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<p class="MsoNormal">I guess it depends if the goal is to have a spin lock class, or a sharded lock table.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">If it’s a spin lock class, then someone needs to just make the class and get it into some header and/or static lib somewhere. The specifics aren’t super important.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">If it’s a sharded lock table, then you need to figure out which dynamic library will provide it, and how that will interact with libc++ when it is used atop libsupc++ or built with GCC.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b>From:</b> jfbastien@apple.com <jfbastien@apple.com> <br>
<b>Sent:</b> Monday, May 20, 2019 3:59 PM<br>
<b>To:</b> Mitch Phillips <mitchphillips@outlook.com><br>
<b>Cc:</b> Ben Craig <ben.craig@ni.com>; Olivier Giroux <OGiroux@nvidia.com>; libcxx-dev@lists.llvm.org; kostyak@google.com<br>
<b>Subject:</b> [EXTERNAL] Re: [libcxx-dev] Is a compiler-rt shared spinlock implementation of any use to libcxx?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">“Freestanding” is meant for a few things, of which: codebases that can’t have external dependencies, “bare metal” code or other embedded code. This sounds exactly like what you’re after.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">The projects are all in the same repository. You can absolutely depend on having it: it’s in the same repo. You therefore don’t need to duplicate anything.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On May 20, 2019, at 1:55 PM, Mitch Phillips <<a href="mailto:mitchphillips@outlook.com">mitchphillips@outlook.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I'm a little confused about how said aformentioned freestanding implementation would work. We (GWP-ASan + Scudo) can't depend on having libcxx (we actually
<a href="https://urldefense.proofpoint.com/v2/url?u=https-3A__reviews.llvm.org_D62048&d=DwMFaQ&c=I_0YwoKy7z5LMTVdyO6YCiE2uzI1jjZZuIPelcSjixA&r=y8mub81SfUi-UCZRX0Vl1g&m=0JDRKupnKG2FyAYi69CfJqswn5ys3q3D8nrjL8eqtQw&s=AgRXbJ1EHKQtYIn_0Ku-RB1v_LizN2Q4nVeZ595pZa8&e=">
explicitly disable</a> using of the c++ standard library for these libraries), and I don't know where we could have a shared implementation live (that could be used by both us and libcxx).<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Would we have a spinlock implementation in libcxx and have that guarded by a macro definition before the include of <mutex>, and then keep a synchronised version in compiler-rt?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Mon, May 20, 2019 at 10:10 AM Ben Craig <<a href="mailto:ben.craig@ni.com">ben.craig@ni.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Wouldn’t all the Scudo and sanitizer things count as part of the toolchain implementation though? Having pieces of the toolchain use an alternative name seems pretty reasonable
to me.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">If the goal is to have a freestanding std::mutex that is implemented as a spin lock, well, that’s different. I could be convinced that’s a good thing to have, but I’m still a bit
uneasy about it.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><b>From:</b>
<a href="mailto:jfbastien@apple.com" target="_blank">jfbastien@apple.com</a> <<a href="mailto:jfbastien@apple.com" target="_blank">jfbastien@apple.com</a>>
<br>
<b>Sent:</b> Monday, May 20, 2019 11:27 AM<br>
<b>To:</b> Ben Craig <<a href="mailto:ben.craig@ni.com" target="_blank">ben.craig@ni.com</a>><br>
<b>Cc:</b> Olivier Giroux <<a href="mailto:OGiroux@nvidia.com" target="_blank">OGiroux@nvidia.com</a>>; Mitch Phillips <<a href="mailto:mitchphillips@outlook.com" target="_blank">mitchphillips@outlook.com</a>>;
<a href="mailto:libcxx-dev@lists.llvm.org" target="_blank">libcxx-dev@lists.llvm.org</a>;
<a href="mailto:kostyak@google.com" target="_blank">kostyak@google.com</a><br>
