https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70849

            Bug ID: 70849
           Summary: Loop can be vectorized through gathers on AVX2
                    platforms.
           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: ---

Simple test which will be attached is not vectorized as not profitable:
test.c:11:5: note: cost model: the vector iteration cost = 2061 divided by the
scalar iteration cost = 9 is greater or equal to the vectorization factor = 8.
test.c:11:5: note: not vectorized: vectorization not profitable.
test.c:11:5: note: not vectorized: vector version will never be profitable.

but it can be vectorized as icc does using gathers:
   LOOP BEGIN at test.c(11,5)
      remark #15388: vectorization support: reference c1[j] has aligned access 
 [ test.c(12,7) ]
      remark #15388: vectorization support: reference c2[j] has aligned access 
 [ test.c(13,7) ]
      remark #15388: vectorization support: reference c1[j] has aligned access 
 [ test.c(12,7) ]
      remark #15388: vectorization support: reference c2[j] has aligned access 
 [ test.c(13,7) ]
      remark #15415: vectorization support: gather was generated for the
variable <f[j+base]>, strided by 256   [ test.c(12,16) ]
      remark #15415: vectorization support: gather was generated for the
variable <f[j+base+1]>, strided by 256   [ test.c(13,16) ]
      remark #15415: vectorization support: gather was generated for the
variable <f[j+base]>, strided by 256   [ test.c(12,16) ]
      remark #15415: vectorization support: gather was generated for the
variable <f[j+base+1]>, strided by 256   [ test.c(13,16) ]
      remark #15305: vectorization support: vector length 8
      remark #15300: LOOP WAS VECTORIZED
      remark #15449: unmasked aligned unit stride stores: 4 
      remark #15460: masked strided loads: 4 
      remark #15475: --- begin vector loop cost summary ---
      remark #15476: scalar loop cost: 18 
      remark #15477: vector loop cost: 12.000 
      remark #15478: estimated potential speedup: 1.500 
      remark #15488: --- end vector loop cost summary ---
   LOOP END

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