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Add -fdirectives-only for gcc to speed up preprocessor

mbitsnbites requested to merge github/fork/roboteacher1/use-gcc-fdirectives-only into master

Created by: roboteacher1

Also adds -frewrite-includes for clang

This can drastically reduce the amount of work needed to be done by the preprocessor by limiting the preprocessor to only a subset of the directives and avoiding many macro expansions. #include and #define and #ifdef are still resolved

On a large codebase with thousands of cpp files, this sped up local, warm, cache hits 4x when in preprocessor mode (direct mode is still much faster)

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