[cfe-dev] Separate preprocess and compile: hack or feature?

[Nico Weber via cfe-dev]

Most distributed build systems I know about end up writing their own custom
preprocessor to very quickly discover which .h files are included by a cc
file (you don't need a full preprocessor for getting just that, and so you
can be faster than clang -E), and then send .h and .cc files to the server
based on content hashes, so that you don't need to send the full
preprocessed text, but can send source files before preprocessing.
https://github.com/facebookarchive/warp was a somewhat recent example of
this (but also, as you say, pump mode, and proprietary systems). (Your
thread mentions that you do this for -M / /showIncludes, but you can just
do this as part of regular compilation – not sure why you need this in a
separate process?)

So while this doesn't answer your question, I'd expect that you won't need
it, eventually :-)

On Thu, May 11, 2017 at 5:05 AM, Boris Kolpackov via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hi,
>
> In the build system I am working on we are looking at always performing
> the preprocessing and then C/C++ compilation as two separate clang/clang++
> invocations. The main reason is support for distributed compilation but
> see here[1] for other reasons.
>
> I realize that tools like ccache/distcc have been relying on this for
> a while (though see the 'direct' mode in ccache and 'pump' in distcc).
> However, some compilers apparently do not support this (for example,
> VC; see the above link for details).
>
> So I wonder, in the context of Clang, if this is just a hack that
> happens to work "for now" or if this is a feature that is expected
> to continue to work?
>
> Also, has anyone seen/heard of any real-world issues with compiling
> preprocessed source code?
>
> [1] https://www.reddit.com/r/cpp/comments/6abi99/rfc_issues_
> with_separate_preprocess_and_compile/
>
>
> Thanks,
> Boris
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