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Date: Tue, 15 Apr 2003 22:36:00 -0000
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From: Wolfgang Bangerth <bangerth@ices.utexas.edu>
Subject: Re: middle-end/10415: allocated stack space non optimimal
Reply-To: Wolfgang Bangerth <bangerth@ices.utexas.edu>
X-SW-Source: 2003-04/txt/msg00722.txt.bz2
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The following reply was made to PR middle-end/10415; it has been noted by GNATS.

From: Wolfgang Bangerth <bangerth@ices.utexas.edu>
To: gcc-gnats@gcc.gnu.org
Cc:  
Subject: Re: middle-end/10415: allocated stack space non optimimal
Date: Tue, 15 Apr 2003 17:35:03 -0500 (CDT)

 ---------- Forwarded message ----------
 Date: Tue, 15 Apr 2003 22:42:42 +0200
 From: GrzegorzB <b.grzes@interia.pl>
 To: Wolfgang Bangerth <bangerth@ices.utexas.edu>
 Subject: Re: middle-end/10415: allocated stack space non optimimal
 
 Wolfgang Bangerth wrote:
 >>Program test.c:
 >>
 >>void f() {
 >>	char buf[3];
 >>}
 >>
 >>main() {
 >>	f();
 >>}
 >>
 >>I compile this:
 >>gcc -S -o test test.c
 > 
 > 
 > What happens if you switch on optimization? Is stack allocation better 
 > then?
 > 
 > [I think that the compiler will just optimize away everything in that 
 > case, but you might prevent this by doing something like
 > 
 > void p(char *x);
 > void f() {
 >   char buf[3];
 >   p(buf);
 > }
 > 
 > and simply not defining f().]
 > 
 > W.
 > 
 If I compile test.c with option -O0, I have (for "char buf[3]"):
 
   pushl   %ebp
          movl    %esp, %ebp
          subl    $24, %esp
          leave
 
 for option -O3 is:
 
 pushl   %ebp
          movl    %esp, %ebp
          subl    $24, %esp
          andl    $-16, %esp
          leave
 If is "char buff[4]" and -O3 is:
 
 pushl   %ebp
          movl    %esp, %ebp
          subl    $8, %esp
          andl    $-16, %esp
          leave
 
 and for -O0 option (and "char buf[4]"):
 
 pushl   %ebp
          movl    %esp, %ebp
          subl    $4, %esp
          leave
 
 So sorry, I thought it is a bug.
 
 
 
 
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