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.
