[Bug tree-optimization/72772] New: Missed SCEV after pass reordering@236440

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72772

            Bug ID: 72772
           Summary: Missed SCEV after pass reordering@236440
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: amker at gcc dot gnu.org
  Target Milestone: ---

For below case
int foo (int flag, char *a)                                                    
{                                                                              
  short i, j;                                                                  
  short l = 0;                                                                 
  if (flag == 1)                                                               
    l = 3;                                                                     

  for (i = 0; i < 4; i++)                                                      
    {                                                                          
      for (j = l - 1; j > 0; j--)                                              
        a[j] = a[j - 1];                                                       
      a[0] = i;                                                                
    }                                                                          
}

GCC can't recognize &a[j]/&a[j-1] as SCEVs after pass reordering @commit:

commit 410372fef14173261ce8e547db98eafb3174921f
Author: rguenth <rguenth@138bc75d-0d04-0410-961f-82ee72b054a4>
Date:   Thu May 19 07:39:52 2016 +0000

    2016-05-19  Richard Biener  <rguenther@suse.de>

        PR tree-optimization/70729
        * passes.def: Move LIM pass before PRE.  Remove no longer
        required copyprop and move first DCE out of the loop pipeline.

        * gcc.dg/autopar/outer-6.c: Adjust to avoid redundant store.
        * gcc.dg/graphite/scop-18.c: Likewise.
        * gcc.dg/pr41783.c: Disable LIM.
        * gcc.dg/tree-ssa/loadpre10.c: Likewise.
        * gcc.dg/tree-ssa/loadpre23.c: Likewise.
        * gcc.dg/tree-ssa/loadpre24.c: Likewise.
        * gcc.dg/tree-ssa/loadpre25.c: Likewise.
        * gcc.dg/tree-ssa/loadpre4.c: Likewise.
        * gcc.dg/tree-ssa/loadpre8.c: Likewise.
        * gcc.dg/tree-ssa/ssa-pre-16.c: Likewise.
        * gcc.dg/tree-ssa/ssa-pre-18.c: Likewise.
        * gcc.dg/tree-ssa/ssa-pre-20.c: Likewise.
        * gcc.dg/tree-ssa/ssa-pre-3.c: Likewise.
        * gfortran.dg/pr42108.f90: Likewise.


    git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@236440
138bc75d-0d04-0410-961f-82ee72b054a4


This is because of changed IR, from:

Loop_guard:
  if (prephitmp_41 > 0)
    goto <bb 8>;
  else
    goto <bb 6>;

Preheader:
  <bb 8>:

Loop:
  <bb 9>:
  # j_29 = PHI <prephitmp_41(8), j_23(11)>
  _3 = (sizetype) j_29;
  _4 = a_21(D) + _3;
  _5 = _3 + 18446744073709551615;
  _6 = a_21(D) + _5;
  _7 = *_6;
  *_4 = _7;
  j.2_8 = (unsigned short) j_29;
  _9 = j.2_8 + 65535;
  j_23 = (short int) _9;
  if (j_23 > 0)
    goto <bb 11>;
  else
    goto <bb 10>;

Latch:
  <bb 11>:
  goto <bb 9>;


To:
Loop_guard:
  if (prephitmp_43 > 0)
    goto <bb 6>;
  else
    goto <bb 10>;

Preheader:
  <bb 6>:
  # j_16 = PHI <prephitmp_43(5)>

Loop:
  <bb 7>:
  # j_29 = PHI <j_16(6), j_23(8)>
  _3 = (sizetype) j_29;
  _4 = a_21(D) + _3;
  _5 = _3 + 18446744073709551615;
  _6 = a_21(D) + _5;
  _7 = *_6;
  *_4 = _7;
  j.2_8 = (unsigned short) j_29;
  _9 = j.2_8 + 65535;
  j_23 = (short int) _9;
  if (j_23 > 0)
    goto <bb 8>;
  else
    goto <bb 9>;

Latch:
  <bb 8>:
  goto <bb 7>;

Specifically, simplif_using_initial_conditions failed in this case because it
missed expanding simple operations in single-arg PHI in preheader.