[Bug tree-optimization/71437] New: [7 regression' Performance regression after r235817

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71437

            Bug ID: 71437
           Summary: [7 regression' Performance regression after r235817
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ysrumyan at gmail dot com
  Target Milestone: ---

We noticed ~10% slowdown on one important benchmark used for Silvermont
testing. I can reproduced this performance gap using attached test-case on
SandyBridge:

before r235817

time ./good.exe                             
W[100]=10

real    0m0.761s

r235817
W[100]=10

real    0m0.863s

THere exist another optimization opportunnty, which can be illustrated by the
following test fragment:

        if( i == ( I - 1 ) ) 
          L = pL[i] ; 
        LD = (float)( L - pL[i] ) /
                        (float)( pL[i + 1] - pL[i] ) ; 

It is clear that LD value is 0 if L == pL[i], i.e. we can move the second
statement inside the hammock and perform simplification.