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

[Antoni Boucher]

Hi.
Thanks for the review, David!

I talked to Arthur and he's OK with having a file to include in both 
gccrs and libgccjit.

I sent the patch to gccrs to move the code in a new file that we can 
include in both frontends: https://github.com/Rust-GCC/gccrs/pull/3195

I also renamed gcc_jit_target_info_supports_128bit_int to 
gcc_jit_target_info_supports_target_dependent_type because a subsequent 
patch will allow to check if other types are supported like _Float16 and 
_Float128.

Here's the patch for libgccjit updated to include this file.

Thanks.

Le 2024-06-26 à 17 h 55, David Malcolm a écrit :
> On Sun, 2024-03-10 at 12:05 +0100, Iain Buclaw wrote:
>> Excerpts from David Malcolm's message of März 5, 2024 4:09 pm:
>>> 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.
>>>
>>
>> That's certainly the case with the configure/make rules. Itself I
>> think
>> is copied originally from the {cpu_type}-protos.h machinery.
>>
>> It might be worth pointing out that the c-family of front-ends don't
>> have separate headers because their per-target macros are defined in
>> {cpu_type}.h directly - for better or worse.
>>
>>> 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).
>>>
>>
>> As far as I understand, the configure parts should all be identical
>> between tm_p, tm_d, tm_rust, ..., so would benefit from being
>> templated
>> to aid any other front-ends adding in their own per target hooks.
>>
>>> 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 don't know whether libgccjit can just pull in directly the
>> implementation of the rust target hooks here.
> 
> Sorry for the delay in responding.
> 
> I don't want to be in the business of maintaining a copy of the per-
> target code for "jit", and I think it makes sense for libgccjit to
> return identical information compared to gccrs.
> 
> So I think it would be ideal for jit to share code with rust for this,
> rather than do a one-time copy-and-paste followed by a ongoing "keep
> things updated" treadmill.
> 
> Presumably there would be Makefile.in issues given that e.g. Makefile
> has i386-rust.o listed in:
> 
> # Target specific, Rust specific object file
> RUST_TARGET_OBJS= i386-rust.o linux-rust.o
> 
> One approach might be to move e.g. the i386-rust.cc code into, say,  a
> i386-rust-and-jit.inc file and #include it from i386-rust.cc and i386-
> jit.cc (with a similar approach for other targets)
> 
> Antoni, Arthur, would you each be OK with that?
> 
> 
>>    The per-frontend target
>> hooks usually also make use of code specific to that front-end -
>> TARGET_CPU_CPP_BUILTINS and others can't be used by a non-c-family
>> front-end without adding a plethora of stubs, for example.
>>
>> Whether or not libgccjit wants to give identical information as as
>> rust
>> I think is a decision for you as the maintainer of its API.
> 
> Also a question for Antoni and Arthur: is that desirable for the two
> gcc rust approaches?
> 
> Thanks
> Dave
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