[Bug middle-end/35305] Speculative PRE support missing

[Bug middle-end/35305] Speculative PRE support missing

[xinliangli at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35305

--- Comment #4 from davidxl <xinliangli at gmail dot com> 2012-10-30 18:57:18 UTC ---
The suggested transformation can be useful in some cases, but won't be general
enough. The listed example is an extreme case. For instance, the second a+b
instance does not have to be in the hot trace but still still hot enough to be
PREed. Or there is no one dominating traces available -- the expression is
available in all incoming paths except for one rare path.

switch (a)
{
   case 1:
     g[1] = a + b;
     break;
   case 2:
     g[2] = a + b;
     break;
   case 3: 
     g[3] = a + b;
   ...
   default:
     g[0] = 0;
 }

switch (b)
{
   case 1:
     ... a + b; // partially redundant
   case 2:
      ... a + b; // redundant

   default:
     // does nothing
     break;
}


Regarding handling dereferences, the availability and down-safety analysis
needs to be extended to to recognize safe speculation candidates.


Example 1:  dereference of same pointer fully available -- 

int g1, g2;
struct A
{
  int a;
  int b;
};

void foo(struct A* ap, int k,int m)
{

   if (__builtin_expect (k, 1))
      g1 = ap->b;
   else
      g2 = ap->a;


   if (__builtin_expect (m, 1))
   {
       g2 = ap->b;        // Good safe speculative PRE candidate
   }
}


Example 2:  deference of ap fully anticipated



int g1, g2;
struct A
{
  int a;
  int b;
};

void foo(struct A* ap, int k,int m)
{

   if (__builtin_expect (k, 1))
      g1 = ap->b;


   if (__builtin_expect (m, 1))
       g2 = ap->b;             // Safe to speculatively hoist across the branch
into the else of the previous branch
   else
       g1 = ap->a;
}


David


(In reply to comment #3)
> Wouldn't this be a candidate for forming a superblock from hot traces of
> a function?  Thus in the testcase
> 
>   if (k && m)
>     {
>       g1 = a + b;
>       g2 = a + b;
>     }
>   else
>     {
> ... old code
>     }
> 
> which would also handle the case where we cannot speculatively move code
> (like dereferences)?

[Bug middle-end/35305] Speculative PRE support missing

[rguenth at gcc dot gnu dot org]

------- Comment #3 from rguenth at gcc dot gnu dot org  2009-03-21 10:53 -------
Wouldn't this be a candidate for forming a superblock from hot traces of
a function?  Thus in the testcase

  if (k && m)
    {
      g1 = a + b;
      g2 = a + b;
    }
  else
    {
... old code
    }

which would also handle the case where we cannot speculatively move code
(like dereferences)?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35305

[Bug middle-end/35305] Speculative PRE support missing

[steven at gcc dot gnu dot org]

------- Comment #2 from steven at gcc dot gnu dot org  2009-03-21 00:54 -------
I'm considering implementing/experimenting with something along the lines of
this:
http://www.cs.ualberta.ca/~amaral/cascon/CDP05/slides/CDP05-horspool.pdf


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35305

[Bug middle-end/35305] Speculative PRE support missing

[steven at gcc dot gnu dot org]

------- Comment #1 from steven at gcc dot gnu dot org  2009-02-06 21:53 -------
Could be added to PPRE.


-- 

steven at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dberlin at gcc dot gnu dot
                   |                            |org
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2009-02-06 21:53:02
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35305

[Bug middle-end/35305] Speculative PRE support missing

[pinskia at gcc dot gnu dot org]

-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
           Keywords|                            |missed-optimization


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35305