[Bug tree-optimization/42108] [4.4/4.5 Regression] 50% performance regression

["rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #32 from rguenther at suse dot de  2009-12-14 12:27 -------
Subject: Re:  [4.4/4.5 Regression] 50% performance
 regression

On Mon, 14 Dec 2009, matz at gcc dot gnu dot org wrote:

> ------- Comment #26 from matz at gcc dot gnu dot org  2009-12-14 04:55 -------
> And if I fix this problem (so that only one reference to *n_9) remains
> I hit the problem that the fortran frontend emits the computation of countm1
> after the loop bound test.  No pass is moving code in front of that test as
> this is potentially a regression (more evaluations in out-of-bound case).
> 
> And if I fix _that_ I hit the problem of the fix for PR38819.  PRE won't move
> the division at all, because it could trap :-/  If I disable this I get back
> the 4.3 performance.

Well.  VRP should mark divisions as non-trapping if possible.  I see
(after fixing the FRE issue):

  # iftmp.12_8 = PHI <-1(30), 1(12)>
  D.1588_67 = iftmp.12_8 * D.1529_119;
  D.1589_68 = (character(kind=4)) D.1588_67;
  countm1.6_69 = D.1583_64 / D.1589_68;

with

D.1529_119: [2, +INF]
iftmp.12_8: [1, 1]
D.1588_67: [2, +INF]
D.1589_68: [2, 2147483647]
countm1.6_69: [0, 2147483647]

so as D.1589_68 is never -1 or zero the division doesn't trap.
Now it's easy to mark the stmt in VRP this way but non-trivial
to keep track of it in the SCCVN IL.

Richard.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42108