[cfe-dev] Flag for generating LLVM IR from forward declarations

[Alex L via cfe-dev]

On 30 November 2016 at 13:51, Gianluca Stivan <me at yawnt.com> wrote:

> @mats Yeah, you're right, an LLVM Pass seems to be the correct approach
> here, however Alex is bringing up a fair point
>
> @Alex
>
> I was just checking by compiling the header files for libgit2 (in my clang
> fork) and you seem to be correct, there are some cases where informations
> are lost. Take the following example:
>
> -------------------------------------------------
> struct xyz { int x; int y; int z; } xyz;
>
> struct xyz fill(int x, int y, int z);
>
> void pointer(struct xyz* d);
> -------------------------------------------------
>
> I do have correct informations for `pointer` (LLVM returns `declare void
> @pointer(%struct.xyz*)`), but not for `fill` (LLVM returns `declare {
> i64, i32 } @fill(i32, i32, i32)`). I suppose there's no way to get those
> informations back as AST -> IR is a lossy transformation.
>
> If I had some way to link the generated `declare` back to the function AST
> node, that would obviously help in generating my frontend stub. But even
> so, I would still need to follow the function's signature to get the
> parameters' type definition, and so on. I'm beginning to wonder if it's
> just more sane to use libclang, after all.
>

Yeah, the lowering to the IR by clang is lossy, and AFAIK there's no way to
go in the reverse direction. Maybe the debug information could allow you to
reverse the layout? In any case I think that's doing this reverse approach
isn't worth it if you can use clang directly.

Btw, there has been discussion recently about making a separate library
that handles this transformation process:
http://lists.llvm.org/pipermail/llvm-dev/2016-October/106660.html.


>
>
>
> 2016-11-30 12:12 GMT+00:00 Alex L <arphaman at gmail.com>:
>
>>
>>
>> On 30 November 2016 at 11:32, Gianluca Stivan <me at yawnt.com> wrote:
>>
>>> True,
>>>
>>> I was toying with the idea of having a separate step in the pipeline
>>> that plugs in after AST is turned into a Module[*] which generates frontend
>>> code.
>>> But to do so, I would have to have forward declarations be actually
>>> emitted, hence my proposal. Otherwise I'd just be iterating over an empty
>>> set of instructions :)
>>> My theory is that LLVM IR at that point is going to be way more
>>> straightforward (just basic types, structs and `declare @..`s) to deal with.
>>> Could this approach make sense?
>>>
>>
>> I see, so are you thinking about basically looking at the LLVM IR with
>> the C function declarations and generating the calls based on the IR
>> declarations yourself instead or relying on clang? I suppose that could
>> work, but I'm not 100% sure that it will be correct in all cases. How will
>> your front-end deal with functions that take and return C aggregate types?
>>
>>
>>>
>>> Cheers,
>>>
>>> -yawnt
>>>
>>> [*] a Clang plugin? Can they be run over Modules, that is LLVM IR,
>>> instead of AST?
>>>
>>>
>>> 2016-11-30 10:59 GMT+00:00 Alex L <arphaman at gmail.com>:
>>>
>>>>
>>>>
>>>> On 30 November 2016 at 09:12, Gianluca Stivan via cfe-dev <
>>>> cfe-dev at lists.llvm.org> wrote:
>>>>
>>>>> Hi everyone,
>>>>>
>>>>> I was playing around with the LLVM toolchain and it occurred to me
>>>>> that, should I decide to target LLVM IR, it would be neat to have Clang
>>>>> generate declarations from C header files for easier FFI. Someone seems to
>>>>> have had my same idea [1] [2]. This is kind of the opposite of what other
>>>>> projects have gone for, where they use libclang to parse header files to
>>>>> achieve the same result [3].
>>>>>
>>>>
>>>> Sounds like an interesting idea. I how one question though, if you're
>>>> generating only the C function declarations in LLVM IR, how will your
>>>> frontend that targets LLVM IR call these declarations? Maybe I'm
>>>> misunderstanding something, but I don't see how can you generate the calls
>>>> to these functions without getting clang involved.
>>>>
>>>> Cheers,
>>>> Alex
>>>>
>>>>
>>>>>
>>>>> Now, forward declaration generation is doable by patching
>>>>> CodeGenModule.cpp (I kinda have a PoC working locally, except it generates
>>>>> *all* the forward declarations, instead of just the ones from the header
>>>>> files I care about)
>>>>>
>>>>> Is this something that you guys would be interested in merging into
>>>>> Clang through a flag? Or what would the proper way to do this be?
>>>>> I saw there's a `-femit-all-decls`, but it doesn't really emit all
>>>>> declarations.
>>>>>
>>>>> Apologies if this is a dumb idea, kinda new to the whole Clang thing
>>>>> :)!
>>>>>
>>>>> Cheers,
>>>>>
>>>>> -yawnt
>>>>>
>>>>> [1] http://stackoverflow.com/questions/24728901/clang-compiling-
>>>>> a-c-header-to-llvm-ir-bitcode
>>>>> [2] http://stackoverflow.com/questions/14032496/how-can-i-co
>>>>> de-generate-unused-declarations-with-clang?noredirect=1&lq=1
>>>>> [3] https://github.com/tjfontaine/node-ffi-generate
>>>>>
>>>>> _______________________________________________
>>>>> cfe-dev mailing list
>>>>> cfe-dev at lists.llvm.org
>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>>>
>>>>>
>>>>
>>>
>>
>
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