[Bug tree-optimization/49749] Reassociation rank algorithm does not include all non-NULL operands

["wschmidt at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49749

William J. Schmidt <wschmidt at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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             Status|NEW                         |ASSIGNED
         AssignedTo|unassigned at gcc dot       |wschmidt at gcc dot gnu.org
                   |gnu.org                     |

--- Comment #9 from William J. Schmidt <wschmidt at gcc dot gnu.org> 2011-07-20 16:28:49 UTC ---
Created attachment 24798
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24798
Proposed patch

I'm attaching a patch that solves this issue.  The patch was produced against
the ibm/gcc-4_6-branch, but should apply OK to trunk -- I'll verify that later
if the direction of this patch is acceptable.  This regains the 20% performance
loss we had experienced in 410.bwaves, and also gives a 5% boost to 444.namd. 
No significant degradations were observed on SPEC cpu2006.

In addition to fixing the operand access problems, the patch looks for
loop-carried dependencies in innermost loops, and biases the reassociation so
that the phi target is summed last.  The purpose of this is to identify
accumulator variables in inner loops and make each iteration of their
accumulations independent.  When these loops are unrolled, multiple independent
iterations can be interleaved for improved performance.

The optimization is restricted to innermost loops to avoid unnecessarily
raising register pressure.

There may be a better way to achieve the bias than what I've chosen here, so
please comment.  If the general approach is acceptable, I'll apply comments and
submit the patch for approval.