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* [Bug tree-optimization/18181] New: vectorizer: problem in the peeling mechanism in the presence of loop invariants that are used after the loop
@ 2004-10-27 15:04 dorit at il dot ibm dot com
  2004-11-04  9:17 ` [Bug tree-optimization/18181] " dorit at il dot ibm dot com
                   ` (4 more replies)
  0 siblings, 5 replies; 6+ messages in thread
From: dorit at il dot ibm dot com @ 2004-10-27 15:04 UTC (permalink / raw)
  To: gcc-bugs

The loop peeling mechanism in the vectorizer (which is used to force alignment 
of stores - by peeling the first few iterations, and to support unknown loop 
bound - by peeling te last few iterations) does not handle correctly the case 
where a loop invariant is used after the loop; there are two problems:
- When updating ssa-names the peeling utility assumes that anything that needs 
to be considered has a phi node at the loop header (which is not the case for 
loop invariants).
- The peeling utility creats a path that did not exist before - from the loop 
prolog to the loop exit - and there is no value to assign for this variable on 
this path (as it is only defined inside the loop).

This situation should rarely occur, but it did come up in the following case 
(complete testcase attached in 
http://gcc.gnu.org/ml/gcc-patches/2004-10/msg00981.html):

for i
  for j
    a[i][j]= [i+1][j]

This is represented as follows:

outer_loop_start
	i = phi (0, i')
inner_loop_start
	j = phi (0, j')
	k = i + 1
	a[i][j] = a[k][j]
	j' = j + 1
inner_loop_end
	i' = phi (k) 

k is the invariant defined in the inner-loop and used after the inner-loop (it 
was actually moved into the inner loop by PRE...).

This problem does not show up currently because it needs Devang's dependence-
distance patch (without it the vectorizer does not attempt to vectorize this 
loop). But as soon as his patch goes in -
http://gcc.gnu.org/ml/gcc-patches/2004-10/msg01046.html, 
the problem in this testcase will show up.

A patch to solve this is in the works.

-- 
           Summary: vectorizer: problem in the peeling mechanism in the
                    presence of loop invariants that are used after the loop
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: tree-optimization
        AssignedTo: dorit at il dot ibm dot com
        ReportedBy: dorit at il dot ibm dot com
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: powerpc-apple-darwin7.0.0
  GCC host triplet: powerpc-apple-darwin7.0.0
GCC target triplet: powerpc-apple-darwin7.0.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18181


^ permalink raw reply	[flat|nested] 6+ messages in thread
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2004-10-27 15:04 [Bug tree-optimization/18181] New: vectorizer: problem in the peeling mechanism in the presence of loop invariants that are used after the loop dorit at il dot ibm dot com
2004-11-04  9:17 ` [Bug tree-optimization/18181] " dorit at il dot ibm dot com
2004-11-04 13:43 ` pinskia at gcc dot gnu dot org
2004-11-04 14:45 ` dorit at il dot ibm dot com
2004-11-19 19:40 ` cvs-commit at gcc dot gnu dot org
2004-11-19 20:14 ` pinskia at gcc dot gnu dot org

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