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Re: [RFA] [tree-optimization/80576] Handle non-constant sizes in DSE


On Fri, Aug 23, 2019 at 9:19 PM Jeff Law <law@redhat.com> wrote:
>
> On 8/22/19 4:46 AM, Richard Biener wrote:
> >>> Also you seem to use this info to constrain optimization when you
> >>> might remember that types of addresses do not carry such information...
> >>> Thus it should be "trivially" possible to write a testcase that is miscompiled
> >>> after your patch.  I also don't see this really exercised in the
> >>> testcases you add?
> >> Arggh.  You're absolutely correct.  I must be blocking out that entire
> >> discussion from last summer due to the trama :-)
> >>
> >> If the destination is the address of a _DECL node, can we use the size
> >> of the _DECL?
> >
> > Yes, but this should already happen for both invariant ones like &a.b.c
> > and variant ones like &a.b[i].c in ao_ref_init_from_ptr_and_size.
> I don't see that in ao_ref_init_from_ptr_and_size.  AFAICT if you don't
> know the size when you call that routine (size == NULL), then you end up
> with the ref->size and ref->max_size set to -1.
>
> Am I missing something here?

Ah, of course.  ao_ref_from_ptr_and_size would need to be extended
to constrain max_size.  So what I was
saying is that ao_ref_init_from_ptr_and_size should get you
a DECL ao_ref_base () from which you could constrain max_size with.
Or rather ao_ref_from_ptr_and_size should be extended do that,
mimicing what get_ref_base_and_extent does at the end in the
if (DECL_P (exp)) case (mind flag_unconstrained_commons!).

Richard.

>
> Jeff


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