[Bug tree-optimization/69297] New: [6 Regression] Performance regression after r230020

["ysrumyan at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69297

            Bug ID: 69297
           Summary: [6 Regression] Performance regression after r230020
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ysrumyan at gmail dot com
  Target Milestone: ---

This regression was found on spec2006/464.h264ref. The problem is related to
SLP vectorization of BB's and caused by the wrong calculation of scalar cost,
e.g. for attached test-case:
  Cost model analysis: 
  Vector inside of basic block cost: 188
  Vector prologue cost: 0
  Vector epilogue cost: 0
  Scalar cost of basic block: 512

although the basic block contains only 96 statements.
I found out that vect_bb_slp_scalar_cost takes into account the same stmt
several times and results in non-profitable SLP vectorization.