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

Mostyn Bramley-Moore via cfe-dev cfe-dev at lists.llvm.org
Tue Apr 10 08:06:59 PDT 2018

On Tue, Apr 10, 2018 at 4:52 PM, Nico Weber <thakis at chromium.org> wrote:

> On Tue, Apr 10, 2018 at 10:44 AM, David Blaikie <dblaikie at gmail.com>
> wrote:
>> What sort of significant investment are you thinking of regarding modules
>> - the build system support, I would imagine, wouldn't be any less than the
>> support being proposed here for jumbo builds, no?
>> But making header files modules-clean is some work, for sure. I'd imagine
>> doing this the same way we're kind of motivated to do it inside Google -
>> provide the feature, then migrate the most impactful libraries.
>> Teams/projects <https://teams.googleplex.com/u/projects> then have an
>> incentive to cleanup/modularize code in whatever areas are the most
>> important.
> Mostly this work. And it's best to start at the bottom of the dependency
> stack, and we don't control our SDK headers everywhere. And even if we get
> modules working, they're not pure win since they serialize the build graph
> more, and touching a header in a module now requires rebuilds of all
> targets depending on the module instead of just all all translation units
> including that specific header (I think this can be fixed, but that too is
> work.)

In addition to these downsides, I suspect that modules would cause trouble
for tools like icecc which distribute preprocessed sources and a toolchain
caching system.  Though I have not explored this in detail.


> But, yeah, I see where you're coming from - that maybe a tidy jumbo build
>> support might not be too bad.
> Yup.
>> On Tue, Apr 10, 2018 at 7:34 AM Nico Weber <thakis at chromium.org> wrote:
>>> On Tue, Apr 10, 2018 at 10:27 AM, 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.
>>> I feel the same way. However, modules need significant investment to get
>>> going, and people do jumbo builds if we want it or not. WebKit is doing the
>>> same thing for example https://blogs.gnome.or
>>> g/mcatanzaro/2018/02/17/on-compiling-webkit-now-twice-as-fast/
>>> People will use this, if we want them to or not (I have some influence
>>> in chrome land and wasn't able to talk them out of it, since it does
>>> provide huge benefits), and the workarounds needed without compiler support
>>> are gnarly.
>>> So I think we might want to revisit our "you don't really want this"
>>> stance on this topic we've had historically and instead try to make this
>>> work well.
>>>> A few sort of background questions:
>>>> * Have you tried Clang header modules ( https://clang.llvm.org/docs/Mo
>>>> dules.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.
>>>> * 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 :)
>>>> 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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