[Bug tree-optimization/38497] PRE missing a load PRE which causes a loop to have two BBs

[Bug tree-optimization/38497] PRE missing a load PRE which causes a loop to have two BBs

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=38497

--- Comment #10 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
CLANG/LLVM does this while ICC peels off the first iteration of the loop.

[Bug tree-optimization/38497] New: PRE missing a load PRE which causes a loop to have two BBs

[pinskia at gcc dot gnu dot org]

Testcase:
void DoHuffIteration(int);
int f(int *a)
{
  int i;
  int plaintextlen=*a;
  for(i = 0; i< 10000; i++)
     DoHuffIteration(*a);
return *a - plaintextlen;
}

--- CUT ---
There is a load of *a after that loop even though we could have placed the load
inside the loop after the call.


-- 
           Summary: PRE missing a load PRE which causes a loop to have two
                    BBs
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: pinskia at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38497

[Bug tree-optimization/38497] PRE missing a load PRE which causes a loop to have two BBs

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2008-12-11 23:24 -------
This is a reduced testcase from The huffman byte benchmark.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38497

[Bug tree-optimization/38497] PRE missing a load PRE which causes a loop to have two BBs

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2010-03-02 19:01 -------
Still happens as of today on the trunk.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38497

[Bug tree-optimization/38497] PRE missing a load PRE which causes a loop to have two BBs

[rguenth at gcc dot gnu dot org]

------- Comment #3 from rguenth at gcc dot gnu dot org  2010-03-03 10:43 -------
This isn't PRE but code hoisting which we have a dup for.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |stevenb dot gcc at gmail dot
                   |                            |com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38497

[Bug tree-optimization/38497] PRE missing a load PRE which causes a loop to have two BBs

[steven at gcc dot gnu dot org]

------- Comment #4 from steven at gcc dot gnu dot org  2010-03-03 10:57 -------
I think pinskia means we could transform the test case of comment #0 to:

void DoHuffIteration(int);
int f(int *a)
{
  int i;
  int plaintextlen=*a;
  pretmp = plaintextlen;
  for(i = 0; i< 10000; i++)
     {
       DoHuffIteration(pretmp);
       pretmp = *a;
      }
  return pretmp - plaintextlen;
}

which makes this PRE, not hoisting.


-- 

steven at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|stevenb dot gcc at gmail dot|
                   |com                         |
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2010-03-03 10:57:32
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38497

[Bug tree-optimization/38497] PRE missing a load PRE which causes a loop to have two BBs

[rguenth at gcc dot gnu dot org]

------- Comment #5 from rguenth at gcc dot gnu dot org  2010-03-03 11:03 -------
I'm not so sure.  We start with

<bb 2>:
  plaintextlen_3 = *a_2(D);

<bb 3>:
  # i_13 = PHI <i_6(5), 0(2)>
  D.2725_5 = *a_2(D);
  DoHuffIteration (D.2725_5);
  i_6 = i_13 + 1;
  if (i_6 <= 9999)
    goto <bb 5>;
  else
    goto <bb 4>;

<bb 5>:
  goto <bb 3>;

<bb 4>:
  D.2725_7 = *a_2(D);

so *a_2 (4) is antic-out in bb3 but dies in bb3 (thus it's not antic-in there).
*a_2 (3) is antic-in in bb5 and available in bb2.  Thus we insert in bb5:

<bb 2>:
  plaintextlen_3 = *a_2(D);

<bb 3>:
  # i_13 = PHI <i_6(5), 0(2)>
  # prephitmp.3_14 = PHI <pretmp.2_1(5), plaintextlen_3(2)>
  D.2725_5 = prephitmp.3_14;
  DoHuffIteration (D.2725_5);
  i_6 = i_13 + 1;
  if (i_6 <= 9999)
    goto <bb 5>;
  else
    goto <bb 4>;

<bb 5>:
  pretmp.2_1 = *a_2(D);
  goto <bb 3>;

<bb 4>:
  D.2725_7 = *a_2(D);

now what is left is hoisting *a_2(D) to after the call.  But that's not PRE.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38497

[Bug tree-optimization/38497] PRE missing a load PRE which causes a loop to have two BBs

[steven at gcc dot gnu dot org]

------- Comment #6 from steven at gcc dot gnu dot org  2010-03-03 14:26 -------
Well, it is not hoisting, either.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38497

[Bug tree-optimization/38497] PRE missing a load PRE which causes a loop to have two BBs

[rguenth at gcc dot gnu dot org]

------- Comment #7 from rguenth at gcc dot gnu dot org  2010-05-05 10:30 -------
As *a_2(D) is ANTIC_OUT in bb3 (but not ANTIC_IN in bb3 because it dies in
there)
can we insert in bb3 instead of in bb5?


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38497