VTable Optimizations

[jamescourtney]

• Use structs for small vtables (<= 8 fields). This takes the same amount of memory as we consume today and avoids a layer of indirection on access for most table sizes. This also helps to remove a branch on the hot path.
• Extend IInputBuffer to expose AsSpan() and AsReadOnlySpan() methods. These are more efficient than their memory counterparts.
  • For .NET Core 3.1+, this is done using a default interface implementation with custom overrides.
  • For .NET standard 2.0, this is done with a sub-interface IInputBuffer2 and an associated pair of extension methods.

Collectively, this means that usage of the new methods is the same, but for best performance, those with custom IInputBuffer implementations should either implement IInputBuffer2 or provide implementations for the new methods.

[codecov[bot]]

Codecov Report

Merging #265 (0d8adf7) into main (dde473d) will increase coverage by 0.21%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #265      +/-   ##
==========================================
+ Coverage   95.46%   95.67%   +0.21%     
==========================================
  Files         109      119      +10     
  Lines        7272     7633     +361     
  Branches      680      705      +25     
==========================================
+ Hits         6942     7303     +361     
  Misses        230      230              
  Partials      100      100              

Impacted Files	Coverage Δ
src/FlatSharp/TypeModel/ItemMemberModel.cs	90.54% <ø> (ø)
src/FlatSharp/TypeModel/StructMemberModel.cs	80.64% <ø> (ø)
src/FlatSharp.Compiler/SchemaModel/RootModel.cs	100.00% <100.00%> (ø)
src/FlatSharp.Runtime/IO/InputBufferExtensions.cs	95.65% <100.00%> (ø)
src/FlatSharp.Runtime/VTables/VTable0.cs	100.00% <100.00%> (ø)
src/FlatSharp.Runtime/VTables/VTable1.cs	100.00% <100.00%> (ø)
src/FlatSharp.Runtime/VTables/VTable2.cs	100.00% <100.00%> (ø)
src/FlatSharp.Runtime/VTables/VTable3.cs	100.00% <100.00%> (ø)
src/FlatSharp.Runtime/VTables/VTable4.cs	100.00% <100.00%> (ø)
src/FlatSharp.Runtime/VTables/VTable5.cs	100.00% <100.00%> (ø)
... and 10 more

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[jamescourtney]

Increases complexity without any meaningful impact on perf. Leaving branch as-is. Will consider IInputBuffer changes separately.

[jamescourtney]

Found a case where this does help.