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Subject: Re: [boost] [preprocessing] Feedback requested on a C99 preprocessor written in pure universal Python
From: Edward Diener (eldiener_at_[hidden])
Date: 2017-03-07 15:38:21

On 3/7/2017 5:55 AM, Niall Douglas via Boost wrote:
>>> Given that preprocessor checks are often used for compiler
>>> workarounds, and pcpp is not a full C++ frontend, one would have to
>>> make sure pcpp defines the same set of predefined macros the compiler
>>> does. In the particular case of MSVC that would make libraries like
>>> Boost.PP and Boost.VMD treat pcpp the same way they do MSVC, which is
>>> probably suboptimal, if at all functional. I guess, for such tandem to
>>> be workable, pcpp has to define its own predefined macros, and PP and
>>> VMD have to test it before they test other compiler-specific macros.
> You've struck exactly at the main use case for pcpp, and why I didn't
> start from Wave or an existing preprocessor but instead reinvented the
> wheel and in Python, not C. I specifically am implementing a "partially
> executing" pre-preprocessor which can be programmatically and
> dynamically instructed to transform a set of input files with
> preprocessing commands into other files with some of those preprocessor
> commands executed or expanded or replaced, and some passed through.
> pcpp by default acts as a straight preprocessor, but it can also be told
> to pass through #if logic it can't fully execute due to unknowns, or
> partially execute #if logic. It can be told to pass through #define and
> #undef but also execute them (or not) on a per-macro basis. It would be
> easy enough to tell it to not execute any preprocessing commands except
> #include and include guards for example.
> I'm sure you can see the big benefits to pregenerating canned
> preprocessed files such that #including a Boost library is much faster
> than before because most of the preprocessing work is already done.
> Right now generating those is tedious work using hacky scripts on source
> files with splicing metadata injected via comments etc which are
> brittle. pcpp will allow for a FAR more robust solution which can be
> safely left to a CI to run per commit if desired.
> pcpp is basically Facebook's Warp
> ( but done much more flexibly
> and usefully (no offence intended to Warp's developers, but Warp isn't
> very useful outside a very limited use case).
> My ideal end goal is for a download page for a Boost library to provide
> a set of tick boxes and drop down menus that let a user pre-preprocess a
> Boost library into a custom "drop in and go" single file edition for
> their particular use case, just like you can with say jQuery downloads.
> Again choosing Python instead of C makes that safe and secure. I don't
> know if I'll have the time to reach that, but I'm an awful lot closer
> now than I was a month ago.

The practical problem with this is that source files with preprocessor
directives often depend on the compiler being used, with its predefined
macros, to generate the correct output.

> snip...
> Niall

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