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Stop consuming result of GetPinnedReference() #1280

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merged 2 commits into from
Apr 15, 2020
Merged

Stop consuming result of GetPinnedReference() #1280

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d9d55c2
Stop consuming pinned references
kunalspathak Apr 7, 2020
02db979
Update src/benchmarks/micro/libraries/System.Memory/ReadOnlySpan.cs
adamsitnik Apr 15, 2020
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36 changes: 22 additions & 14 deletions src/benchmarks/micro/libraries/System.Memory/ReadOnlySpan.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -13,25 +13,33 @@ namespace System.Memory
public class ReadOnlySpan
{
private readonly string _sampeString = "this is a very nice sample string";
private readonly Consumer _consumer = new Consumer();

[Benchmark]
public ReadOnlySpan<char> StringAsSpan() => _sampeString.AsSpan();

[Benchmark(OperationsPerInvoke = 16)]
public void GetPinnableReference()
[Benchmark(OperationsPerInvoke = 16 * 2)]
public char GetPinnableReference()
{
ReadOnlySpan<char> span = _sampeString.AsSpan();
var consumer = _consumer;

consumer.Consume(span.GetPinnableReference()); consumer.Consume(span.GetPinnableReference());
consumer.Consume(span.GetPinnableReference()); consumer.Consume(span.GetPinnableReference());
consumer.Consume(span.GetPinnableReference()); consumer.Consume(span.GetPinnableReference());
consumer.Consume(span.GetPinnableReference()); consumer.Consume(span.GetPinnableReference());
consumer.Consume(span.GetPinnableReference()); consumer.Consume(span.GetPinnableReference());
consumer.Consume(span.GetPinnableReference()); consumer.Consume(span.GetPinnableReference());
consumer.Consume(span.GetPinnableReference()); consumer.Consume(span.GetPinnableReference());
consumer.Consume(span.GetPinnableReference()); consumer.Consume(span.GetPinnableReference());
char c;

c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
Comment on lines +26 to +41
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is there a possibility that in the future JIT is going to replace all 32 calls with a single one?

@EgorBo or perhaps it can happen as of today with Mono using LLVM?

If so, we should most probably xor the results

Suggested change
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c = span.GetPinnableReference();
c = span.GetPinnableReference(); c ^= span.GetPinnableReference();
c ^= span.GetPinnableReference(); c ^= span.GetPinnableReference();
c ^= span.GetPinnableReference(); c ^= span.GetPinnableReference();
c ^= span.GetPinnableReference(); c ^= span.GetPinnableReference();
c ^= span.GetPinnableReference(); c ^= span.GetPinnableReference();
c ^= span.GetPinnableReference(); c ^= span.GetPinnableReference();
c ^= span.GetPinnableReference(); c ^= span.GetPinnableReference();
c ^= span.GetPinnableReference(); c ^= span.GetPinnableReference();
c ^= span.GetPinnableReference(); c ^= span.GetPinnableReference();
c ^= span.GetPinnableReference(); c ^= span.GetPinnableReference();
c ^= span.GetPinnableReference(); c ^= span.GetPinnableReference();
c ^= span.GetPinnableReference(); c ^= span.GetPinnableReference();
c ^= span.GetPinnableReference(); c ^= span.GetPinnableReference();
c ^= span.GetPinnableReference(); c ^= span.GetPinnableReference();
c ^= span.GetPinnableReference(); c ^= span.GetPinnableReference();
c ^= span.GetPinnableReference(); c ^= span.GetPinnableReference();

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@adamsitnik I've just checked - Mono-LLVM optimizes out all the dead stores (basicaly, does span.GetPinnableReference() only once). The "^=" trick doesn't help 🙂 (with the proposed change it optimized the whole function to return 0

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@EgorBo thank you! Do you know how can we trick the Mono-LLVM ? Mix + with ^ ?

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No idea, GetPinnableReference is just too simple and returns a pointer. I guess only if you hide span to a non-inlineable method e.g. GetSpan().GetPinnableReference() ^ .. but I guess the overhead of that method will be bigger than the actual GetPinnableReference

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Weird. Surprisingly RYUJit doesn't eliminate dead stores (at least for this example). The code that I pasted above is precisely that gets generated today. I will investigate and possibly open an issue to track this.

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Updated existing issue with my comments. dotnet/runtime#13727 (comment)

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Perhaps we can do something like this?

performance/src/benchmarks/micro/libraries/System.Memory/Slice.cs

Line 293 in 8aed638

private static void Consume(in System.Span<T> _) { }

But yeah, that would have method call overhead too.

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In such case, I think that I am going to merge it as it is.

If it becomes a problem in the future, we might consider removing this benchmark because it looks like it might be impossible to properly benchmark this method (it's too simple).

/cc @billwert @DrewScoggins

return c;
}

[Benchmark(OperationsPerInvoke = 16)]
Expand Down Expand Up @@ -97,4 +105,4 @@ private static string GenerateInputString(char source, int count, char replaceCh
return new string(str);
}
}
}
}