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

[Antoni Boucher <bouanto@zoho.com>]

David: Ping.

Le 2024-04-26 à 09 h 51, Antoni Boucher a écrit :
> Now that we have a more general way to check if target-dependent types 
> are supported (see this commit: 
> https://github.com/rust-lang/gcc/commit/1c9a9b2f1fd914cad911467ec1d29f158643c2ce#diff-018089519ab2b14a34313ded0ae1a2f9fcab5f7bcb2fa31f147e1dc757bbdd7aR4016), perhaps we should remove gcc_jit_target_info_supports_128bit_int from this patch, or change it to include the more general way.
> 
> David, what are your thoughts on this?
> 
> Le 2024-04-19 à 08 h 34, Antoni Boucher a écrit :
>> 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
>>>>>>>>
>>>>>>>
>>>>>>