Re: Question about IPA-PTA and build_alias

[Erick Ochoa <erick.ochoa@theobroma-systems.com>]

On 24/08/2020 09:40, Richard Biener wrote:
> On Mon, Aug 17, 2020 at 3:22 PM Erick Ochoa
> <erick.ochoa@theobroma-systems.com> wrote:
>>
>> Hello,
>>
>> I'm looking to understand better the points-to analysis (IPA-PTA) and
>> the alias analysis (build_alias).
>>
>> How is the information produced by IPA-PTA consumed?
>>
>> Are alias sets in build_alias computed by the intersections of the
>> points_to_set(s) (computed by IPA-PTA)?
>>
>> My intuition tells me that it could be relatively simple to move
>> build_alias to be an SIMPLE_IPA_PASS performed just after IPA-PTA, but I
>> do not have enough experience in GCC to tell if this is correct. What
>> could be some difficulties which I am not seeing? (Either move, or
>> create a new IPA-ALIAS SIMPLE_IPA_PASS.) This pass would have the same
>> sensitivity as IPA-PTA { flow-insensitive, context-insensitive,
>> field-sensitive } because the alias sets could be computed by the
>> intersection of points-to-sets.
> 
> Both IPA-PTA and build_alias do the same, they build PTA constraint
> sets, solve them and attach points-to info to SSA names.  Just IPA-PTA
> does this for the whole TU while build_alias does it for a function at a time.
> 
> So I guess I do not understand your question.

Hi Richard,

I'm just trying to imagine what a data-layout optimization would look 
like if instead of using the type-escape analysis we used the points-to 
analysis to find out which variables/memory locations escape and what 
that would mean for the transformation itself.

One of the things that I think would be needed are alias-sets. I thought 
that build_alias was building alias sets but I was mistaken. However, 
computing the alias sets should not be too difficult.

Also continuing imagining what a data-layout optimization would look 
like in GCC, since IPA-PTA is a SIMPLE_IPA_PASS and if alias sets are 
indeed needed, I was asking what would be the reception to a 
SIMPLE_IPA_PASS that computes alias sets just after IPA-PTA. (As opposed 
to a full ipa pass).



> 
> Richard.
> 
>>
>> Thanks!