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Subject: [Bug tree-optimization/26944] [4.1/4.2 Regression] -ftree-ch generates worse code
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From: "steven at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>
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------- Comment #7 from steven at gcc dot gnu dot org  2006-05-03 21:33 -------
Re. comment #5, user code could also have a CFG like that, so we should handle
this case properly (and we do, tree-ch is doing the right thing afaict).  Re.
comment #6, I don't see what the register allocator has to do with this at all. 

The bottom line is that for the case where we produce good code, IVOPTs selects
a simple addressing mode and produces a simple loop exit condition; and for the
complicated code, IVOPTs picks an addressing mode that requires a lea and an
extra register.

Look back at that loop for a moment. With tree-ch, ignoring dead code (the sets
to SSA names 5[456] are dead!), the .cunroll dump (i.e. just before IVOPTs)
looks like this:

  # Int_Index_37 = PHI <Int_Index_58(6), Int_Loc_3(4)>;
<L0>:;
  (*pretmp.28_49)[Int_Index_37] = Int_Loc_3;
  Int_Index_58 = Int_Index_37 + 1;
  if (D.1563_41 >= Int_Index_58) goto <L8>; else goto <L9>;

<L8>:;
  goto <bb 5> (<L0>);

That looks rather nice to me. But just after IVOPTs (in the .ivopts dump) we
have turned that simple nice code into this mess:

  # ivtmp.38_26 = PHI <ivtmp.38_35(6), 0(4)>;
<L0>:;
  D.1622_34 = (int *) pretmp.28_49;
  D.1623_33 = (int *) Int_1_Par_Val_2;
  D.1624_22 = (int *) ivtmp.38_26;
  D.1625_21 = D.1623_33 + D.1624_22;
  MEM[base: D.1622_34, index: D.1625_21, step: 4B, offset: 20B] = Int_Loc_3;
  ivtmp.38_35 = ivtmp.38_26 + 1;
  D.1626_20 = (unsigned int) Int_1_Par_Val_2;
  D.1627_17 = D.1626_20 + ivtmp.38_35;
  D.1628_16 = D.1627_17 + 5;
  Int_Index_15 = (One_Fifty) D.1628_16;
  if (D.1563_41 >= Int_Index_15) goto <L8>; else goto <L9>;

<L8>:;
  goto <bb 5> (<L0>);

If this is caused by the register allocator, I'd like to know why you'd think
that.  And if this is the doing of tree-ch, then I'd like to know what you
expect tree-ch to do instead.  But as far as I can tell, this is just a very
poor choice by IVOPTs.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26944