Optimize bounds checks through early slicing #119254
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EgorBo
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Sep 2, 2025
| @@ -637,7 +637,8 @@ internal static void GetBytes(in decimal d, Span<byte> buffer) | |||
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| internal static decimal ToDecimal(ReadOnlySpan<byte> span) | |||
| { | |||
| Debug.Assert(span.Length >= 16); | |||
| span = span.Slice(0, 16); | |||
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| int lo = BinaryPrimitives.ReadInt32LittleEndian(span); | |||
| int mid = BinaryPrimitives.ReadInt32LittleEndian(span.Slice(4)); | |||
| int hi = BinaryPrimitives.ReadInt32LittleEndian(span.Slice(8)); | |||
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should we just re-order it with int flags = BinaryPrimitives.ReadInt32LittleEndian(span.Slice(12));?
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I think a reordered version would be less readable. You’d probably need something like:
int flags = BinaryPrimitives.ReadInt32LittleEndian(span.Slice(12, 4));
int hi = BinaryPrimitives.ReadInt32LittleEndian(span.Slice(8, 4));
int mid = BinaryPrimitives.ReadInt32LittleEndian(span.Slice(4, 4));
int lo = BinaryPrimitives.ReadInt32LittleEndian(span.Slice(0, 4));Doing the slicing up front (by replacing the Debug.Assert) makes the intent more obvious, avoids repetition, and doesn’t require bending the code to work around JIT limitations. It also gives the most efficient codegen in practice.
EgorBo
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Sep 2, 2025
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Diffs: MihuBot/runtime-utils#1422