<b>Subject:</b> [EXTERNAL] Re: [libcxx-dev] Is a compiler-rt shared spinlock implementation of any use to libcxx?<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I think what developers *use* should definitely be named std::mutex. We indeed want to consider how the ABI is exposed, that’s a good question for libc++ maintainers.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">On May 20, 2019, at 6:23 AM, Ben Craig <<a href="mailto:ben.craig@ni.com" target="_blank">ben.craig@ni.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I think it’s fine to give such a class the same interface as std::mutex, just please don’t actually name it std::mutex. I would be pretty concerned about ABI compatibility in that
situation, as well as subtle interactions with things like condition_variable::wait. cxxabi::__mutex, or almost any other identifier would be fine.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><b>From:</b><span class="gmail-m-1501418984181503871apple-converted-space"> </span>libcxx-dev <<a href="mailto:libcxx-dev-bounces@lists.llvm.org" target="_blank">libcxx-dev-bounces@lists.llvm.org</a>><span class="gmail-m-1501418984181503871apple-converted-space"> </span><b>On
Behalf Of<span class="gmail-m-1501418984181503871apple-converted-space"> </span></b>Olivier Giroux via libcxx-dev<br>
<b>Sent:</b><span class="gmail-m-1501418984181503871apple-converted-space"> </span>Friday, May 17, 2019 7:55 PM<br>
<b>To:</b><span class="gmail-m-1501418984181503871apple-converted-space"> </span>JF Bastien <<a href="mailto:jfbastien@apple.com" target="_blank">jfbastien@apple.com</a>>; Mitch Phillips <<a href="mailto:mitchphillips@outlook.com" target="_blank">mitchphillips@outlook.com</a>><br>
<b>Cc:</b><span class="gmail-m-1501418984181503871apple-converted-space"> </span><a href="mailto:kostyak@google.com" target="_blank">kostyak@google.com</a>;
<a href="mailto:libcxx-dev@lists.llvm.org" target="_blank">libcxx-dev@lists.llvm.org</a><br>
<b>Subject:</b><span class="gmail-m-1501418984181503871apple-converted-space"> </span>[EXTERNAL] Re: [libcxx-dev] Is a compiler-rt shared spinlock implementation of any use to libcxx?<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">That’s what makes the most sense to me, include <mutex> with a macro that freestanding would also use. Inside of that, I would have an implementation of standard mutex backed by
atomics and the atomic_wait/atomic_notify_* functions, themselves configured with a macro to not rely on the OS in freestanding implementations.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Olivier<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><b><span style="font-size:12.0pt">From:<span class="gmail-m-1501418984181503871apple-converted-space"> </span></span></b><span style="font-size:12.0pt">libcxx-dev <<a href="mailto:libcxx-dev-bounces@lists.llvm.org" target="_blank"><span style="color:purple">libcxx-dev-bounces@lists.llvm.org</span></a>>
on behalf of JF Bastien via libcxx-dev <<a href="mailto:libcxx-dev@lists.llvm.org" target="_blank"><span style="color:purple">libcxx-dev@lists.llvm.org</span></a>><br>
<b>Reply-To:<span class="gmail-m-1501418984181503871apple-converted-space"> </span></b>JF Bastien <<a href="mailto:jfbastien@apple.com" target="_blank"><span style="color:purple">jfbastien@apple.com</span></a>><br>
<b>Date:<span class="gmail-m-1501418984181503871apple-converted-space"> </span></b>Friday, May 17, 2019 at 4:40 PM<br>
<b>To:<span class="gmail-m-1501418984181503871apple-converted-space"> </span></b>Mitch Phillips <<a href="mailto:mitchphillips@outlook.com" target="_blank"><span style="color:purple">mitchphillips@outlook.com</span></a>><br>
<b>Cc:<span class="gmail-m-1501418984181503871apple-converted-space"> </span></b>"<a href="mailto:kostyak@google.com" target="_blank"><span style="color:purple">kostyak@google.com</span></a>" <<a href="mailto:kostyak@google.com" target="_blank"><span style="color:purple">kostyak@google.com</span></a>>,
