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References: <bug-26944-1008@http.gcc.gnu.org/bugzilla/>
Subject: [Bug tree-optimization/26944] [4.1/4.2 Regression] -ftree-ch generates worse code
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From: "dann at godzilla dot ics dot uci dot edu" <gcc-bugzilla@gcc.gnu.org>
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------- Comment #5 from dann at godzilla dot ics dot uci dot edu  2006-05-03 18:54 -------
IMO Comment #4 does not look close enough at what is actually happening.
IMO tree-ch is the root cause here.

The code looks like this before .ch
Before .ch
  goto <bb 2> (<L1>);

<L0>:;
  D.1301_54 = Int_Loc.0_4 * 200;
  D.1302_55 = (int[50] *) D.1301_54;
  D.1303_56 = Arr_2_Par_Ref_30 + D.1302_55;
  (*D.1303_56)[Int_Index_1] = Int_Loc_3;
  Int_Index_58 = Int_Index_1 + 1;

  # Int_Index_1 = PHI <Int_Loc_3(0), Int_Index_58(1)>;
<L1>:;
  D.1305_26 = Int_Loc_3 + 1;
  if (Int_Index_1 <= D.1305_26) goto <L0>; else goto <L2>;

<L2>:;


after .ch it looks like this: 
  D.1305_41 = Int_Loc_3 + 1;
  if (Int_Loc_3 <= D.1305_41) goto <L0>; else goto <L2>; <-- this just
complicates the CFG. Look below to see what are the effects of doing this in
later passes. Plus just look at the comparison ...

  # Int_Index_37 = PHI <Int_Index_58(1), Int_Loc_3(0)>;
<L0>:;
  D.1301_54 = Int_Loc.0_4 * 200;
  D.1302_55 = (int[50] *) D.1301_54;
  D.1303_56 = Arr_2_Par_Ref_30 + D.1302_55;
  (*D.1303_56)[Int_Index_37] = Int_Loc_3;
  Int_Index_58 = Int_Index_37 + 1;
  D.1305_26 = Int_Loc_3 + 1;
  if (D.1305_26 >= Int_Index_58) goto <L0>; else goto <L2>;

<L2>:;

Given the above CFG, critical edge splitting transforms this into:
  D.1305_41 = Int_Loc_3 + 1;
  if (Int_Loc_3 <= D.1305_41) goto <L6>; else goto <L7>;

<L7>:;
  goto <bb 2> (<L2>);

<L6>:;

  # Int_Index_37 = PHI <Int_Index_58(5), Int_Loc_3(3)>;
<L0>:;
  D.1301_54 = Int_Loc.0_4 * 200;
  D.1302_55 = (int[50] *) D.1301_54;
  D.1303_56 = Arr_2_Par_Ref_30 + D.1302_55;
  (*D.1303_56)[Int_Index_37] = Int_Loc_3;
  Int_Index_58 = Int_Index_37 + 1;
  if (D.1305_41 >= Int_Index_58) goto <L8>; else goto <L9>;

<L8>:;
  goto <bb 1> (<L0>);

<L9>:;

<L2>:;

Given the above CFG PRE will dutifully fill with code a lot of the empty basic
blocks: 

after pre
  D.1305_41 = Int_Loc_3 + 1;
  if (Int_Loc_3 <= D.1305_41) goto <L6>; else goto <L7>;

<L7>:;
  pretmp.34_45 = Int_Loc.0_4 * 200;
  pretmp.36_57 = (int[50] *) pretmp.34_45;
  pretmp.38_25 = Arr_2_Par_Ref_30 + pretmp.36_57;
  goto <bb 2> (<L2>);

<L6>:;
  pretmp.30_26 = Int_Loc.0_4 * 200;
  pretmp.31_19 = (int[50] *) pretmp.30_26;
  pretmp.32_1 = pretmp.31_19 + Arr_2_Par_Ref_30;

  # Int_Index_37 = PHI <Int_Index_58(5), Int_Loc_3(3)>;
<L0>:;
  D.1301_54 = pretmp.30_26;
  D.1302_55 = pretmp.31_19;
  D.1303_56 = pretmp.32_1;
  (*D.1303_56)[Int_Index_37] = Int_Loc_3;
  Int_Index_58 = Int_Index_37 + 1;
  if (D.1305_41 >= Int_Index_58) goto <L8>; else goto <L9>;

<L8>:;
  goto <bb 1> (<L0>);

<L9>:;

  # prephitmp.39_23 = PHI <D.1303_56(6), pretmp.38_25(4)>;
  # prephitmp.37_53 = PHI <D.1302_55(6), pretmp.36_57(4)>;
  # prephitmp.35_49 = PHI <D.1301_54(6), pretmp.34_45(4)>;
<L2>:;


Now when using -fno-tree-ch 

before critical edge splitting the code looks like this:
  goto <bb 2> (<L1>);

<L0>:;
  D.1301_54 = Int_Loc.0_4 * 200;
  D.1302_55 = (int[50] *) D.1301_54;
  D.1303_56 = Arr_2_Par_Ref_30 + D.1302_55;
  (*D.1303_56)[Int_Index_1] = Int_Loc_3;
  Int_Index_58 = Int_Index_1 + 1;

  # Int_Index_1 = PHI <Int_Loc_3(0), Int_Index_58(1)>;
<L1>:;
  D.1305_26 = Int_Loc_3 + 1;
  if (Int_Index_1 <= D.1305_26) goto <L0>; else goto <L2>;

<L2>:;


after crited it looks like this: (i.e. no change) 

  goto <bb 2> (<L1>);

<L0>:;
  D.1301_54 = Int_Loc.0_4 * 200;
  D.1302_55 = (int[50] *) D.1301_54;
  D.1303_56 = Arr_2_Par_Ref_30 + D.1302_55;
  (*D.1303_56)[Int_Index_1] = Int_Loc_3;
  Int_Index_58 = Int_Index_1 + 1;

  # Int_Index_1 = PHI <Int_Loc_3(0), Int_Index_58(1)>;
<L1>:;
  D.1305_26 = Int_Loc_3 + 1;
  if (Int_Index_1 <= D.1305_26) goto <L0>; else goto <L2>;

<L2>:;

and after PRE

  goto <bb 2> (<L1>);

<L0>:;
  D.1301_54 = pretmp.31_49;
  D.1302_55 = pretmp.32_45;
  D.1303_56 = pretmp.33_41;
  (*D.1303_56)[Int_Index_1] = Int_Loc_3;
  Int_Index_58 = Int_Index_1 + 1;

  # Int_Index_1 = PHI <Int_Loc_3(0), Int_Index_58(1)>;
<L1>:;
  D.1305_26 = pretmp.30_19;
  if (Int_Index_1 <= D.1305_26) goto <L0>; else goto <L2>;

<L2>:;


-- 


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