__start_section + 64.
> 
> I narrowed it down to the following example:
> 
> <<<
> struct {
>         int xxx[NINT];
> } aaa __attribute__((section(".foo")));
> <<<<
> 
> When compiled, GCC selects the following alignments:
> 
> $ gcc -o - -S gg.c -DNINT=7 | grep align
>         .align 4
> $ gcc -o - -S gg.c -DNINT=8 | grep align
>         .align 32
> $ gcc -o - -S gg.c -DNINT=9 | grep align
>         .align 32
> 
> That's especially strange since __alignof__ reports the alignment of
> this structure as 4. It seems natural that the size of the structure
> should be a multiple of its alignment.
> 
> For now, I circumvented it by adding __attribute__((aligned(4))) to
> these structures. However, it may not be good if this structure gets a
> new member which would have a 8-byte alignment.
> 
> The question is, why does GCC perform such 32-byte alignment and is it
> possible to turn off such behavior globally?
> 
> P.S. GCC version:
> 
> $ gcc -v
> Using built-in specs.
> Target: i386-redhat-linux
> Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
> --infodir=/usr/share/info --enable-shared --enable-threads=posix
> --enable-checking=release --with-system-zlib --enable-__cxa_atexit
> --disable-libunwind-exceptions
> --enable-languages=c,c++,objc,obj-c++,java,fortran,ada
> --enable-java-awt=gtk --disable-dssi --enable-plugin
> --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-1.5.0.0/jre
> --enable-libgcj-multifile --enable-java-maintainer-mode
> --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --with-cpu=generic
> --host=i386-redhat-linux
> Thread model: posix
> gcc version 4.1.2 20070925 (Red Hat 4.1.2-33)
> 
> Best regards,
> Alexey.