[Bug c/40845] New: malign-double witout effect for long double type

[Bug c/40845] New: malign-double witout effect for long double type

[bugtrack at roumenpetrov dot info]

According to documentation (quote):
-----
-malign-double
-mno-align-double
    Control whether GCC aligns double, long double, and long long variables on
a two word boundary or a one word boundary. Aligning double variables on a two
word boundary will produce code that runs somewhat faster on a `Pentium' at the
expense of more memory.
-----

Note for cygwin/mingw targets this options is enabled by default but it seems
to me do not work on long double type. The following code:
======================
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <sys/types.h>


typedef struct {
  char   a;
  char   x;
  double b;
} offset_d;

typedef struct {
  char        a;
  char        x;
  long double b
#if (defined(__MINGW32__) || defined(__CYGWIN__)) && 0
__attribute__((aligned(8)))
#endif
;
} offset_ld;

typedef struct {
  char a;
  char x;
  long b;
} offset_l;

typedef long long       longlong;

typedef struct {
  char      a;
  char      x;
  longlong  b;
} offset_ll;


#define OFFSET(a,b)     ((char*)(a)-(char*)(b))

int main()
{
  offset_d   o_d ;
  offset_ld  o_ld;
  offset_l   o_l ;
  offset_ll  o_ll;


  printf("sizeof/offset:");
  printf(" o_d=%02d/%02d" , sizeof(o_d ), OFFSET(&o_d .b, &o_d .a));
  printf(" o_ld=%02d/%02d", sizeof(o_ld), OFFSET(&o_ld.b, &o_ld.a));
  printf(" o_l=%02d/%02d" , sizeof(o_l ), OFFSET(&o_l .b, &o_l .a));
  printf(" o_ll=%02d/%02d", sizeof(o_ll), OFFSET(&o_ll.b, &o_ll.a));
  printf("\n");

  exit(0);
  return(0);
}
======================
output following (see table):
version+flag                         o_d    o_ld   o_l    o_ll  
gcc4.2.4(linux                    ): 12/04  16/04  08/04  12/04
gcc4.2.4(linux+malign_double      ): 16/08  16/04  08/04  16/08
gcc3.4.5(mingw32                  ): 16/08  16/04  08/04  16/08
gcc4.4.0(mingw32                  ): 16/08  16/04  08/04  16/08
gcc3.4.5(mingw32+-mno-align-double): 12/04  16/04  08/04  12/04
gcc4.4.0(mingw32+-mno-align-double): 12/04  16/04  08/04  12/04


Issue impacts:
- http://mail.python.org/ pipermail/python-dev/ 2009-July/090724.html


-- 
           Summary: malign-double witout effect for long double type
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: bugtrack at roumenpetrov dot info


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40845

[Bug c/40845] malign-double witout effect for long double type

[bugtrack at roumenpetrov dot info]

------- Comment #1 from bugtrack at roumenpetrov dot info  2009-07-24 12:40 -------
P.S.: issue is tested on 32-bit x86 platform


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40845

[Bug c/40845] malign-double without effect for long double type

[rguenth at gcc dot gnu dot org]

------- Comment #2 from rguenth at gcc dot gnu dot org  2009-07-24 15:44 -------
long double is 12 bytes, C requires arrays to pack dense, thus a 12 byte
long double has to be aligned to 4 bytes at least in arrays.

GCC 3.3 to 4.5 do

sizeof/offset: o_d=12/08 o_ld=16/04 o_l=08/04 o_ll=12/08
sizeof/offset: o_d=16/08 o_ld=16/04 o_l=08/04 o_ll=16/08

while 2.95 did

sizeof/offset: o_d=12/04 o_ld=16/04 o_l=08/04 o_ll=12/04
sizeof/offset: o_d=16/08 o_ld=24/08 o_l=08/04 o_ll=16/08


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40845