[Bug target/19131] alloca returning unnecessarily aligned pointer and uses too much memory

["rguenth at tat dot physik dot uni-tuebingen dot de" <gcc-bugzilla at gcc dot gnu dot org>]

------- 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.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19131