[Bug c/13355] New: Suboptimal code generated

[Bug c/13355] New: Suboptimal code generated

[arjanv at redhat dot com]

Attached .c file generates quite suboptimal code with tree-ssa including
* excessive stack slot usage
* moving data from a reg to a stack slot and then immediately moving it back to    
  the same reg
* cltd does not modify EAX so the XOR of eax after that instruction with the
previously-saved value is guaranteed to yield 0 so redundant

-- 
           Summary: Suboptimal code generated
           Product: gcc
           Version: tree-ssa
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: arjanv at redhat dot com
                CC: dnovillo at redhat dot com,gcc-bugs at gcc dot gnu dot
                    org
 GCC build triplet: i686-redhat-linux
  GCC host triplet: i686-redhat-linux
GCC target triplet: i686-redhat-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13355

[Bug c/13355] Suboptimal code generated

[arjanv at redhat dot com]

------- Additional Comments From arjanv at redhat dot com  2003-12-08 15:13 -------
Created an attachment (id=5301)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=5301&action=view)
example .c file that shows behavior with  gcc -march=i686 -mtune=i686
-fomit-frame-pointer -O2 test.c


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13355

[Bug optimization/13355] Suboptimal code generated

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2003-12-08 16:04 -------
The second one also happens on the mainline:
        movl    %eax, (%esp)
        movl    (%esp), %ecx

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c                           |optimization
           Keywords|                            |pessimizes-code


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13355

[Bug c/13355] Suboptimal code generated

[arjanv at redhat dot com]

------- Additional Comments From arjanv at redhat dot com  2003-12-08 16:56 -------
fwiw expected code should be something like:
    __asm__ volatile (
    "    movl    (%%ebp), %%eax \n"
    "    imull   %%ebx \n"
    "    movl    %%edx, %%ecx \n"
    "    movl    %%eax, (%%ebp) \n"
    "    movl    %%edx, 8(%%ebp) \n"
    "    cltd \n"
    "    xorl    %%ecx, %%edx \n"
    "    setne   %%al \n"
    "    movzbl  %%al, %%eax \n"
    "    movl    %%eax, 0x28(%%ebp) \n"
                                                                               
                             
    : );


-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|optimization                |c


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13355

[Bug optimization/13355] Suboptimal code generated

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c                           |optimization


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13355

[Bug optimization/13355] Suboptimal code generated

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2003-12-08 19:36 -------
Looks very target specific bugs now and looks like the register allocator is not doing its job.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
   Last reconfirmed|0000-00-00 00:00:00         |2003-12-08 19:36:55
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13355

[Bug optimization/13355] Suboptimal code generated

[arjanv at redhat dot com]

------- Additional Comments From arjanv at redhat dot com  2003-12-09 08:42 -------
compiling with -fnew-ra actually makes the code worse not better fwiw

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13355

[Bug optimization/13355] Suboptimal code generated

[kazu at cs dot umass dot edu]

------- Additional Comments From kazu at cs dot umass dot edu  2003-12-24 04:16 -------
A reduced testcase for the second case.
On mainline, i686-pc-linux-gnu.

/* -march=i686 -mtune=i686 -fomit-frame-pointer -O2 */

register int ebx asm ("ebx");
register int dummy1 asm ("esi");
register int dummy2 asm ("ebp");

int
foo (int arg)
{
  long long res = (long long) arg * (long long) ebx;
  return (int) res;
}

Global variables take up three registers from IA-32!


-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |kazu at cs dot umass dot edu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13355

[Bug optimization/13355] Suboptimal code generated

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13355

[Bug optimization/13355] Suboptimal code generated with global register variables

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-03-12 16:39 -------
This is related to bug 12395 which might get fixed when that gets fixed.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
  BugsThisDependsOn|                            |12395
            Summary|Suboptimal code generated   |Suboptimal code generated
                   |                            |with global register
                   |                            |variables


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13355

[Bug rtl-optimization/13355] Suboptimal code generated with global register variables

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-03-29 01:55 -------
Some of this is already fixed.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13355