[Bug c/11232] New: stack allocation bug?

["joshabbott at gmx dot net" <gcc-bugzilla at gcc dot gnu dot org>]

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11232

           Summary: stack allocation bug?
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: joshabbott@gmx.net
                CC: gcc-bugs@gcc.gnu.org
 GCC build triplet: " "
  GCC host triplet: (see report content)
GCC target triplet: " "

We are inquiring about how GCC determines the stack size after noticing what appears 
to be some inconsistencies. While word size is 4 bytes, a buffer declared as 
buffer[5] would in some instances have 24 bytes allocated for it when only 8 are 
needed. In other instances with multiple buffers and a single printf() in the code, 
the stack allocation would be x times larger than it would appear to need.
Two examples that work on redhat8 and redhat7.3 distro releases, are attached.
The gcc versions we've tested on are as follows: 

Reading specs from /usr/lib/gcc-lib/i386-redhat-linux/3.2/specs
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info 
--enable-shared --enable-threads=posix --disable-checking --host=i386-redhat-linux --with-
system-zlib --enable-__cxa_atexit
Thread model: posix
gcc version 3.2 20020903 (Red Hat Linux 8.0 3.2-7)

and

Reading specs from /usr/lib/gcc-lib/i386-redhat-linux/2.96/specs
 gcc version 2.96 20000731 (Red Hat Linux 7.3 2.96-113

We are interested in whether or not this is the desired behavior of gcc?

joshabbott@gmx.net
and
andrewg@d2.net.au


----------------------
example1.c
----------------------
void function(void) {
  char buffer1[5];
  char buffer2[10];
 } 
 void main() {
  function();
 }

----------------------
compiled with:  gcc example1.c -o example1
----------------------
gdb example1, disassemble function:

 0x80482f4 : push   %ebp
 0x80482f5 : mov    %esp,%ebp
 0x80482f7 : sub    $0x28,%esp
 0x80482fa : leave  
 0x80482fb : ret 
 

it allocates 40 bytes when it should only use 20.


------------------------
example2.c
------------------------

void function(void) {
  char buffer1[5];
  printf("buffer1: 0x%x\n",buffer1);
 } 
 void main() {
  function();
 }

-------------------------
compiled with: gcc example2.c -o example2
-------------------------
gdb example2, disassemble function

 0x8048328 : push   %ebp
 0x8048329 : mov    %esp,%ebp
 0x804832b : sub    $0x18,%esp
 0x804832e : sub    $0x8,%esp
 0x8048331 : lea    0xffffffe8(%ebp),%eax
 0x8048334 : push   %eax
 0x8048335 : push   $0x80483bc
 0x804833a : call   0x8048268 
 0x804833f : add    $0x10,%esp
 0x8048342 : leave  
 0x8048343 : ret


in this example it takes 24 bytes for buffer1[5].