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

[Büke Beyond via cfe-dev]

Thank you for your fine analysis Mr. Blaikie and Mr. Smith.

Perhaps the most practical and the fastest to implement solution, is to
give clang a -generate-module-interface-hash command that takes in a pcm
file, scans the AST, skips the inplace implementation code and outputs a
simple hash.

Without such a facility, current build systems will be unable to build
efficiently, further delaying the adoption of modules.

There are the common benefits that modules will resolve macro collisions,
but those are not too common or too unsettled in practice.  And that they
will speed up compilation, but real world tests show a slight gain over
previous pch (precompiled header) techniques.

Thus, the feature they can modernize and clean up traditional C++ source
code is perhaps the most significant potential of modules.  Not only can
this accelerate future development for seasoned experts, giving them extra
focus on complexer problems, but also will make the language more
attractive to a wider audience of math, science, and art fields.

My thoughts on the all-in-one pcm files containing compiled machine
instructions and hefty debug symbols are mixed.  Currently, lldb demands
full -gdwarf generation, otherwise it will skip over stepping into the
module calls.  Such heft in the pcm file, may slow down the compilation of
module importing code.  Certainly, there are some savings on the parsing
and the generation of the AST tree from the module source just once,
however the current ability of compiling the modules to the object files
separately and with varying release options seems like a simpler and more
flexible design.  Furthermore, how are the inline functions encoded in the
pcm?  Hopefully they are still in AST form, and not machine instruction
form, so they can be better fused, vectorized and optimized at the usage
site.

With these design considerations, it seems the pcm file encoding will be in
a state of flux for a while, and that is fine.  Even the clangd team is
reporting crashes at loading pcm files of the slightest variation of the
version .

So before the pcm format reaches stability and standardisation one day,
there needs to be something like the above hash facility to complete the
build usability for the primetime adoption of modules.

Microsoft's aging compiler is significantly behind clang at code
generation.  They will likely try to compensate for the gap with modern
usability features like modules.  Currently, Intellisense is failing at
parsing Concepts, even with the recent update promising to do so.
Intellisense is also having significant bugs and problems with modules.
Clangd is almost there, although it is crashing on using templates from
modules on Windows.

Looking forward to hearing your thoughts.

Sincerely,
Büke Beyond


On Tue, Nov 17, 2020 at 5:50 PM Richard Smith <richard at metafoo.co.uk> wrote:

> +Iain Sandoe, who has been looking at C++ Modules implementation issues in
> Clang.
>
> I think it would make sense to be able to emit a "cut-down" pcm file that
> omits information that an importer of the module never needs (such as
> definitions of non-inline functions), in order to keep the file sizes
> smaller. However, that alone won't be enough to avoid rebuilds when the
> .cppm files change -- we also encode source location information in the pcm
> file that would be invalidated whenever implementation details change -- or
> at least whenever they change size. In principle, there are techniques we
> could use here to avoid rebuilds when only those locations change, such as
> splitting the location information out into a separate file that is not
> listed as a dependency of downstream compilations (eg, according to -M),
> but that would need investigation.
>
> Another promising idea that has not been investigated is the possibility
> of generating two different pcm's for each module: one containing only
> cut-down 'forward declaration'-level information (no class definitions, no
> inline function bodies, and so on), and one containing full information.
> The idea would be that we initially load only the cut-down version, and
> pull in the full information (and include the additional file as a
> dependency according to -M) only if the dependent compilation needs that
> information. Then we can avoid rebuilds if (for example) a class definition
> in a module interface changes but the consumers of that module interface
> didn't actually use the class definition.
>
> But I don't think anyone has done any work to implement these approaches.
>
> On Tue, 17 Nov 2020 at 13:46, David Blaikie <dblaikie at gmail.com> wrote:
>
>> 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
>> > _______________________________________________
>> > cfe-dev mailing list
>> > cfe-dev at lists.llvm.org
>> > https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>
>
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