[API Proposal]: System.Threading.Relaxed.Read

[VSadov]

Background and motivation

Unlike System.Threading.Volatile.Read, the System.Threading.Relaxed.Read only defeats compiler optimizations.

This is basically an officialVolatileReadWithoutBarrier (see:

runtime/src/libraries/System.Threading/tests/InterlockedTests.cs

Line 682 in 118a162

if (VolatileReadWithoutBarrier(ref _current) == entry)

)

The read will keep its order with respect to other memory operations (in singlethreaded/program order, not in the order of sideeffects), and used reads will not be coalesced with other reads from the same location.

Such read would be paired with Interlocked.MemoryBarrierProcessWide or could be used to poll a local that could be changed on another thread.
A Volatile.Read could be used for the same purpose, but will add unnecessary guarantees of the order of the actual read.

We can, optionally, add a requirement that the read is atomic.
A better name than "Relaxed" may exist too.

API Proposal

namespace System.Threading;

public class Relaxed
{
    public T Read<T>(ref T location) where T: class;

    public int Read(ref int location);

    public double Read(ref double location);
. . .

}

API Usage

Alternative Designs

Can use Volatile.Read or hand-written uninlinable methods with sole purpose of reading some variable.

Risks

No risk, this is completely additive API.

[ghost]

Tagging subscribers to this area: @mangod9
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Background and motivation

Unlike System.Threading.Volatile.Read, the System.Threading.Relaxed.Read only defeats compiler optimizations.

This is basically an officialVolatileReadWithoutBarrier (see:

runtime/src/libraries/System.Threading/tests/InterlockedTests.cs

Line 682 in 118a162

if (VolatileReadWithoutBarrier(ref _current) == entry)

)

The read will not be reordered with other memory operations (in program order, not in the order of sideeffects), and used read will not be coalesced with reads from the same location.

Such read would be paired with Interlocked.MemoryBarrierProcessWide or could be used to poll a local that could be changed on another thread.
A Volatile.Read could be used of the same purpose, but will add unnecessary guarantees of the order of the actual read.

API Proposal

namespace System.Threading;

public class Relaxed
{
    [Intrinsic]
    public T Read(ref T location);
}

API Usage

// Fancy the value
var c = new MyFancyCollection<int>();
c.Fancy(42);

// Getting the values out
foreach (var v in c)
    Console.WriteLine(v);

Alternative Designs

Can use Volatile.Read or hand-written uninlinable methods with sole purpose of reading some variable.

Risks

No response

Author:	VSadov
Assignees:	-
Labels:	api-suggestion, area-System.Threading
Milestone:	-