[cfe-dev] JumboSupport: making unity builds easier in Clang

David Blaikie via cfe-dev cfe-dev at lists.llvm.org
Tue Apr 10 08:56:42 PDT 2018

On Tue, Apr 10, 2018 at 8:52 AM Mostyn Bramley-Moore <mostynb at vewd.com>

> On Tue, Apr 10, 2018 at 4:27 PM, David Blaikie <dblaikie at gmail.com> wrote:
>> I haven't looked at the patches in detail - but generally a jumbo build
>> feels like a bit of a workaround & maybe there are better long-term
>> solutions that might fit into the compiler. A few sort of background
>> questions:
>> * Have you tried Clang header modules (
>> https://clang.llvm.org/docs/Modules.html )? (explicit (granted, explicit
>> might only be practical at the moment using Google's internal version of
>> Bazel - but you /might/ get some comparison numbers from a Google Chrome
>> developer) and implicit)
>>   * The doc talks about maybe disabling jumbo builds for a single target
>> for developer efficiency, with the risk that a header edit would maybe be
>> worse for the developer than the jumbo build - this is where modules would
>> help as well, since it doesn't have this tradeoff property of two different
>> dimensions of "more work" you have to choose from.
> There are ways to minimise this- an earlier proprietary jumbo build system
> used at Opera would detect when you're modifying and rebuilding files, and
> compile these in "normal" mode.  This gave fast full/clean build times but
> also short modify+rebuild times.  We have not attempted to implement this
> in the Chromium Jumbo build configuration.

Building that kind of infrastructure seems like a pretty big hammer
compared to modularizing the codebase... (maybe still less work - but a lot
of work to workaround things & produce some rather quirky behavior (in
terms of how the build functions based on looking at exactly how the source
files have changed & changing the build action graph depending on that) -
but enough that I'd be inclined to reconsider going in the modular
direction again)

>> * I was going to ask about the lack of parallelism in a jumbo build - but
>> reading the doc I see it's not a 'full' jumbo build, but chunkifying the
>> build - so there's still some/enough parallelism. Cool :)
> I have heard rumours of some codebases in the games industry using a
> single jumbo source file for the entire build, but this is generally
> considered to be taking things too far and not our intended use case.

Ah, my understanding was that jumbo builds were often/mainly used for
optimized builds to get cross-module optimizations (LTO-esque) & so it'd be
likely to be the whole program.

> The size of Chromium's jumbo compilation units is tunable- you can simply
> #include fewer real source files per jumbo source file- the bigger your
> build farm is, the smaller you want this number to be.  The optimal setup
> depends on things like the shape of the dependency graph and the relative
> costs of the original source files.  IIRC we currently only have build-wide
> "jumbo_file_merge_limit" setting, though that might have changed since I
> last looked (V8 would benefit from this, since its source files compile
> more slowly than most Chromium source files).
> -Mostyn.
>> On Tue, Apr 10, 2018 at 5:12 AM Mostyn Bramley-Moore via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>>> *Hi,I am a member of a small group of Chromium developers who are
>>> working on adding a unity build[1] setup to Chromium[2], in order to reduce
>>> the project's long and ever-increasing compile times.  We're calling these
>>> "jumbo" builds, because this term is not as overloaded as "unity".We're
>>> slowly making progress, but find that a lot of our time is spent renaming
>>> things in anonymous namespaces- it would be much simpler if it was possible
>>> to automatically treat these as if they were file-local.   Jens Widell has
>>> put together a proof-of-concept which appears to work reasonably well, it
>>> consists of a clang plugin and a small clang
>>> patch:https://github.com/jensl/llvm-project-20170507/tree/wip/jumbo-support/v1
>>> <https://github.com/jensl/llvm-project-20170507/tree/wip/jumbo-support/v1>https://github.com/jensl/llvm-project-20170507/commit/a00d5ce3f20bf1c7a41145be8b7a3a478df9935f
>>> <https://github.com/jensl/llvm-project-20170507/commit/a00d5ce3f20bf1c7a41145be8b7a3a478df9935f>After
>>> building clang and the plugin, you generate jumbo source files that look
>>> like:jumbo_source_1.cc:#pragma jumbo#include
>>> "real_source_file_1.cc"#include "real_source_file_2.cc"#include
>>> "real_source_file_3.cc"Then, you compile something like this:clang++ -c
>>> jumbo_source_1.cc -Xclang -load -Xclang lib/JumboSupport.so -Xclang
>>> -add-plugin -Xclang jumbo-supportThe plugin gives unique names[3] to the
>>> anonymous namespaces without otherwise changing their semantics, and also
>>> #undef's macros defined in each top-level source file before processing the
>>> next top-level source file.  That way header files can still define macros
>>> that are used in multiple source files in the jumbo translation unit.
>>> Collisions between macros defined in header files and names used in other
>>> headers and other source files are still possible, but less likely.To show
>>> how much these two changes help, here's a patch to make Chromium's network
>>> code build in jumbo
>>> mode:https://chromium-review.googlesource.com/c/chromium/src/+/966523
>>> <https://chromium-review.googlesource.com/c/chromium/src/+/966523>
>>> (+352/-377 lines)And here's the corresponding patch using the
>>> proof-of-concept JumboSupport
>>> plugin:https://chromium-review.googlesource.com/c/chromium/src/+/962062
>>> <https://chromium-review.googlesource.com/c/chromium/src/+/962062> (+53/-52
>>> lines)It seems clear that the version using the JumboSupport plugin would
>>> require less effort to create, review and merge into the codebase.  We have
>>> a few other feature ideas, but these two changes seem to do most of the
>>> work for us.So now we're trying to figure out the best way forward- would a
>>> feature like this be welcome to the Clang project?  And if so, how would
>>> you recommend that we go about it? We would prefer to do this in a way that
>>> does not require a locally patched Clang and could live with building a
>>> custom plugin, although implementing this entirely in Clang would be even
>>> better.Thanks,-Mostyn.[1] If you're not familiar with unity builds, the
>>> idea is to compile multiple source files per compiler invocation, reducing
>>> the overhead of processing header files (which can be surprisingly high).
>>> We do this by taking a list of the source files in a target and generating
>>> "jumbo" source files that #include multiple "real" source files, and then
>>> we feed these jumbo files to the compiler one at a time.  This way, we
>>> don't prevent the usage of valuable build tools like ccache and icecc that
>>> only support a single source file on the command line.[2] Daniel Bratell
>>> has a summary of our progress jumbo-ifying the Chromium codebase
>>> here:https://docs.google.com/document/d/19jGsZxh7DX8jkAKbL1nYBa5rcByUL2EeidnYsoXfsYQ/edit#
>>> <https://docs.google.com/document/d/19jGsZxh7DX8jkAKbL1nYBa5rcByUL2EeidnYsoXfsYQ/edit#>[3]
>>> The JumboSupport plugin assigns names to the anonymous namespaces in a
>>> given file:  foo::(anonymous namespace)::bar is replaced with a symbol name
>>> of the form foo::__anonymous_<number>::bar where <number> is unique to the
>>> file within the jumbo translation unit.  Due to the internal linkage of
>>> these symbols, <number> does not need to be unique across multiple object
>>> files/jumbo source files.*
>>> --
>>> Mostyn Bramley-Moore
>>> Vewd Software
>>> mostynb at vewd.com <mostynb at opera.com>
>>> _______________________________________________
>>> cfe-dev mailing list
>>> cfe-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> --
> Mostyn Bramley-Moore
> Vewd Software
> mostynb at vewd.com <mostynb at opera.com>
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