[libcxx-commits] [PATCH] D157298: [RFC][libc++] Reduce the size of translation units

[Mark de Wever via Phabricator via libcxx-commits]

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The C++ standard library adds new features in every C++ version. To
implement these new features libc++ adds more includes to a header. This
means the number of transitive includes grows with newer language version.

The growing size is an issue for users. This change is motivated by a
report of the Chromium team where properly enabling C++23's
std::formatter<std::vector<bool>::reference> had a significant impact on
their build time.

In D149543 <https://reviews.llvm.org/D149543> Hans Wennborg reported the following

  In Chromium we noticed that this almost doubled the preprocessed size
  of <vector>, from ca 1.6 MB to 3.2 MB. Since it's a widely included
  header, that results in ca 8 GB (2.5%) of extra code to compile during a
  full build.

This is an experiment to see how libc++ can reduce this size by only
"enabling" headers per language version. This is mainly done at the level
of the granularized includes, since they often have features for one or a
limited set of minimum language versions.

The compare header is an exception, its inclusion is often mandated by the
standard so this header is entirely guarded by C++20 or newer.

At the moment the removed transitive includes are not conditionally
restored like we usually do; this is intentional for the RFC. It gives a
good view what changes.

For this patch only vector is adjusted and mainly based on what I recall
being based on a language version.

I did some testing with the size of a preprocessed file that contains

  #include <vector>

I used the following command

  clang++ -E test.cpp -nostdinc++ -I <build>/include/c++/v1" -I -std=$std |wc -l

| Version                    | Before | After |
| ---------+--------+------- |
| C++03                      | 53153  | 38721 |
| C++11                      | 55665  | 40914 |
| C++14                      | 56709  | 41896 |
| C++17                      | 60591  | 44957 |
| C++20                      | 75905  | 75863 |
| C++23                      | 61867  | 61828 |
| C++26                      | 61867  | 61828 |
|

The interesting value is C++23 (before) there the size drops a lot. In
C++23 some transitive includes are removed unconditionally. These can be
removed with a build option. The Chromimum team uses this option. This is
done by the following command

  clang++ -E test.cpp -nostdinc++ -I <build>/include/c++/v1" -I -std=$std -D_LIBCPP_REMOVE_TRANSITIVE_INCLUDES |wc -l

| Version                    | Before | After |
| ---------+--------+------- |
| C++03                      | 45883  | 18482 |
| C++11                      | 48366  | 19352 |
| C++14                      | 49352  | 19745 |
| C++17                      | 51590  | 22331 |
| C++20                      | 61259  | 61217 |
| C++23                      | 61867  | 61828 |
| C++26                      | 61867  | 61828 |


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D157298

Files:
  libcxx/include/__algorithm/lexicographical_compare_three_way.h
  libcxx/include/__concepts/same_as.h
  libcxx/include/__format/enable_insertable.h
  libcxx/include/__format/formatter.h
  libcxx/include/__format/formatter_bool.h
  libcxx/include/__memory_resource/polymorphic_allocator.h
  libcxx/include/__ranges/access.h
  libcxx/include/__ranges/concepts.h
  libcxx/include/__ranges/container_compatible_range.h
  libcxx/include/__ranges/from_range.h
  libcxx/include/__ranges/size.h
  libcxx/include/compare
  libcxx/test/libcxx/transitive_includes/cxx03.csv
  libcxx/test/libcxx/transitive_includes/cxx11.csv
  libcxx/test/libcxx/transitive_includes/cxx14.csv
  libcxx/test/libcxx/transitive_includes/cxx17.csv

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