[Bug c/34001] New: Incorrect __attribute__ fastcall behavior

[zia at aracnet dot com]

4byte structures that are passed as parameters to a fastcall routine will get
passed as registers. This does not match the original fastcall convention,
which passes such structures on the stack.

Compiling this code:
#include <stdio.h>

typedef struct {
  int a;
} my_struct;

int __attribute__((fastcall)) foo(my_struct s, int b)
{
  return s.a + b;
}


main()
{
  my_struct ss;
  ss.a = 1;
  printf("\nfoo = %d.\n",foo(ss,2));
}

Results in:
.
.
        movl    $1, -12(%ebp)
        movl    -12(%ebp), %ecx
        movl    $2, %edx
        call    _foo
.
.
and
.
.
.globl _foo
_foo:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $24, %esp
        movl    %ecx, -12(%ebp)
        movl    %edx, -16(%ebp)
        movl    -12(%ebp), %eax
        addl    -16(%ebp), %eax
        leave
        ret

With GCC.

Using other compilers that support fastcall will pass the struct value on the
stack.


-- 
           Summary: Incorrect __attribute__ fastcall behavior
           Product: gcc
           Version: 4.0.1
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: zia at aracnet dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34001