[Bug rtl-optimization/26190] New: combine misses some distributivity

[Bug rtl-optimization/26190] New: combine misses some distributivity

[bonzini at gnu dot org]

In this code,

  unsigned short
  crc (unsigned short crc, unsigned char data)
  {
    unsigned char i, x16;
    for (i = 0; i < 8; i++)
      {
        x16 = (data ^ (unsigned char) crc) & 1;
        data >>= 1;
        if (x16)
          {
            crc ^= 0x4002;
            crc >>= 1;
            crc |= 0x8000;
          }
        else
          crc >>= 1;
      }
    return crc;
  }

the `then' branch of the if-statement should become

  crc >>= 1;
  crc ^= 0xA001;

Seems related to bug 16798.

Paolo


-- 
           Summary: combine misses some distributivity
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: bonzini at gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26190

[Bug rtl-optimization/26190] combine misses some distributivity

[bonzini at gnu dot org]

-- 

bonzini at gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
  BugsThisDependsOn|                            |16798, 18395
           Severity|normal                      |enhancement
           Priority|P3                          |P1


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26190

[Bug rtl-optimization/26190] combine misses some distributivity

[pinskia at gcc dot gnu dot org]

-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
           Priority|P1                          |P5


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26190

[Bug rtl-optimization/26190] combine misses some distributivity

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2006-02-25 19:40 -------
Confirmed.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2006-02-25 19:40:32
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26190

[Bug rtl-optimization/26190] combine misses some distributivity

[wilson at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=26190

Jim Wilson <wilson at gcc dot gnu.org> changed:

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

--- Comment #2 from Jim Wilson <wilson at gcc dot gnu.org> ---
Created attachment 35398
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35398&action=edit
prototype patches for ior/shift/xor optimization

[Bug rtl-optimization/26190] combine misses some distributivity

[wilson at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=26190

--- Comment #3 from Jim Wilson <wilson at gcc dot gnu.org> ---
This ior/shift/xor optimization be done during combine with a simplify-rtx.c
patch.  I wrote a prototype and tested it.  Combine canonicalizes shift/logical
as logical/shift, so we actually have to look for ior/xor/shift.  I see that
the optimization does not happen on arm and aarch64 because the 0x4002 value
does not fit into the immediate range for logical ops, gets loaded into a
register, pulled out of the loop, and hence is not available to combine.  We
would have to perform the optimization earlier for it to work for arm/aarch64. 
I tried MIPS, and see that because MIPS promotes HImode to SImode we don't have
enough info to prove that the opt is safe.  We need type info to make this
work.  This prototype does work for the x86 target which has both 16-bit
immediates and HImode instructions.

Next I wrote a prototype for match-and-simplify.  This one works for all 4
targets.  We have to handle ior/shift/xor as match-and-simplify does not
canonicalize logical/shift.  I noticed that if I change the xor to an ior, then
it gets optimized in combine because of the canonicalization for mips and x86,
but not for arm/aarch64 again because the constant was pulled out of the loop. 
So it seems that match-and-simplify should canonicalize shift/logical to
logical/shift too.  That would also reduce the number of patterns we need to
match when performing this ior/shift/xor optimization.

These are prototype patches that need a bit more work to handle more cases, and
to prove that they test all conditions necessary to make them safe.  This is my
first attempt to use the wi:: functions, so there might be a better way to do
this.

[Bug rtl-optimization/26190] combine misses some distributivity

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=26190

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
      Known to work|                            |10.1.0
         Resolution|---                         |FIXED
   Target Milestone|---                         |10.0

--- Comment #9 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Fixed in GCC 10.1.0.

Part of it was fixed with r6-3841-g84ff66b884e9 . The other part which fixed
this was improving EVRP.