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Optimize Span<T>.Fill implementation #51365

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merged 5 commits into from
Apr 17, 2021
Merged

Optimize Span<T>.Fill implementation #51365

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a44adae
Span.Fill perf improvements
GrabYourPitchforks Apr 16, 2021
4e7b47d
PR feedback, slight perf tweaks
GrabYourPitchforks Apr 16, 2021
ba5ce88
Fix > to >=
GrabYourPitchforks Apr 16, 2021
0bf26c3
PR feedback
GrabYourPitchforks Apr 17, 2021
22ad8fa
Work around initblk null check on mono
GrabYourPitchforks Apr 17, 2021
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17 changes: 12 additions & 5 deletions src/libraries/System.Private.CoreLib/src/System/Span.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -284,11 +284,18 @@ public void Fill(T value)
{
if (Unsafe.SizeOf<T>() == 1)
{
// Special-case single-byte types like byte / sbyte / bool.
// The runtime eventually calls memset, which can efficiently support large buffers.
// We don't need to check IsReferenceOrContainsReferences because no references
// can ever be stored in types this small.
Unsafe.InitBlockUnaligned(ref Unsafe.As<T, byte>(ref _pointer.Value), Unsafe.As<T, byte>(ref value), (uint)_length);
#if MONO
// Mono runtime's implementation of initblk performs a null check on the address.
// We'll perform a length check here to avoid passing a null address in the empty span case.
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Is that a compatibility thing? Should we change it?

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See comment at #51365 (comment). I think I know where to make the change, but I was concerned that it would have broader implications and that I'd risk inadvertently breaking some other component. Putting the check here seemed the safest option. (This code had a check originally, so it's not a behavioral change from before this PR.)

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File an issue? Making the runtimes behave the same would be best if possible.

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Already filed - #51411 😄.

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Is that the same bug?

if (_length != 0)
#endif
{
// Special-case single-byte types like byte / sbyte / bool.
// The runtime eventually calls memset, which can efficiently support large buffers.
// We don't need to check IsReferenceOrContainsReferences because no references
// can ever be stored in types this small.
Unsafe.InitBlockUnaligned(ref Unsafe.As<T, byte>(ref _pointer.Value), Unsafe.As<T, byte>(ref value), (uint)_length);
}
}
else
{
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