[Bug rtl-optimization/40327] New: Use less instructions to add some constants to register

[Bug rtl-optimization/40327] New: Use less instructions to add some constants to register

[carrot at google dot com]

Compiling this simple function in thumb mode:

int add_const(int x)
{
 return x+400;
}

I got:

       mov     r1, #200
       lsl     r3, r1, #1
       add     r0, r0, r3

A better code sequence should be:

    add r0, r0, 200
    add r0, r0, 200

In order to apply this optimization, the constant should be less than 2 times
of the largest immediate value in the target ISA. So this optimization should
also useful to other architecture with limited immediate operand range.

It can also be applied to sub instruction.


-- 
           Summary: Use less instructions to add some constants to register
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: carrot at google dot com
 GCC build triplet: i686-linux
  GCC host triplet: i686-linux
GCC target triplet: arm-eabi


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40327

[Bug target/40327] Use less instructions to add some constants to register

[steven at gcc dot gnu dot org]

------- Comment #1 from steven at gcc dot gnu dot org  2009-06-03 10:36 -------
This code comes from the split2 (split insns after reload).  So this is target
specific.  It surprises me that non-legitimate constants are accepted before
reload on ARM (at least in thumb mode).  It may be (and IMHO, probably is)
better to split the constant early, e.g. during expand.  A REG_EQUAL note can
be used to remember the original value added.


-- 

steven at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
          Component|rtl-optimization            |target
     Ever Confirmed|0                           |1
  GCC build triplet|i686-linux                  |
   GCC host triplet|i686-linux                  |
   Last reconfirmed|0000-00-00 00:00:00         |2009-06-03 10:36:14
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40327

[Bug target/40327] Use less instructions to add some constants to register

[steven at gcc dot gnu dot org]

------- Comment #2 from steven at gcc dot gnu dot org  2009-06-03 10:57 -------
FWIW it looks like the define_split at arm.c:5060 (of r147729) is triggered.

Richard E., is there a reason for lowering load-immediates so late in the
pipeline?


-- 

steven at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rearnsha at gcc dot gnu dot
                   |                            |org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40327

[Bug target/40327] Use less instructions to add some constants to register

[rearnsha at gcc dot gnu dot org]

------- Comment #3 from rearnsha at gcc dot gnu dot org  2009-06-09 22:06 -------
Working on a patch


-- 

rearnsha at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |rearnsha at gcc dot gnu dot
                   |dot org                     |org
             Status|NEW                         |ASSIGNED
   Last reconfirmed|2009-06-03 10:36:14         |2009-06-09 22:06:44
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40327

[Bug target/40327] Use less instructions to add some constants to register

[rearnsha at gcc dot gnu dot org]

------- Comment #4 from rearnsha at gcc dot gnu dot org  2009-06-13 12:49 -------
Subject: Bug 40327

Author: rearnsha
Date: Sat Jun 13 12:49:25 2009
New Revision: 148452

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=148452
Log:
        PR target/40327
        * arm/constraints.md (Pa, Pb): New constraints.
        * arm/arm.md (thumb1_addsi3): Support more complex additions.  Add a 
        split pattern to deal with them.

Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/config/arm/arm.md
    trunk/gcc/config/arm/constraints.md


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40327

[Bug target/40327] Use less instructions to add some constants to register

[rearnsha at gcc dot gnu dot org]

------- Comment #5 from rearnsha at gcc dot gnu dot org  2009-06-13 13:04 -------
fixed


-- 

rearnsha at gcc dot gnu dot org changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40327

[Bug target/40327] Use less instructions to add some constants to register

[steven at gcc dot gnu dot org]

------- Comment #6 from steven at gcc dot gnu dot org  2009-06-13 23:45 -------
I would still like to know why these constants can't be lowered earlier. 
Exposing the split-up insns may allow CSE to do a better job, etc.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40327