[llvm-dev] Meaning of loads/stores marked both atomic and volatile

Tim Northover via llvm-dev llvm-dev at lists.llvm.org
Tue Nov 21 03:18:30 PST 2017


Hi,

On 21 November 2017 at 10:27, PaweĊ‚ Batko <pawel.batko at gmail.com> wrote:
> Ok, let's say in Example 1. monotonic atomic prevents loop
> optimization because compiler assumes existence of other threads.
> (Compiler (effectively) assumes existence of other threads the moment
> one starts using at least monotonic atomic load/stores?)

I think that's pretty accurate, though the assumptions are of course
limited to those monotonic (or stronger) stores.

> In Example. 2 let's consider signal handler in a single thread situation.
> If monotonic atomic prevents loop optimization as in Example 1., then
> I say it does the same Example 2.

I agree, but in certain theoretical situations a general monotonic
operation might be stronger than what's actually needed there. You
could imagine a GPU or something needing a real barrier instruction to
guarantee even a monotonic store becomes visible to other cores, but
not necessarily for one in synchscope("singlethread").

Cheers.

Tim.


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