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Implicit HeaderMaps #3712

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wants to merge 3 commits into from
Closed

Implicit HeaderMaps #3712

wants to merge 3 commits into from

Commits on Mar 19, 2018

  1. Implicit HeaderMaps 

    Request for feedback on an implementation of C++ HeaderMaps in Bazel.

A HeaderMap is a data structure that allows the compiler to lookup included
headers in constant time.

Traditionally, the compiler has to parse a large string of `iquote` includes,
and then search these directories for a given header. This is slow for many
reasons.

The protocol of HeaderMap is implemented within compilers. Please find the
Lexer implementation in Clang.
https://clang.llvm.org/doxygen/HeaderMapTypes_8h.html
https://clang.llvm.org/doxygen/HeaderMap_8cpp_source.html

Use case:

I'm seeing a massive increase in build performance by using this. It cut my
clean build time in half.

Performance data:

Build time before HeaderMap:
Target //Pinterest/iOS/App:PinterestDevelopment up-to-date:
bazel-bin/Pinterest/iOS/App/PinterestDevelopment.ipa
____Elapsed time: 373.588s, Critical Path: 18.86s

Build time after header maps on the entire project:
Target //Pinterest/iOS/App:PinterestDevelopment up-to-date:
bazel-bin/Pinterest/iOS/App/PinterestDevelopment.ipa
____Elapsed time: 188.971s, Critical Path: 17.11s

Additionally, this solves the problem of having namespaced headers which is used
in CocoaPods all over. Using a namespace makes includes more clear since it is
easier for the user to distinguish where the header was derived.

Implementation:

At the ObjC level, headermaps are created with a namespace of the given target.
In `objc_library` it is possible for the user to override the value of the
namespace via the new attribute, `header_namespace`.

By using 2 headermaps the headersearchs are most efficient: a headermap for the
current target, and a header map with namespaced includes.

Users can include headers from ObjC targets in the convention of
`Namespace/Header.h`. Projects that don't use namespacing should see benefits as
well: includes of the form `Header.h` will be read from the headermap.

`HeaderMapInfo` contains all of the transitive info for dependent header maps,
and is merged together into a single map. This yields much better performance
than multiple headermaps.

This is my first PR to the Bazel repo, so any suggestions or feedback is greatly
appreciated!
    @jerrymarino
    jerrymarino committed Mar 19, 2018
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  2. Start integrating include prefix headers for Objc

    @jerrymarino
    jerrymarino committed Mar 19, 2018
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  3. Integrate flatten_virtual_headers 

    When flatten virtual headers is set, we add all headers to the
headermap.

This allows a user to include a Header, "path/to/header" as "header.h"

Together with include_prefix, this allows bazel to support Xcode style
includes out of the box.

As a followup, we'll want to make this work without headermaps.

+ misc cleanups from rebasing
    @jerrymarino
    jerrymarino committed Mar 19, 2018
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