[llvm-dev] [cfe-dev] Clang/LLVM function ABI lowering (was: Re: [RFC] Refactor Clang: move frontend/driver/diagnostics code to LLVM)

Wed Jun 3 21:54:17 PDT 2020

While MLIR may be one part of the solution, I think it's also the case that
the function-ABI interface between Clang and LLVM is just wrong and should
be fixed -- independently of whether Clang might use MLIR in the future.

I've mentioned this idea before, I think, but never got around to writing
up a real proposal. And I still haven't. Maybe this email could inspire
someone else to work on that.

Essentially, I'd like to see the code in Clang responsible for function
parameter-type mangling as part of its ABI lowering deleted. Currently,
there is a secret "LLVM IR" ABI used between Clang and LLVM, which involves
expanding some arguments into multiple arguments, adding a smattering of
"inreg" or "byval" attributes, and converting some types into other types.
All in a completely target-dependent, complex, and undocumented manner.

So, while the IR function syntax appears at first glance to be generic and
target-independent, that's not at all true. Sadly, in some cases, clang
must even know how many registers different calling conventions use, and
count numbers of available registers left, in order to choose the right set
of those "generic" attributes to put on a parameter.

So: not only does a frontend need to understand the C ABI rules, they also
need to understand that complex dance for how to convert that into LLVM IR
-- and that's both completely undocumented, and a huge mess.

Instead, I believe clang should always pass function parameters in a
"naive" fashion. E.g. if a parameter type is "struct X", the llvm function
should be lowered to LLVM IR with a function parameter of type %struct.X.
The decision on whether to then pass that in a register (or multiple
registers), on the stack, padded and then passed on the stack, etc, should
be the responsibility of LLVM. Only in the case of C++ types which *must* be
passed indirectly for correctness, independent of calling convention ABI,
should clang be explicitly making the decision to pass indirectly.

Of course, the tricky part is that LLVM doesn't -- and shouldn't -- have
the full C type system available to it, and the full C type system
typically is required to evaluate the ABI rules (e.g., distinguishing a
"_Complex float" from a struct containing two floats).

Therefore, in order to communicate the correct ABI information to LLVM, I'd
like clang to also emit *explicitly-ABI-specific* data (metadata?),
reflecting the extra information that the ABI rules require the backend to
know about the type. E.g., for X86_64, clang needs to inform LLVM of the
classification for each parameter's type into MEMORY, INTEGER, SSE, SSEUP,
X87, X87UP, COMPLEX_X87. Or, for PPC64 elfv2, Clang needs to inform LLVM
when a structure should be treated as a "homogenous aggregate" of
floating-point or vector type. (In both cases, that information cannot
correctly be extracted from the LLVM IR struct type, only from the C type
system.)

We should document what data is needed, for each architecture/abi. This
required data should be as straightforward an application of the ABI
document's rules as possible -- and be only the minimum data necessary.

If this is done, frontends (either a new one, or Clang itself) who want to
use the C ABI have a significantly simpler task. It remains non-trivial --
you do still need to understand ABI-specific rules, and write ABI-specific
code to generate ABI-specific metadata. But, at least the interface
boundary has become something which is readily-understandable and
implementable based on the ABI documents.

All that said, an MLIR encoding of the C type system can still be useful --
it could contain the code which distills the C types into the ABI-specific
metadata. But, I  see that as less important than getting the fundamentals
in LLVM-IR into a better shape. Even frontends without a C type system
representation should still be able to generate LLVM IR which conforms in
their own manner to the documented ABIs -- without it being super painful.
Also, the code in Clang now is really confusing, and nearly unmaintainable;
it would be a clear improvement to be able to eliminate the majority of it,
not just move it into an MLIR dialect.

On Wed, Jun 3, 2020 at 7:26 PM Chris Lattner via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> On Jun 2, 2020, at 4:21 PM, comex via cfe-dev <cfe-dev at lists.llvm.org>
> wrote:
>
> While this is a different area of the codebase, another thing that
> would benefit greatly from being moved out of Clang is function call
> ABI handling.  Currently, that handling is split awkwardly between
> Clang and LLVM proper, forcing frontends that implement C FFI to
> either recreate the Clang parts themselves (like Rust does), depend on
> Clang (like Swift does), or live with FFI just not working with some
> function signatures.  I'm not sure what Flang currently does, but my
> understanding is that Flang does support C FFI, so it would probably
> benefit from this as well.  Just something to consider. :)
>
>
> For what its worth, I think there is a pretty clear path on this, but it
> hinges on Clang moving to MLIR as its code generation backend (an
> intermediary to generating LLVM IR).
>
> The approach is to factor the ABI lower part of clang out of Clang itself
> into a specific dialect lowering pass, that works on a generic C type
> system (plus callout to extended type systems).  MLIR has all the infra to
> support this, it is just a massive job to refactor all the things to change
> clang’s architecture.
>
> I also don’t think there is broad consensus on the direction for Clang
> here, but given that Flang is already using MLIR for this, maybe it would
> make sense to start work there.
>
> If you’re curious, I co-delivered a talk about this recently, the slides are
> available here
> <https://docs.google.com/presentation/d/11-VjSNNNJoRhPlLxFgvtb909it1WNdxTnQFipryfAPU/edit#slide=id.g7d334b12e5_0_4>
> .
>
> -Chris
>
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