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

David Blaikie via cfe-dev cfe-dev at lists.llvm.org
Tue Apr 10 10:01:45 PDT 2018

On Tue, Apr 10, 2018 at 9:58 AM Nico Weber <thakis at chromium.org> wrote:

> On Tue, Apr 10, 2018 at 11:56 AM, David Blaikie <dblaikie at gmail.com>
> wrote:
>> On Tue, Apr 10, 2018 at 8:52 AM Mostyn Bramley-Moore <mostynb at vewd.com>
>> wrote:
>>> 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...
> Modularizing the codebase doesn't give you the same build time impact,
> linearizes your build more,

Not sure I follow - it partially linearizes (as you say, due to the module
dependency rather than header dependency issue), as does the jumbo build.

> and slows down incremental builds.

Compared to a traditional build? I wouldn't think so (I mean, yes,
reading/writing modules has some overhead - but also some gains) on
average. I'd expect slower builds if you modify a header at the very base
of the dependency (the STL), but beyond that I would've thought the
reading/writing modules overhead would be saved by reusing modules for
infrequently modified files (like the STL).

(wonder what the combination would be like - modularizing headers, and also
jumbo-ifying .cpp files together... - whether there's much to be saved in
the reading modules part of the work, reading them in fewer times - that
gets into some of the ideas of compiler as a service I guess)

> Even if it wasn't a lot more work to get modules going, it's not
> completely clear to me that that would address the use case that the people
> working on the jumbo build have.
>> (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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