[Bug tree-optimization/60172] [4.9/4.10 Regression] ARM performance regression from trunk@207239

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60172

--- Comment #17 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Thomas Preud'homme from comment #16)
> Hi Richard,
> 
> could you expand on what you said in comment #13? I don't see how reassoc
> could help cse here. From what I understood, reassoc tries to group per
> rank. In our case, we have (view of the source with loop unrolling):
> 
> Arr_2_Par_Ref [Int_Loc] [Int_Loc] = Int_Loc;
> /* some stmts */
> Arr_2_Par_Ref [Int_Loc+10] [Int_Loc] = Arr_1_Par_Ref [Int_Loc];
> 
> If I'm not mistaken, in the first case you'd have:
> 
> Int_Loc * 4
> Int_Loc * 100
> Arr_2_Par_Ref
> 
> that would be added together with several statements. However in the second
> case you'd have:
> 
> Int_Loc * 4
> Int_Loc * 100
> 1000
> Arr_2_Par_Ref
> 
> that would be added together with several statements. I don't see how could
> 1000 being added first or last, it seems to me that it's always going to be
> in an intermediate statement and thus not all redanduncy would be eliminated
> by CSE.
> 
> Please let me know if my reasonning is flawed so that I can progress toward
> a solution.

Citing myself:

On the GIMPLE level before expansion we have

 +40 = Arr_2_Par_Ref_22(D) + (_41 + pretmp_20);

 _51 = Arr_2_Par_Ref_22(D) + (_41 + (pretmp_20 + 1000));

so if _51 were Arr_2_Par_Ref_22(D) + ((_41 + pretmp_20) + 1000);

then _41 + pretmp_20 would be fully redundant with the expression needed
by _40.

Note that IIRC one issue with TER is that it is no longer happening as
there are dead stmts around that confuse its has_single_use logic.  Thus
placing a dce pass right before expand would fix that and might be a good
idea anyway (see comment #3).  Implementing a "proper" poor-mans SSA-based
DCE would be a good way out (out-of-SSA already has one to remove dead
PHIs).