My hunch is that, similar to struct ordering, the order of declaration is the order on the stack. I do not know this for sure. If that's the case, you can get the best packing by putting the highest alignment items first. For example: __m128 a; double b; float c; short d; uint8_t e; Brian On Wed, Aug 26, 2009 at 12:28 AM, Rohit Garg<rpg.314@xxxxxxxxx> wrote: > Hi, > > What is the default stack alignment for gcc 4.4 on x86-64 linux? I > assume it is 16 bytes so that SSE can use aligned loads and stores. I > have googled for it a little and found some references for it that say > so in passing, but I'd prefer to get some official blessing for it. > > My real issue is that I would like to declare certain variables on > stack as aligned on 16 byte boundaries, so I can load/store them > quickly. Of course, I'd prefer if this didn't involve _any_ overhead. > > http://gcc.gnu.org/onlinedocs/gcc/i386-and-x86_002d64-Options.html > > This here says that stack can be aligned to 16 bytes, which is also > default. If I wish to avoid any code size increase, is there any > particular order of declaring these variables which I should follow so > that no extra space is consumed for padding. I would have thought that > GCC would be smart enough to reorder variables to minimize code bloat, > but..... > > I am using fedora 11, so all code is compiled using gcc 4.4. Not sure > what the compilation options are. > > Thanks, > > -- > Rohit Garg > > http://rpg-314.blogspot.com/ > > Senior Undergraduate > Department of Physics > Indian Institute of Technology > Bombay >
