[cfe-dev] C++20 Modules, PCM files encoding implementation

[David Blaikie via cfe-dev]

I don't think anyone's actively looking at this right now - perhaps
partly because there's still significant benefit to separating the
interface and implementation, even when using modules (no extraneous
rebuilds when you change the implementation - even if that rebuild
only rebuilds the interface and then you have a hash (rather than
timestamp) based build system that finds the interface to be identical
and so nothing else downstream is touched). Also at least with Clang's
model, I think the idea is to build the object file from the pcm
rather than from the cppm file. Though the possibility of having two
output files has some potential benefits, to be sure - I /think/ maybe
MSVC is doing something more like that two file model, but I don't
know for sure.

On Tue, Nov 17, 2020 at 4:51 AM Büke Beyond via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
>
> Dear LLVM team,
>
> I am new.  Please forgive me for a bit, if I bump into any established protocol.
>
> I was referred to this mailing list to infer about the direction of the C++20 Modules, as we are considering an evolution of our codebase to the feature.
>
> First let me reiterate some appreciation for what your team is doing for the world of computer science with the outstanding and pioneering work on clang.  We have successfully switched our codebase from MSVC to clang for the superior code generation and the most modern features of C++.  We have on average, doubled our compilation speed, tripled our execution speed, and halved the binary size. We have converted to C++20 Concepts from old SFINAE hacks, use direct builtins for modern instructions, and continue to marvel at the excellence of SIMD vectorization.
>
> Our work is focused on exploring algorithmic frontiers in film and pro audio production.  Artistic quality, precision and execution speed are paramount.  This type of  algorithm research and development typically demands fast iteration of the implementation code, often a few formulaic pages of DSP, rapidly changing to meet the speed and quality needs of the production team.  The interface to consuming that code changes much less often.
>
> Our question is about the current encoding of the pcm files generated from the module cppm files.  We envision accelerating and simplifying development by converging most h files and cpp files into single module files.  Currently, clang can compile a cppm file to a pcm file, to be consumed by the module importing code and the code editor enhancement clangd.
>
> The implementation code is inside the module, inplace (not to be confused with the inline keyword for function inlining).  The cppm file can be independently compiled to an object file and normally linked to produce function calls from consumers.
>
> The current naive build systems can use the pcm file as a dependency, when the interface and layout of the classes change inside the module, to trigger efficient recompilations of the consuming code.
>
> However, we have observed that the pcm file is growing in size as the inplace implementation code is growing in size.  We envisioned the pcm would only extract the class interface and the memory layout, but that does not seem to be the case.  Perhaps, it would take more LLVM effort to extract and isolate that from the AST tree.
>
> This of course has the unfortunate side effect of triggering redundant rebuilds of large portions of the codebase, making the iteration times unacceptable versus older conventions.  The older conventions of splitting .h and .cpp files involve repeating yourself, wasting developer focus on simultaneously editing and managing of 2 files at once, and often resorting to messy pimpl techniques that have to heap allocate a backend and manage 2 references throughout the formulas, etc.  Considering these overheads, there are 100s and an ever growing number of small and large plugins (modular effects) that can benefit from convergence to modules as single and succinct files focusing on clean formulation.
>
> Are there any future plans at LLVM, the pcm files may encode the interface only?  Or are there any tools and functions you can recommend to extract the module interface to signal the build system more efficiently?
>
> Thank you for your time.
>
> Sincerely,
> Büke Beyond
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