[Bug target/11877] New: gcc should use xor trick with -Os

[Bug target/11877] New: gcc should use xor trick with -Os

[debian-gcc at lists dot debian dot org]

PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11877

           Summary: gcc should use xor trick with -Os
           Product: gcc
           Version: 3.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: debian-gcc at lists dot debian dot org
                CC: gcc-bugs at gcc dot gnu dot org,herbert at gondor dot
                    apana dot org dot au
 GCC build triplet: 386-linux
  GCC host triplet: i386-linux
GCC target triplet: 386-linux

[forwarded from http://bugs.debian.org/204687]

The program 
 
-- 
#include <linux/time.h> 
 
void foo(struct timespec *t, struct timespec *u) 
{ 
        struct timespec zero = {0, 0}; 
 
        *t = zero; 
        *u = zero; 
} 
-- 
 
produces the following code on i386 with -Os 
 
-- 
        .file   "b.c" 
        .text 
.globl foo 
        .type   foo, @function 
foo: 
        pushl   %ebp 
        movl    %esp, %ebp 
        movl    8(%ebp), %eax 
        movl    $0, (%eax) 
        movl    $0, 4(%eax) 
        movl    12(%ebp), %eax 
        movl    $0, (%eax) 
        movl    $0, 4(%eax) 
        leave 
        ret 
        .size   foo, .-foo 
        .ident  "GCC: (GNU) 3.3.1 20030626 (Debian prerelease)" 
-- 
 
It would be much better size-wise if it stored a zero in a register 
and then stored the register into those locations.

[Bug target/11877] gcc should use xor trick with -Os

[pinskia at gcc dot gnu dot org]

PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11877


pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
   Last reconfirmed|0000-00-00 00:00:00         |2003-08-10 16:03:50
               date|                            |


------- Additional Comments From pinskia at gcc dot gnu dot org  2003-08-10 16:03 -------
I can confirm that the xor trick will be a size win.

The movl still happens with the mainline (20030810).

[Bug target/11877] gcc should use xor trick with -Os

[dhazeghi at yahoo dot com]

PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11877


dhazeghi at yahoo dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
  GCC build triplet|386-linux                   |i386-linux
 GCC target triplet|386-linux                   |i386-linux
   Target Milestone|3.4                         |---

[Bug target/11877] gcc should use xor trick with -Os

[kazu at cs dot umass dot edu]

------- Additional Comments From kazu at cs dot umass dot edu  2003-12-31 21:19 -------
(set (mem:DI ...) (const_int 0)) is split into two moves in SImode after reload.
We could delay the split until after peephole2.
In peephole2, if a scratch reg is available,
load 0 into it with XOR and then copy that reg to two mem:SI locations.

Reduced to:

void
foo (long long *p)
{
  *p = 0;
}

The reduction from 13 bytes down to 7 bytes sounds impressive.
My proposed solution would still leave two XORs, though.


-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |kazu at cs dot umass dot edu
         AssignedTo|unassigned at gcc dot gnu   |kazu at cs dot umass dot edu
                   |dot org                     |
             Status|NEW                         |ASSIGNED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11877

[Bug target/11877] gcc should use xor trick with -Os

[kazu at cs dot umass dot edu]

------- Additional Comments From kazu at cs dot umass dot edu  2004-01-04 06:55 -------
Patch posted:

http://gcc.gnu.org/ml/gcc-patches/2004-01/msg00153.html


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11877

[Bug target/11877] gcc should use xor trick with -Os

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-01-04 07:50 -------
What about expanding (set (mem:DI ...) (const_int 0)) at expand time, this will cause more 
opportunities to happen and then the discusion is up to other parts of the compiler.
It looks like an easy change to ix86_expand_move.
Also interesting is that this testcase:
void
foo (long *p)
{
  *p = 0;
  p[1] = 0;
}
Does not use the xor trick either.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11877

[Bug target/11877] gcc should use xor trick with -Os

[kazu at cs dot umass dot edu]

------- Additional Comments From kazu at cs dot umass dot edu  2004-02-01 17:24 -------
Then we need something like an un-cse pass.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11877

[Bug target/11877] gcc should use xor trick with -Os

[kazu at cs dot umass dot edu]

------- Additional Comments From kazu at cs dot umass dot edu  2004-03-25 19:16 -------
Even if you split the long move in ix86_expand_move,
the constant 0 is propagated into the two moves.
I guess the right way may be uncse sometime after register allocation.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11877

[Bug target/11877] gcc should use xor trick with -Os

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-08-12 05:27 -------
*** Bug 23338 has been marked as a duplicate of this bug. ***

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dann at godzilla dot ics dot
                   |                            |uci dot edu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11877

[Bug target/11877] gcc should use xor trick with -Os

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-08-12 05:28 -------
PR 23102 is the bug for multiple xors.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
  BugsThisDependsOn|                            |23102


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11877