This is the mail archive of the
gcc-bugs@gcc.gnu.org
mailing list for the GCC project.
[Bug tree-optimization/26944] [4.1/4.2 Regression] -ftree-ch generates worse code
- From: "steven at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: 3 May 2006 21:33:09 -0000
- Subject: [Bug tree-optimization/26944] [4.1/4.2 Regression] -ftree-ch generates worse code
- References: <bug-26944-1008@http.gcc.gnu.org/bugzilla/>
- Reply-to: gcc-bugzilla at gcc dot gnu dot org
------- 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