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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/61757] [4.10 Regression] genmodes failure with enable-checking
Date: Mon, 14 Jul 2014 09:48:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61757

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |law at gcc dot gnu.org

--- Comment #26 from Richard Biener <rguenth at gcc dot gnu.org> ---
DOM first rotates the loop by threading the entry over the first part of
the conditional, producing

 if (i + 1 < n && q == a[i + 1].p)
   {
     ++i;
     do
       {
         if (q != a[i].p)
           break;
         ++i;
         if (i >= n)
           break;
       }
     while (1);

the 2nd DOM pass then tries to complete the rotate but manages to record
two threadings that are incompatible:

  Threaded jump 6 --> 12 to 13
Jump threading proved probability of edge 12->4 too small (it is 5114, should
be 10000).
  Threaded jump 7 --> 12 to 14

the 2nd one over the loop entry would be ok, completing the rotate.  But
the first one is odd - and I can't see why we register it:

Optimizing block #6

1>>> COND 1 = n_4(D) ge_expr i_12
1>>> COND 1 = n_4(D) ne_expr i_12
1>>> COND 1 = n_4(D) gt_expr i_12
1>>> COND 0 = n_4(D) le_expr i_12
Optimizing statement pretmp_11 = a[i_12].p;
LKUP STMT pretmp_11 = a[i_12].p
          pretmp_11 = a[i_12].p;
2>>> STMT pretmp_11 = a[i_12].p
          pretmp_11 = a[i_12].p;
  Registering jump thread: (6, 12) incoming edge;  (12, 4) normal;
<<<< STMT pretmp_11 = a[i_12].p
          pretmp_11 = a[i_12].p;
<<<< COND 0 = n_4(D) le_expr i_12
<<<< COND 1 = n_4(D) gt_expr i_12
<<<< COND 1 = n_4(D) ne_expr i_12
<<<< COND 1 = n_4(D) ge_expr i_12

bb6 is just

<bb 6>:
pretmp_11 = a[i_12].p;

and its single predecessor

<bb 4>:
i_12 = i_9 + 1;
if (n_4(D) > i_12)
  goto <bb 6>;
else
  goto <bb 5>;

with the record_equality change we now record q_8(D) == prephitmp_16
instead of the other way around:

Optimizing block #4

0>>> COPY q_8(D) = prephitmp_16
1>>> COND 1 = q_8(D) le_expr prephitmp_16
1>>> COND 1 = q_8(D) ge_expr prephitmp_16
1>>> COND 1 = q_8(D) eq_expr prephitmp_16
1>>> COND 0 = q_8(D) ne_expr prephitmp_16

but when threading over the backedge this doesn't get invalidated.

So this canonicalization happens to be a correctness issue (by accident?
Jeff?)