[Bug c++/40874] New: Function object abstraction penalty

[Bug c++/40874] New: Function object abstraction penalty

[dave at boost-consulting dot com]

The following program shows that wrapping a simple class around a function
pointer costs important optimizations.  If you change the #if 1 to #if 0,
you'll see that the call is completely optimized away.  

#if 1
struct pf
{
  inline pf(int(*x)(int)) : x(x) {}

  inline int operator()(int a) const
  {
    return x(a);
  }

  int (*x)(int);
};
#else
typedef int(*pf)(int);
#endif

inline int g(int x) { return x/x - 1; }

int main(int argc, char* argv[])
{
  pf f(g);
  return f(3);
}


-- 
           Summary: Function object abstraction penalty
           Product: gcc
           Version: 4.4.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: dave at boost-consulting dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40874

[Bug tree-optimization/40874] Function object abstraction penalty

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2009-07-27 15:36 -------
So one problem is that nothing after SRA (before inlining) constant props &g
into the call expression.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pinskia at gcc dot gnu dot
                   |                            |org
           Severity|normal                      |enhancement
             Status|UNCONFIRMED                 |NEW
          Component|c++                         |tree-optimization
     Ever Confirmed|0                           |1
           Keywords|                            |missed-optimization
   Last reconfirmed|0000-00-00 00:00:00         |2009-07-27 15:36:36
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40874

[Bug tree-optimization/40874] Function object abstraction penalty with inline functions.

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2009-07-27 15:45 -------
(In reply to comment #1)
> So one problem is that nothing after SRA (before inlining) constant props &g
> into the call expression.

So there is no abstraction penalty really just a missing inlining.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Function object abstraction |Function object abstraction
                   |penalty                     |penalty with inline
                   |                            |functions.


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40874

[Bug tree-optimization/40874] Function object abstraction penalty with inline functions.

[dave at boost-consulting dot com]

------- Comment #3 from dave at boost-consulting dot com  2009-07-27 16:26 -------
The missing inlining is the cause, abstraction penalty is the symptom.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40874

[Bug tree-optimization/40874] Function object abstraction penalty with inline functions.

[paolo dot carlini at oracle dot com]

------- Comment #4 from paolo dot carlini at oracle dot com  2009-07-27 17:57 -------
I would say let's add Martin (and Honza) in CC, he did a lot of improvements
for similar issues (which I pointed out a lot of time ago to Honza).


-- 

paolo dot carlini at oracle dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hubicka at ucw dot cz,
                   |                            |jamborm at gcc dot gnu dot
                   |                            |org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40874

[Bug tree-optimization/40874] Function object abstraction penalty with inline functions.

[rguenth at gcc dot gnu dot org]

------- Comment #5 from rguenth at gcc dot gnu dot org  2009-07-28 11:22 -------
After early SRA we get

  f$x_8 = g;
  D.2142_6 = f$x_8;
  D.2141_7 = D.2142_6 (3);

which now misses a constant propagation of &g into the call which is why
inlining doesn't catch this opportunity.  Put one in and the abstraction
goes away.

Puting FRE into early optimizations also would get this.


-- 

rguenth at gcc dot gnu dot org changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40874

[Bug tree-optimization/40874] Function object abstraction penalty with inline functions.

[dave at boost-consulting dot com]

------- Comment #6 from dave at boost-consulting dot com  2009-07-28 18:42 -------
The next step would be to verify that the penalty is eliminated when using
boost::function / tr1::function


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40874

[Bug tree-optimization/40874] Function object abstraction penalty with inline functions.

[paolo dot carlini at oracle dot com]

------- Comment #7 from paolo dot carlini at oracle dot com  2009-07-28 19:38 -------
One step at a time, Dave ;)


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40874

[Bug tree-optimization/40874] Function object abstraction penalty with inline functions.

[jamborm at gcc dot gnu dot org]

------- Comment #8 from jamborm at gcc dot gnu dot org  2009-07-28 21:33 -------
I can confirm that if we schedule pass_ccp right after pass_sra_early,
g gets inlined.  Moreover, if we schedule one more pass_forwprop right
afterwards, even the testcase for PR 3713, comment #12 gets optimized
as it should :-)

So, like with PR 3713, we either have to schedule ccp or add some
specific pattern matching to the inlining preparation phase.  I guess
that people will find running one more ccp and fwprop unacceptable and
so some pattern matching will have to be done anyway for the other PR
(and we already do some awkward stuff like that for indirect member
pointer calls).  Perhaps we can match both, this one would be very
easy.  (Or is scheduling the two extra passes an option?)


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40874

[Bug tree-optimization/40874] Function object abstraction penalty with inline functions.

