[Bug rtl-optimization/22568] Should use cmov in some stituations

[Bug rtl-optimization/22568] Should use cmov in some stituations

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22568

--- Comment #12 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-02-08 20:59:51 UTC ---
I have a patch to tree-ssa-phiopt.c to fix comment #1 though it needs another
patch to expr.c to produce the cmov directly from COND_EXPR.  I hope to post
both patches for 4.8.0.

[Bug rtl-optimization/22568] Should use cmov in some stituations

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=22568

--- Comment #16 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #15)
> The expr.c patch yes.  The tree-ssa-phiopt.c patch, no.  Tree-ssa-phiopt.c
> needs more code rework because of the new infrastructures so I have not
> gotten around to improving the patch yet; been busy with other stuff.

The patch for PR 25290 includes the most of the infrastructure to implement
cmov at the gimple level (See match_simplify_replacement and late_p argument)
but is not enabled because I think it might require a cost model.

[Bug rtl-optimization/22568] Should use cmov in some stituations

[pinskia at gcc dot gnu dot org]

------- Comment #9 from pinskia at gcc dot gnu dot org  2006-01-15 03:14 -------
This might be fixed by (I am checking right now):
2006-01-14  Ian Lance Taylor  <ian@airs.com>

        * ifcvt.c (noce_init_if_info): New static function, broken out of
        noce_process_if_block.
        (noce_process_if_block): Call noce_init_if_info.
        (check_cond_move_block): New static function.
        (cond_move_process_if_block): New static function.
        (process_if_block): Call cond_move_process_if_block.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22568

[Bug rtl-optimization/22568] Should use cmov in some stituations

[pinskia at gcc dot gnu dot org]

------- Comment #10 from pinskia at gcc dot gnu dot org  2006-01-15 04:05 -------
This was not fixed.

If I rewrite the function like:
ulong f1(ulong a, ulong b)
{
  ulong tmp = a<b?b:a;
  ulong tmp1 = a<b?a:b;
  a = tmp;
  b = tmp1;
  return use (a, b)+use(a,b);
}

It works.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22568

[Bug rtl-optimization/22568] Should use cmov in some stituations

[rguenth at gcc dot gnu dot org]

------- Comment #11 from rguenth at gcc dot gnu dot org  2006-01-15 19:11 -------
Created an attachment (id=10647)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=10647&action=view)
patch aft the point I stopped working on this


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22568

[Bug c++/22568] New: Should use cmov in some stituations

[arndt at jjj dot de]

With (ulong a, ulong b) and code like
if ( a<b )  { ulong t=a; a=b; b=t; }
gcc should (if a and b are in registers already)
not emit a conditional jump but code like
MOV a, t;
CMP a, b;
CMOVcc b, a;
CMOVcc t, b;

Other such examples are easily found.  If I got it correctly, gcc
emits CMOVcc in just one situation, the conditional assignment.

Machine is AMD64, OS is SuSE Linux 9.3
gcc (GCC) 3.3.5 20050117 (prerelease) (SUSE Linux)

-- 
           Summary: Should use cmov in some stituations
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: arndt at jjj dot de
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22568

[Bug rtl-optimization/22568] Should use cmov in some stituations

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-07-20 15:11 -------
IIRC ifcvt is not smart enough to do this.

Confirmed.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
          Component|c++                         |rtl-optimization
     Ever Confirmed|                            |1
           Keywords|                            |missed-optimization
   Last reconfirmed|0000-00-00 00:00:00         |2005-07-20 15:11:41
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22568

[Bug rtl-optimization/22568] Should use cmov in some stituations

[rguenth at gcc dot gnu dot org]

------- Additional Comments From rguenth at gcc dot gnu dot org  2005-08-31 09:40 -------
With new tree-codes instead of using COND_EXPR we may use the tree-vectorizers
if-conversion and make expand preserve the conditional moves.

Also it shouldn't be too hard to hack rtl if-conversion to handle the case of
exactly two set's in the then/else block, too, and that may turn out to be
profitable always.  Is there any arch whose conditional move will kill
condition codes?  RTL is not my best friend (yet), but ifcvt doens't look too bad
either ;)

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22568

[Bug rtl-optimization/22568] Should use cmov in some stituations

[rguenth at gcc dot gnu dot org]

------- Additional Comments From rguenth at gcc dot gnu dot org  2005-09-01 12:38 -------
I have a patch that for

int a,b;
void foo(void)
{
  int x;
  if (a<b) x=a,a=b,b=x;
}

produces

bar:
        movl    a, %eax
        movl    b, %edx
        cmpl    %edx, %eax
        movl    %eax, %ecx
        cmovl   %edx, %ecx
        cmovge  b, %eax
        movl    %ecx, a
        movl    %eax, b
        ret

but it produces horrible code if the stuff is not available in pseudos
already.  (i.e. we miss load/store sinking)

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22568

[Bug rtl-optimization/22568] Should use cmov in some stituations

[bonzini at gcc dot gnu dot org]

------- Additional Comments From bonzini at gcc dot gnu dot org  2005-09-02 09:47 -------
Richard, can you write a case where it produces awful code?

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22568

[Bug rtl-optimization/22568] Should use cmov in some stituations

[rguenth at gcc dot gnu dot org]

------- Additional Comments From rguenth at gcc dot gnu dot org  2005-09-02 09:54 -------
Blindly applying ifcvt to something like

int a,b;
void foo(int flag)
{
  int x;
  if (flag)
    x=a,a=b,b=x;
}

because we're presented with

  if (flag)
  {
    int reg_a = a;
    x = reg_a;
    int reg_b = b;
    a = reg_b;
    b = x;
  }

and we get cmovs for loading a,b into pseudos instead of
loading them unconditionally.  F.i.

int a,b;
void foobar(int flag)
{
  if (flag)
    a = b;
}

will become

foobar:
        movl    4(%esp), %eax   # flag, flag
        testl   %eax, %eax      # flag
        cmovne  b, %edx # b,, b
        movl    a, %eax # a, tmp61
        cmovne  %edx, %eax      # b,, tmp61
        movl    %eax, a # tmp61, a
        ret

notice how we special-cased the _store_ to use a temporary.  I'll
try applying the same for loads.

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22568