------- Comment #20 from dberlin at gcc dot gnu dot org 2009-02-18 02:54
-------
Subject: Re: loop number of iterations analysis
not working
If the program terminates before i would wrap, then the number of
iterations was not MAXINT.
And since it can't wrap, it is not infinite in any case.
I agree you can't prove the number of iterations (since bar could
exit), but the requiring the assumption i != MAXINT still seems
useless.
On Tue, Feb 17, 2009 at 7:50 PM, rakdver at gcc dot gnu dot org
<gcc-bugzi...@gcc.gnu.org> wrote:
>
>
> ------- Comment #19 from rakdver at gcc dot gnu dot org 2009-02-18 00:50
> -------
>> Smaller testcase:
>>
>> void bar();
>> void foo(int i1)
>> {
>> int i;
>>
>> for (i=0; i<=i1; ++i)
>> bar();
>> }
>
> What the # of iterations analysis does is the following:
>
> -- the number of iterations is i1, unless i1==MAXINT, in which case it is
> infinity
> -- we cannot prove that i1 is not MAXINT (*)
> -- therefore, we must record the assumption i1!=MAXINT
>
> There is not much that can be done about this in general. We could make # of
> iterations analysis to be more specific, e.g. return the assumption i1==MAXINT
> as a condition for the number of iterations to be infinite (similarly as the
> rtl level analysis does), however, I don't think any of the tree level
> optimization passes can use that information.
>
> For some optimizations (final value replacement is the only one that I can
> think about now), we could use the fact that we are only interested in the
> number of iterations under the assumption that the given exit is taken (# of
> iterations analysis already supports that, by the only_exit flag).
>
> I would suggest changing the summary to something more appropriate, as the #
> of
> iterations analysis seems to work just fine for this testcase :-)
>
> (*) it might seem that we can use the fact that the induction variable i does
> not wrap; however, the program might terminate in the function bar before the
> induction variable i would wrap, regardless of whether i1==MAXINT
>
>
> --
>
> rakdver at gcc dot gnu dot org changed:
>
> What |Removed |Added
> ----------------------------------------------------------------------------
> AssignedTo|rakdver at gcc dot gnu dot |unassigned at gcc dot gnu
> |org |dot org
> Status|ASSIGNED |NEW
>
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26939
>
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--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26939