[rguenth at gcc dot gnu dot org]

------- Comment #10 from rguenth at gcc dot gnu dot org  2009-07-29 08:06 -------
I'll take this for now.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |rguenth at gcc dot gnu dot
                   |dot org                     |org
             Status|NEW                         |ASSIGNED
   Last reconfirmed|2009-07-27 15:36:36         |2009-07-29 08:06:04
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40874

[Bug tree-optimization/40874] Function object abstraction penalty with inline functions.

[rguenther at suse dot de]

------- Comment #9 from rguenther at suse dot de  2009-07-29 08:05 -------
Subject: Re:  Function object abstraction penalty
 with inline functions.

On Tue, 28 Jul 2009, paolo dot carlini at oracle dot com wrote:

> ------- Comment #7 from paolo dot carlini at oracle dot com  2009-07-28 19:38 -------
> One step at a time, Dave ;)

Indeed ;)  Just adding a CCP pass is likely not going to happen here.
Moving / adding FRE to early optimizations might get done though.

Dependent on time available for evaluation ...

Richard.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40874

[Bug tree-optimization/40874] Function object abstraction penalty with inline functions.

[hubicka at ucw dot cz]

------- Comment #11 from hubicka at ucw dot cz  2009-07-29 08:08 -------
Subject: Re:  Function object abstraction penalty with inline functions.

> I'll take this for now.
My preferred way of fixing this would be to include FRE pass.
Unfortunately my last benchmarks adding FRE early wasn't showing much of
win on our benchmark suite...  Still it seems right thing to do.

Honza


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40874

[Bug tree-optimization/40874] Function object abstraction penalty with inline functions.

[rguenther at suse dot de]

------- Comment #12 from rguenther at suse dot de  2009-07-29 08:12 -------
Subject: Re:  Function object abstraction penalty
 with inline functions.

On Tue, 28 Jul 2009, jamborm at gcc dot gnu dot org wrote:

> ------- Comment #8 from jamborm at gcc dot gnu dot org  2009-07-28 21:33 -------
> I can confirm that if we schedule pass_ccp right after pass_sra_early,
> g gets inlined.  Moreover, if we schedule one more pass_forwprop right
> afterwards, even the testcase for PR 3713, comment #12 gets optimized
> as it should :-)
> 
> So, like with PR 3713, we either have to schedule ccp or add some
> specific pattern matching to the inlining preparation phase.  I guess
> that people will find running one more ccp and fwprop unacceptable and
> so some pattern matching will have to be done anyway for the other PR
> (and we already do some awkward stuff like that for indirect member
> pointer calls).  Perhaps we can match both, this one would be very
> easy.  (Or is scheduling the two extra passes an option?)

Not really.  Or maybe it is ... at least scheduling FRE is still on
the list of possible things todo (can you check if that fixes 3713 as 
well?)

Richard.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40874

[Bug tree-optimization/40874] Function object abstraction penalty with inline functions.

[jamborm at gcc dot gnu dot org]

------- Comment #13 from jamborm at gcc dot gnu dot org  2009-07-29 10:16 -------
(In reply to comment #12)
> ... at least scheduling FRE is still on the list of possible things
> todo (can you check if that fixes 3713 as well?)
> 

No, it doesn't (unlike the testcase above, for which FRE is enough).
We have to remove the if-statement in (foo is a function):

  p$__pfn_25 = foo;
  D.1739_3 = (int) p$__pfn_25;
  D.1740_4 = D.1739_3 & 1;
  if (D.1740_4 != 0)
    goto <bb 3>;
  else
    goto <bb 4>;

Not even FRE combined with a subsequent fwprop (in their current form)
can make this happen :-/


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40874

[Bug tree-optimization/40874] Function object abstraction penalty with inline functions.

[rguenther at suse dot de]

------- Comment #14 from rguenther at suse dot de  2009-07-29 10:57 -------
Subject: Re:  Function object abstraction penalty
 with inline functions.

On Wed, 29 Jul 2009, jamborm at gcc dot gnu dot org wrote:

> ------- Comment #13 from jamborm at gcc dot gnu dot org  2009-07-29 10:16 -------
> (In reply to comment #12)
> > ... at least scheduling FRE is still on the list of possible things
> > todo (can you check if that fixes 3713 as well?)
> > 
> 
> No, it doesn't (unlike the testcase above, for which FRE is enough).
> We have to remove the if-statement in (foo is a function):
> 
>   p$__pfn_25 = foo;
>   D.1739_3 = (int) p$__pfn_25;
>   D.1740_4 = D.1739_3 & 1;
>   if (D.1740_4 != 0)
>     goto <bb 3>;
>   else
>     goto <bb 4>;
> 
> Not even FRE combined with a subsequent fwprop (in their current form)
> can make this happen :-/

FRE has to be teached the & 1 simplification via looking through the
int cast.

Richard.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40874