<div dir="ltr"><div dir="ltr"><div dir="ltr"><div class="gmail_quote"><div dir="ltr">On Tue, Jan 8, 2019 at 6:04 PM Chris Bieneman <<a href="mailto:chris.bieneman@me.com">chris.bieneman@me.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div style="overflow-wrap: break-word;"><br><div><br><blockquote type="cite"><div>On Jan 8, 2019, at 2:44 PM, Joel E. Denny via cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>> wrote:</div><br class="gmail-m_-2296978515651679138Apple-interchange-newline"><div><div dir="ltr"><div class="gmail_quote"><div dir="ltr">On Tue, Jan 8, 2019 at 5:14 PM David Greene <<a href="mailto:dag@cray.com" target="_blank">dag@cray.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">"Joel E. Denny via cfe-dev" <<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>> writes:<br>
<br>
> Do other Clang developers find BUILD_SHARED_LIBS=True useful? Do you<br>
> see the above failures? Should there be a bot to test that build<br>
> configuration?<br>
<br>
I don't use BUILD_SHARED_LIBS=True</blockquote><div><br></div><div>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.<br></div></div></div></div></blockquote><div><br></div><div>How much of that rebuild time is actually link time? I'd guess most.</div></div></div></blockquote><div><br></div><div>Yes. Even more so because I forgot I have ccache enabled.<br></div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div style="overflow-wrap: break-word;"><div><div>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.</div></div></div></blockquote><div><br></div><div>I just tried LLVM_USE_LINKER=gold and then lld, and lld gave me a better incremental build time, so I'll stick with it.</div><div><br></div><div>The aforementioned ~4 min shrank to ~35s. However, BUILD_SHARED_LIBS=True shrank that to 3s!<br></div><div><br></div><div>Continuing to use lld, when I disable ccache (or make a change ccache hasn't seen), it's ~40s and BUILD_SHARED_LIBS=True shrinks it to ~13s.<br></div><div><br></div><div>The gap is certainly closing, but BUILD_SHARED_LIBS=True still seems appealing.<br></div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div style="overflow-wrap: break-word;"><div><div>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.</div></div></div></blockquote><div><br></div><div>Indeed, ninja check-clang grows from ~10 min to ~20 min when I use BUILD_SHARED_LIBS=True. Either way, I'd switch contexts instead of waiting, so my development time wouldn't necessarily grow with the latter.<br></div><div><br></div><div>Nevertheless, whether to use BUILD_SHARED_LIBS=True is definitely a harder choice to make now.</div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div style="overflow-wrap: break-word;"><div><div><br></div><div>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.</div></div></div></blockquote><div><br></div><div>Seems fine to me except that, if it is supported as a developer workflow, the broken tests are a problem.<br></div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div style="overflow-wrap: break-word;"><div><div><br></div><div>Additionally BUILD_SHARED_LIBS is problematic with some of LLVM's design patterns, like cl::opt's global registration, which can deter its usefulness.</div></div></div></blockquote><div><br></div><div>Could you explain a little more, or point to a previous discussion?<br></div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div style="overflow-wrap: break-word;"><div><div><br></div><div>Hope this helps explain why it is less widely used than you might have anticipated.</div></div></div></blockquote><div><br></div><div>Definitely. Thanks!</div><br></div><div class="gmail_quote">Joel<div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div style="overflow-wrap: break-word;"><div><div><br></div><div>-Chris</div><br><blockquote type="cite"><div><div dir="ltr"><div class="gmail_quote"><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"> but if that's a supported option, it<br>
should be tested.<br>
<br>
This argument will, of course, lead to an explosion of builders, as the<br>
set of combinations of cmake variables is very large. It would be<br>
useful to gather the various configurations people use and see if there<br>
is a set of common-ish combinations that is small enough to regularly<br>
test.<br>
<br>
We build with RTTI and exceptions enabled and I'll bet there are others<br>
out there that do the same. But AFAIK there are no bots testing those<br>
options.<br></blockquote><div><br></div><div>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.<br></div><div><br></div><div>Thanks.<br></div><div><br></div><div>Joel<br></div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<br>
-David<br>
<br>
</blockquote></div></div>
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