[cfe-dev] BUILD_SHARED_LIBS=True breaks some tests

Chris Bieneman via cfe-dev cfe-dev at lists.llvm.org
Tue Jan 8 15:04:21 PST 2019

> On Jan 8, 2019, at 2:44 PM, Joel E. Denny via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> On Tue, Jan 8, 2019 at 5:14 PM David Greene <dag at cray.com <mailto:dag at cray.com>> wrote:
> "Joel E. Denny via cfe-dev" <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> writes:
> > Do other Clang developers find BUILD_SHARED_LIBS=True useful? Do you
> > see the above failures? Should there be a bot to test that build
> > configuration?
> I don't use BUILD_SHARED_LIBS=True
> One point I'd like to understand is why more people don't use it.  Were you not aware of it, or is it just not beneficial enough to you?  I was very happy to reduce a nearly 4-minute rebuild after a minor change to ~31 sec.  Especially when I have to do that repeatedly, that time difference can determine whether I decide to switch contexts or stay focused.

How much of that rebuild time is actually link time? I'd guess most. What linker are you using? ld64, lld and gold are all much faster than bfd, so the performance implications may be smaller to other people. Also, using `BUILD_SHARED_LIBS` has a significant impact on execution performance of the final binaries, which impacts test execution speed. So if you aren't struggling with link time, it can be overall better to generate faster loading binaries for the test suite.

There are a lot of tradeoffs on performance, but I've strongly advocated that BUILD_SHARED_LIBS should never be used when building distributions for performance reasons, which means it is only really supported as a developer workflow.

Additionally BUILD_SHARED_LIBS is problematic with some of LLVM's design patterns, like cl::opt's global registration, which can deter its usefulness.

Hope this helps explain why it is less widely used than you might have anticipated.


> but if that's a supported option, it
> should be tested.
> This argument will, of course, lead to an explosion of builders, as the
> set of combinations of cmake variables is very large.  It would be
> useful to gather the various configurations people use and see if there
> is a set of common-ish combinations that is small enough to regularly
> test.
> We build with RTTI and exceptions enabled and I'll bet there are others
> out there that do the same.  But AFAIK there are no bots testing those
> options.
> To reduce the configuration space, we could consider combining orthogonal options under a single builder.  That could make debugging some fails a little harder, but it might be worthwhile as it would provide more coverage with less builders.
> Thanks.
> Joel
>                              -David
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