[Bug target/49865] New: Unneccessary reload causes small size regression from 4.6.1

[Bug target/49865] New: Unneccessary reload causes small size regression from 4.6.1

[sgunderson at bigfoot dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49865

           Summary: Unneccessary reload causes small size regression from
                    4.6.1
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: sgunderson@bigfoot.com
            Target: i?86-*-*


Comparing 4.6.1 with gcc-snapshot from Debian:

gcc version 4.7.0 20110709 (experimental) [trunk revision 176106] (Debian
20110709-1) 

Given this code:

fugl:~> cat test.cpp 
#include <string.h>

class MyClass {
    void func();

        float f[1024];
        int i;
};

void MyClass::func()
{
    memset(f, 0, sizeof(f));
    i = 0;
}

and compiling with

fugl:~> /usr/lib/gcc-snapshot/bin/g++ -Os -c test.cpp

g++ produces, according to objdump:

00000000 <_ZN7MyClass4funcEv>:
   0:    55                       push   %ebp
   1:    31 c0                    xor    %eax,%eax
   3:    89 e5                    mov    %esp,%ebp
   5:    b9 00 04 00 00           mov    $0x400,%ecx
   a:    57                       push   %edi
   b:    8b 7d 08                 mov    0x8(%ebp),%edi
   e:    f3 ab                    rep stos %eax,%es:(%edi)
  10:    8b 45 08                 mov    0x8(%ebp),%eax
  13:    c7 80 00 10 00 00 00     movl   $0x0,0x1000(%eax)
  1a:    00 00 00 
  1d:    5f                       pop    %edi
  1e:    5d                       pop    %ebp
  1f:    c3                       ret    

while 4.6.1 has a more efficient sequence:

00000000 <_ZN7MyClass4funcEv>:
   0:    55                       push   %ebp
   1:    b9 00 04 00 00           mov    $0x400,%ecx
   6:    89 e5                    mov    %esp,%ebp
   8:    31 c0                    xor    %eax,%eax
   a:    8b 55 08                 mov    0x8(%ebp),%edx
   d:    57                       push   %edi
   e:    89 d7                    mov    %edx,%edi
  10:    f3 ab                    rep stos %eax,%es:(%edi)
  12:    c7 82 00 10 00 00 00     movl   $0x0,0x1000(%edx)
  19:    00 00 00 
  1c:    5f                       pop    %edi
  1d:    5d                       pop    %ebp
  1e:    c3                       ret   

It seems 4.6 is able to take a copy of the "this" pointer from a register
before the "rep stos" operation, which is one byte smaller than reloading it
from the stack when it needs to clear "i".

Of course, the _most_ efficient code sequence here would be doing the i = 0
before the memset, but I'm not sure if this is legal. However, eax should still
contain zero, so the mov could be done from eax instead of from a constant.

[Bug target/49865] Unnecessary reload causes small size regression from 4.6.1

[sgunderson at bigfoot dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49865

--- Comment #1 from sgunderson at bigfoot dot com 2011-07-27 11:38:57 UTC ---
(In reply to comment #0)
> Of course, the _most_ efficient code sequence here would be doing the i = 0
> before the memset, but I'm not sure if this is legal. However, eax should still
> contain zero, so the mov could be done from eax instead of from a constant.

Actually, thinking about it, the most efficient code sequence would be just
giving 4100 to memset instead of 4096, but that's for an enhancement request at
some point.

[Bug target/49865] [4.7 Regression] Unnecessary reload causes small bloat

[sgunderson at bigfoot dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49865

--- Comment #2 from sgunderson at bigfoot dot com 2011-07-27 17:28:19 UTC ---
(In reply to comment #1)
> Actually, thinking about it, the most efficient code sequence would be just
> giving 4100 to memset instead of 4096, but that's for an enhancement request at
> some point.

Filed as bug #49872.

[Bug target/49865] [4.7 Regression] Unnecessary reload causes small bloat

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49865

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |4.7.0

[Bug target/49865] [4.7 Regression] Unnecessary reload causes small bloat

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49865

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2011-10-27
                 CC|                            |vmakarov at gcc dot gnu.org
      Known to work|                            |4.6.2
     Ever Confirmed|0                           |1
      Known to fail|                            |4.7.0
           Severity|normal                      |minor

--- Comment #3 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-10-27 09:54:39 UTC ---
Confirmed.  The load is CSEd but the reg has a REG_EQUIV note pointing
to the MEM and is reloaded.  An IRA regression.

Vlad, can you investigate if there is a bug?

[Bug target/49865] [4.7 Regression] Unnecessary reload causes small bloat

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49865

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-11-24 16:59:03 UTC ---
Started with http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=171649

[Bug target/49865] [4.7 Regression] Unnecessary reload causes small bloat

[vmakarov at redhat dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49865

Vladimir Makarov <vmakarov at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |vmakarov at redhat dot com

--- Comment #5 from Vladimir Makarov <vmakarov at redhat dot com> 2011-12-13 19:38:16 UTC ---
I can not reproduce it on the current trunk (rev. 182263).  The recent ira
patches might fix it.  The code generated on the current trunk is 

        pushl   %ebp
        .cfi_def_cfa_offset 8
        .cfi_offset 5, -8
        movl    %esp, %ebp
        .cfi_def_cfa_register 5
        pushl   %edi
        .cfi_offset 7, -12
        movl    $1024, %ecx
        xorl    %eax, %eax
        movl    8(%ebp), %edi
        rep stosl
        movl    8(%ebp), %eax
        movl    $0, 4096(%eax)
        popl    %edi
        .cfi_restore 7
        popl    %ebp
        .cfi_restore 5
        .cfi_def_cfa 4, 4
        ret

[Bug target/49865] [4.7 Regression] Unnecessary reload causes small bloat

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49865

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-12-15 13:39:58 UTC ---
Well, that is the "bad" sequence.
Until r171648 instead of two movl 8(%ebp), %eX insns (one before rep stosl, one
after) there was just one of those and one movl %eA, %eB register move.

[Bug target/49865] [4.7 Regression] Unnecessary reload causes small bloat

[vmakarov at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49865

--- Comment #7 from Vladimir Makarov <vmakarov at gcc dot gnu.org> 2011-12-20 21:29:40 UTC ---
Author: vmakarov
Date: Tue Dec 20 21:29:36 2011
New Revision: 182553

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=182553
Log:
2011-12-20  Vladimir Makarov  <vmakarov@redhat.com>

    PR target/49865
    * ira-costs.c (find_costs_and_classes): Prefer registers even
          if the memory cost is the same.


Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/ira-costs.c

[Bug target/49865] [4.7 Regression] Unnecessary reload causes small bloat

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49865

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED

--- Comment #8 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-01-03 12:58:17 UTC ---
Fixed, thanks.