fix for pr47837

[Xinliang David Li]

On Fri, Mar 11, 2011 at 11:50 AM, Jeff Law <law@redhat.com> wrote:
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> On 03/09/11 09:24, Xinliang David Li wrote:
>> On Wed, Mar 9, 2011 at 6:03 AM, Jeff Law <law@redhat.com> wrote:
>> On 03/09/11 02:45, Richard Guenther wrote:
>>>>> On Tue, Mar 8, 2011 at 11:04 PM, Jeff Law <law@redhat.com> wrote:
>>
>>>>> True.  I've been repeatedly thinking of building some on-the-side CFG
>>>>> with value-numbered predicates to also catch the CFG vs. scalar-code
>>>>> predicate combinations we have.  On such on-the-side data structure
>>>>> we could do very aggressive jump-threading just for analysis purposes
>>>>> (experiments when working on separating conditions shows that
>>>>> a PRE-like algorithm could drive this).
>> I'm pondering the same kind of thing.  PRE on the predicates with a more
>> structured approach to block copying to isolate the paths seems to be
>> the way to go.
>>
>>
>>> Any simple examples to show how your idea would work?
> Our current jump threading is just a special case of path isolation; the
> biggest problem with path isolation is exploding codesize.

Jump threading can reduce size in some cases -- due to more aggressive
cleanups -- the -Os need to be more intelligent about it.

>
> I'd like to see that code generalized in a few ways based on well known
> algorithms rather than the ad-hoc stuff we've got now keeping a
> reasonable knob on the codesize bloat.
>
> In cases where we want more accurate warnings, but aren't willing to
> accept the size bloat, on the side structures might be the way to go.
>
> I'm still looking a literature on the subject, but there's clearly
> things we can do to improve the path sensitivity of the optimizers and
> warning analysis.

There are actually many papers on this (e.g. IMPACT group)  as a
result of research for EPIC optimizations (predication and if
convert). Most of them talk about predicate query systems (efficient
and accurate), e.g PQS using partition graph, PAS using BDD etc.  The
predicates relation can be built as side data structure, and the
following queries can be made:

 p1 = get_predicate (gimple_stmt1);
 p2 = get_predicate (gimple_stmt2);

if (is_disjoint (p1, p2) )
  ..
else if is_subset(p1, p2).
   ..

Keeping the information up to date with transformations can be difficult.

David


>
> jeff
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