Alignment of large structures in GCC

[Alexey Neyman <stilor at att dot net>]

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.