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

[Antoni Boucher <bouanto@zoho.com>]

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
>>>>>>>
>>>>>>
>>>>>