[libcxx-dev] Is a compiler-rt shared spinlock implementation of any use to libcxx?

Olivier Giroux via libcxx-dev libcxx-dev at lists.llvm.org
Fri May 17 17:54:38 PDT 2019

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.


From: libcxx-dev <libcxx-dev-bounces at lists.llvm.org> on behalf of JF Bastien via libcxx-dev <libcxx-dev at lists.llvm.org>
Reply-To: JF Bastien <jfbastien at apple.com>
Date: Friday, May 17, 2019 at 4:40 PM
To: Mitch Phillips <mitchphillips at outlook.com>
Cc: "kostyak at google.com" <kostyak at google.com>, "libcxx-dev at lists.llvm.org" <libcxx-dev at lists.llvm.org>
Subject: Re: [libcxx-dev] Is a compiler-rt shared spinlock implementation of any use to libcxx?

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.

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>?

On May 17, 2019, at 1:49 PM, Mitch Phillips via libcxx-dev <libcxx-dev at lists.llvm.org<mailto:libcxx-dev at lists.llvm.org>> wrote:

Hi all,

In a recent discussion<https://reviews.llvm.org/D61923#1503272> from the reviews of GWP-ASan<https://reviews.llvm.org/D60593>, it was mentioned that I should probably consult with cxx-dev to see whether they'd be interested in a common spinlock implementation.

The problem is:
 - Scudo hardened allocator (compiler-rt/lib/scudo) requires its own spinlock as it can't use the C++ standard library due to Fuchsia requirements.
 - GWP-ASan (as it's packaged into Scudo) also requires a spinlock.
 - 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.

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?

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