fix for pr47837

[Richard Guenther]

On Fri, Mar 11, 2011 at 8:50 PM, 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.
>
> 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.

Yes, I am also thinking of static analysis stuff people seem to want.
That requires us to not actually do any transform.

Richard.