[Bug target/35271] Stack not aligned at mod 16 byte boundary in x86_64 code

["matz at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #8 from matz at gcc dot gnu dot org  2008-02-22 23:40 -------
I can confirm this, the code in foo is definitely wrong.  When I compile
the (standard compliant, unlike the example from comment #4) with 4.2 or
4.3 on x86-64, with -O or -O2 it breaks:

% cat foo.c
long align;

int foo (int flag)
{
  int variable;
  if (flag == 0)
    return 42;
  align = (long)&variable;
  return foo (flag - 1);
}

The asm code for foo():
foo:
        subq    $16, %rsp
        testl   %edi, %edi
        movl    $42, %eax
        je      .L4
        leaq    12(%rsp), %rax
        subl    $1, %edi
        movq    %rax, align(%rip)
        call    foo
.L4:
        addq    $16, %rsp
        ret

It's clear that this misaligns the stack for the next calls.  On entry it's
8 mod 16, after the rsp adjustment it's still 8 mod 16, so in front of the
call it's also 8 mod 16, and boom.  Just before calls %rsp needs to be
0 mod 16.

It seems to be important that this is a recursive call.  This body does a
'sub $24', i.e. is correct:

int bar(int);
int foo (int flag)
{
  int variable;
  if (flag == 0)
    return 42;
  align = (long)&variable;
  return bar (flag - 1);
}

The same is true, when I instead add a second call in the body (to bar()),
then the incoming argument is stored in %rbx, which needs saving, so there's
a push, and still a 'sub $16', i.e. again adjusting correctly.


-- 

matz at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jh at suse dot cz, matz at
                   |                            |gcc dot gnu dot org
 GCC target triplet|i386-apple-darwin8.11.1     |x86_64


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35271