"<a href="mailto:libcxx-dev@lists.llvm.org" target="_blank"><span style="color:purple">libcxx-dev@lists.llvm.org</span></a>" <<a href="mailto:libcxx-dev@lists.llvm.org" target="_blank"><span style="color:purple">libcxx-dev@lists.llvm.org</span></a>><br>
<b>Subject:<span class="gmail-m-1501418984181503871apple-converted-space"> </span></b>Re: [libcxx-dev] Is a compiler-rt shared spinlock implementation of any use to libcxx?</span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I think it makes sense for libc++ to have a version of mutex which has the same API as the standard one, but for “freestanding” platforms such as yours. A flavor which mostly just
spins and yields as in your review.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I’m not sure how to best turn it on, though. Should it be controlled by a macro, and otherwise it just looks like you’re using <mutex>?<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">On May 17, 2019, at 1:49 PM, Mitch Phillips via libcxx-dev <<a href="mailto:libcxx-dev@lists.llvm.org" target="_blank"><span style="color:purple">libcxx-dev@lists.llvm.org</span></a>>
wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Hi all,<span class="gmail-m-1501418984181503871apple-converted-space"> </span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">In a recent<span class="gmail-m-1501418984181503871apple-converted-space"> </span><a href="https://urldefense.proofpoint.com/v2/url?u=https-3A__reviews.llvm.org_D61923-231503272&d=DwMGaQ&c=I_0YwoKy7z5LMTVdyO6YCiE2uzI1jjZZuIPelcSjixA&r=y8mub81SfUi-UCZRX0Vl1g&m=yw9dJtPZSvMNi00nrRSUlnN4NTQzKvXzHbd5w8ShH8A&s=uW10gtroz30NxGIUU3BvNH5efGnzv2tds3P49eY5ri4&e=" target="_blank"><span style="color:purple">discussion</span></a><span class="gmail-m-1501418984181503871apple-converted-space"> </span>from
the reviews of<span class="gmail-m-1501418984181503871apple-converted-space"> </span><a href="https://urldefense.proofpoint.com/v2/url?u=https-3A__reviews.llvm.org_D60593&d=DwMGaQ&c=I_0YwoKy7z5LMTVdyO6YCiE2uzI1jjZZuIPelcSjixA&r=y8mub81SfUi-UCZRX0Vl1g&m=yw9dJtPZSvMNi00nrRSUlnN4NTQzKvXzHbd5w8ShH8A&s=rKVj5vHVGEsEZRjMkiN2X6e0bAfdTY7iVW0laX2AO8o&e=" target="_blank"><span style="color:purple">GWP-ASan</span></a>,
it was mentioned that I should probably consult with cxx-dev to see whether they'd be interested in a common spinlock implementation.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">The problem is:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> - Scudo hardened allocator (compiler-rt/lib/scudo) requires its own spinlock as it can't use the C++ standard library due to Fuchsia requirements.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> - GWP-ASan (as it's packaged into Scudo) also requires a spinlock.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> - Compiler-rt sanitizer_common also can't use c++ stdlib, and has its own spinlock. We can't reuse the sanitizer_common spinlock for Scudo/GWP-ASan as it's currently tightly coupled
into sanitizer_common, and Scudo+Fuchsia can't afford the code size overhead of pulling the entire sanitizer_common library.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">The plan was to basically write a small standalone spinlock implementation that can be used by all three of these requirements. Would libcxx benefit by us making this an llvm-common
spinlock rather than compiler-rt-common? If so, are there any requirements that we need to be aware of?<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Cheers,<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Mitch.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">_______________________________________________<br>
libcxx-dev mailing list<br>
<a href="mailto:libcxx-dev@lists.llvm.org" target="_blank"><span style="color:purple">libcxx-dev@lists.llvm.org</span></a><br>
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