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

Reid Kleckner via cfe-dev cfe-dev at lists.llvm.org
Tue Apr 10 10:19:04 PDT 2018

On Tue, Apr 10, 2018 at 7:34 AM Nico Weber via cfe-dev <
cfe-dev at lists.llvm.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.
> 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.

I'm also revisiting my position on this. We've discussed unity build
support in the past (I think Ubisoft proposed it), and at the time I felt
that it was very backwards-facing. It's not a long term solution to
reducing the overall cost of C++ compilation, and it can lead to creeping
transitive dependencies between C++ files.

However, more than a year later, we have not produced a solution that is as
easy to deploy and as compelling as unity builds are today. I think we need
to seriously weigh cost of adding features to support unity/jumbo builds.
The initial patches necessary to get things off the ground look small and
relatively low-maintenance. They may be just the tip of the iceberg, so we
need to gather more input, but I think it's worth a try.

FYI, my availability this week is low, so I don't expect to be able to
participate more in this thread.
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