Re: [PATCH] libgccjit: Fix get_size of size_t

[Antoni Boucher <bouanto@zoho.com>]

Le 2024-06-26 à 18 h 01, David Malcolm a écrit :
> On Wed, 2024-02-21 at 14:16 -0500, Antoni Boucher wrote:
>> On Thu, 2023-12-07 at 19:57 -0500, David Malcolm wrote:
>>> On Thu, 2023-12-07 at 17:26 -0500, Antoni Boucher wrote:
>>>> Hi.
>>>> This patch fixes getting the size of size_t (bug 112910).
>>>>
>>>> There's one issue with this patch: like every other feature that
>>>> checks
>>>> for target-specific stuff, it requires a compilation before
>>>> actually
>>>> fetching the size of the type.
>>>> Which means that getting the size before a compilation might be
>>>> wrong
>>>> (and I actually believe is wrong on x86-64).
>>>>
>>>> I was wondering if we should always implicitely do the first
>>>> compilation to gather the correct info: this would fix this issue
>>>> and
>>>> all the others that we have due to that.
>>>> I'm not sure what would be the performance implication.
>>>
>>> Maybe introduce a new class target_info which contains all the
>>> information we might want to find via a compilation, and have the
>>> top-
>>> level recording::context have a pointer to it, which starts as
>>> nullptr,
>>> but can be populated on-demand the first time something needs it?
>>
>> That would mean that we'll need to populate it for every top-level
>> context, right? Would the idea be that we should then use child
>> contexts to have the proper information filled?
>> If so, how is this different than just compiling two contexts like
>> what
>> I currently do?
>> This would also mean that we'll do an implicit compilation whenever
>> we
>> use an API that needs this info, right? Wouldn't that be unexpected?
> 
> I was thinking a compilation with an empty playback::context to lazily
> capture the target data.

I'm still not sure I understand what you mean.
Do you mean having a global context that we can compile to then fetch 
the size of the types?
If not, could you please provide an example with some code?

I'm wondering if we could have something that would also work for custom 
types like structs.
I'm also not sure what would happen if options that change the size of 
types (like -m32) are provided by the user.

Is the way libgccjit currently work (with 2 phases: recording and 
playback) this way because gcc is not thread-safe?
If we could directly create GENERIC trees, we could get the size from 
those, but it seems like this would not be possible.

> 
> My hope was that this would make things easier for users.  But you're
> the one using this API, so if you're more comfortable with the explicit
> initial compilation approach, let's go with that.
> 
> If so, this is OK for trunk - but we might want to add a note to the
> documentation about the double-compilation workaround.
> 
> Dave
> 
> 
>>
>> Thanks for the idea.
>>
>>>
>>>>
>>>> Another solution that I have been thinking about for a while now
>>>> would
>>>> be to have another frontend libgccaot (I don't like that name),
>>>> which
>>>> is like libgccjit but removes the JIT part so that we get access
>>>> to
>>>> the
>>>> target stuff directly and would remove the need for having a
>>>> seperation
>>>> between recording and playback as far as I understand.
>>>> That's a long-term solution, but I wanted to share the idea now
>>>> and
>>>> gather your thoughts on that.
>>>
>>> FWIW the initial version of libgccjit didn't have a split between
>>> recording and playback; instead the client code had to pass in a
>>> callback to call into the various API functions (creating tree
>>> nodes).
>>> See:
>>> https://gcc.gnu.org/legacy-ml/gcc-patches/2013-10/msg00228.html
>>>
>>> Dave
>>>
>>
>