Re: [PATCH] Implement a context aware points-to analyzer for use in evrp.

[Martin Sebor <msebor@gmail.com>]

On 6/7/21 12:29 PM, Aldy Hernandez via Gcc-patches wrote:
> 
> 
> On 6/7/21 3:30 PM, Richard Biener wrote:
>> On Mon, Jun 7, 2021 at 12:10 PM Aldy Hernandez via Gcc-patches
>> <gcc-patches@gcc.gnu.org> wrote:
>>>
>>> The substitute_and_fold_engine which evrp uses is expecting symbolics
>>> from value_of_expr / value_on_edge / etc, which ranger does not provide.
>>> In some cases, these provide important folding cues, as in the case of
>>> aliases for pointers.  For example, legacy evrp may return [&foo, &foo]
>>> for the value of "bar" where bar is on an edge where bar == &foo, or
>>> when bar has been globally set to &foo.  This information is then used
>>> by the subst & fold engine to propagate the known value of bar.
>>>
>>> Currently this is a major source of discrepancies between evrp and
>>> ranger.  Of the 284 cases legacy evrp is getting over ranger, 237 are
>>> for pointer equality as discussed above.
>>>
>>> This patch implements a context aware points-to class which
>>> ranger-evrp can use to query what a pointer is currently pointing to.
>>> With it, we reduce the 284 cases legacy evrp is getting to 47.
>>>
>>> The API for the points-to analyzer is the following:
>>>
>>> class points_to_analyzer
>>> {
>>> public:
>>>    points_to_analyzer (gimple_ranger *r);
>>>    ~points_to_analyzer ();
>>>    void enter (basic_block);
>>>    void leave (basic_block);
>>>    void visit_stmt (gimple *stmt);
>>>    tree get_points_to (tree name) const;
>>> ...
>>> };
>>>
>>> The enter(), leave(), and visit_stmt() methods are meant to be called
>>> from a DOM walk.   At any point throughout the walk, one can call
>>> get_points_to() to get whatever an SSA is pointing to.
>>>
>>> If this class is useful to others, we could place it in a more generic
>>> location.
>>>
>>> Tested on x86-64 Linux with a regular bootstrap/tests and by comparing
>>> EVRP folds over ranger before and after this patch.
>>
>> Hmm, but why call it "points-to" - when I look at the implementation
>> it's really about equivalences.  Thus,
>>
>>   if (var1_2 == var2_3)
>>
>> could be handled the same way.  Also "points-to" implies (to me)
>> that &p[1] and &p[2] point to the same object but your points-to
>> is clearly tracking equivalences only.
>>
>> So maybe at least rename it to pointer_equiv_analyzer?  ISTR
> 
> Good point.  Renaming done.  I've adjusted the changelog and commit 
> message as well.

Mostly just a question of the type choices in the implementation
of the ssa_equiv_stack class: m_stack is an auto_vec while
m_replacements is a plain array.  I'd expect both to be the same
(auto_vec).  Is there a reason for this choice?

If not, I'd suggest to use auto_vec.  That way the ssa_equiv_stack
class shouldn't need a dtor (auto_vec doesn't need to be explicitly
released before it's destroyed), though it should delete its copy
and assignment.

Martin

> 
> Thanks.
> Aldy