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

[Iain Buclaw <ibuclaw@gdcproject.org>]

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.  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.

Iain.