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


On 8/26/19 3:00 AM, Richard Biener wrote:
> 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!).
So I was going to use get_ref_base_and_extent from within
ao_ref_init_from_ptr_and_size to capture these cases, but
get_ref_base_and_extent internally uses TYPE_SIZE to get the maximum
size of the referenced object.

That likely represents a codegen bug waiting to happen.

I'll see if I can refactor just the bits we want so that we're not
duplicating anything.

Jeff


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