------- Additional Comments From rguenth at tat dot physik dot uni-tuebingen
dot de 2004-12-22 18:16 -------
Subject: Re: alloca returning unnecessarily aligned pointer
and uses too much memory
pinskia at gcc dot gnu dot org wrote:
> ------- Additional Comments From pinskia at gcc dot gnu dot org 2004-12-22
> 15:06 -------
> The reason you cannot find anything in the C standard is because this is ABI
> thing so this is invalid
>
> We need to keep the stack aligned sorry.
Inside a function!? Or just at function callsites? Humm, the Intel
compiler produces
..B1.3: # Preds ..B1.2 ..B1.4
movl $4, %eax #5.12
subl %eax, %esp #5.12
andl $-16, %esp #5.12
movl %esp, %eax #5.12
# LOE eax ebx ebp esi edi
..B1.4: # Preds ..B1.3
addl (%eax), %ebx #6.3
addl $1, %esi #4.21
cmpl %edi, %esi #4.2
jl ..B1.3
which looks like it aligns the stack after alloca, but it manages to
waste less space by subtracting $4, not $32.
Also if the ABI says the stack is aligned, why do we not make use of
this and avoid the andl $-16, %esp -- or is the alignment only about
alloca?
I'm a bit confused.
Richard.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19131
