Re: Frontend access to target features (was Re: [PATCH] libgccjit: Add ability to get CPU features)

[Antoni Boucher <bouanto@zoho.com>]

David: Ping.

Le 2024-04-09 à 09 h 21, Antoni Boucher a écrit :
> David: Ping.
> 
> Le 2024-04-01 à 08 h 20, Antoni Boucher a écrit :
>> David: Ping.
>>
>> Le 2024-03-19 à 07 h 03, Arthur Cohen a écrit :
>>> Hi,
>>>
>>> On 3/5/24 16:09, David Malcolm wrote:
>>>> On Thu, 2023-11-09 at 19:33 -0500, Antoni Boucher wrote:
>>>>> Hi.
>>>>> See answers below.
>>>>>
>>>>> On Thu, 2023-11-09 at 18:04 -0500, David Malcolm wrote:
>>>>>> On Thu, 2023-11-09 at 17:27 -0500, Antoni Boucher wrote:
>>>>>>> Hi.
>>>>>>> This patch adds support for getting the CPU features in libgccjit
>>>>>>> (bug
>>>>>>> 112466)
>>>>>>>
>>>>>>> There's a TODO in the test:
>>>>>>> I'm not sure how to test that gcc_jit_target_info_arch returns
>>>>>>> the
>>>>>>> correct value since it is dependant on the CPU.
>>>>>>> Any idea on how to improve this?
>>>>>>>
>>>>>>> Also, I created a CStringHash to be able to have a
>>>>>>> std::unordered_set<const char *>. Is there any built-in way of
>>>>>>> doing
>>>>>>> this?
>>>>>>
>>>>>> Thanks for the patch.
>>>>>>
>>>>>> Some high-level questions:
>>>>>>
>>>>>> Is this specifically about detecting capabilities of the host that
>>>>>> libgccjit is currently running on? or how the target was configured
>>>>>> when libgccjit was built?
>>>>>
>>>>> I'm less sure about this part. I'll need to do more tests.
>>>>>
>>>>>>
>>>>>> One of the benefits of libgccjit is that, in theory, we support all
>>>>>> of
>>>>>> the targets that GCC already supports.  Does this patch change
>>>>>> that,
>>>>>> or
>>>>>> is this more about giving client code the ability to determine
>>>>>> capabilities of the specific host being compiled for?
>>>>>
>>>>> This should not change that. If it does, this is a bug.
>>>>>
>>>>>>
>>>>>> I'm nervous about having per-target jit code.  Presumably there's a
>>>>>> reason that we can't reuse existing target logic here - can you
>>>>>> please
>>>>>> describe what the problem is.  I see that the ChangeLog has:
>>>>>>
>>>>>>>          * config/i386/i386-jit.cc: New file.
>>>>>>
>>>>>> where i386-jit.cc has almost 200 lines of nontrivial code.  Where
>>>>>> did
>>>>>> this come from?  Did you base it on existing code in our source
>>>>>> tree,
>>>>>> making modifications to fit the new internal API, or did you write
>>>>>> it
>>>>>> from scratch?  In either case, how onerous would this be for other
>>>>>> targets?
>>>>>
>>>>> This was mostly copied from the same code done for the Rust and D
>>>>> frontends.
>>>>> See this commit and the following:
>>>>> https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=b1c06fd9723453dd2b2ec306684cb806dc2b4fbb
>>>>> The equivalent to i386-jit.cc is there:
>>>>> https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=22e3557e2d52f129f2bbfdc98688b945dba28dc9
>>>>
>>>> [CCing Iain and Arthur re those patches; for reference, the patch being
>>>> discussed is attached to :
>>>> https://gcc.gnu.org/pipermail/jit/2024q1/001792.html ]
>>>>
>>>> One of my concerns about this patch is that we seem to be gaining code
>>>> that's per-(frontend x config) which seems to be copied and pasted with
>>>> a search and replace, which could lead to an M*N explosion.
>>>
>>> I think this is definitely already the case, and it would be worth 
>>> investigating if C/C++/Rust/jit can reuse a similar set of target 
>>> files, or how to factor them together. I imagine that all of these 
>>> components share similar needs for the targets they support.
>>>
>>>>
>>>> Is there any real difference between the per-config code for the
>>>> different frontends, or should there be a general "enumerate all
>>>> features of the target" hook that's independent of the frontend? (but
>>>> perhaps calls into it).
>>>>
>>>> Am I right in thinking that (rustc with default LLVM backend) has some
>>>> set of feature strings that both (rustc with rustc_codegen_gcc) and
>>>> gccrs are trying to emulate?  If so, is it presumably a goal that
>>>> libgccjit gives identical results to gccrs?  If so, would it be crazy
>>>> for libgccjit to consume e.g. config/i386/i386-rust.cc ?
>>>
>>> I think this would definitely make sense, and it could probably be 
>>> extended to other frontends. For the time being I think it makes 
>>> sense to try it out for gccrs and jit. But finding a fitting name 
>>> will be hard :)
>>>
>>> Best,
>>>
>>> Arthur
>>>
>>>>
>>>> Dave
>>>>
>>>>>
>>>>>>
>>>>>> I'm not at expert at target hooks (or at the i386 backend), so if
>>>>>> we
>>>>>> do
>>>>>> go with this approach I'd want someone else to review those parts
>>>>>> of
>>>>>> the patch.
>>>>>>
>>>>>> Have you verified that GCC builds with this patch with jit *not*
>>>>>> enabled in the enabled languages?
>>>>>
>>>>> I will do.
>>>>>
>>>>>>
>>>>>> [...snip...]
>>>>>>
>>>>>> A nitpick:
>>>>>>
>>>>>>> +.. function:: const char * \
>>>>>>> +              gcc_jit_target_info_arch (gcc_jit_target_info
>>>>>>> *info)
>>>>>>> +
>>>>>>> +   Get the architecture of the currently running CPU.
>>>>>>
>>>>>> What does this string look like?
>>>>>> How long does the pointer remain valid?
>>>>>
>>>>> It's the march string, like "znver2", for instance.
>>>>> It remains valid until we free the gcc_jit_target_info object.
>>>>>
>>>>>>
>>>>>> Thanks again; hope the above makes sense
>>>>>> Dave
>>>>>>
>>>>>
>>>>