Optimize asm size for the biggest corelib method - WebUtility..cctor

[EgorBo]

According to the jit-diff tool WebUtility..cctor is the largest method (with a noticeable delay to compile it) - it's triggered when WebUtility.HtmlDecode public API is called.

This PR shrinks s_lookupTable codegen from 17605 bytes to 10083 277 bytes.

I could use source gen here since we already use them for eventpipe stuff but I don't think it worth the complexity.

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Issue Details

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According to the jit-diff tool WebUtility..cctor is the largest method (with a noticeable delay to compile it) - it's triggered when WebUtility.HtmlDecode public API is called.

This PR shrinks s_lookupTable codegen from to 17605 bytes to 10083 bytes which is still a lot for an immutable compile-time known dictionary - I wonder how we can improve the performance for such cases in general.

I could use source gen here since we already use them for eventpipe stuff but I don't think it worth the complexity.

Author:	EgorBo
Assignees:	-
Labels:	area-System.Net
Milestone:	-

[MichalStrehovsky]

Splitting the data into two static ReadOnlySpans (one for keys, one for values) would probably be the most efficient, space-wise, and codegen-wise. Then just do a for loop, indexing into both spans and adding into dictionary.

[EgorBo]

Splitting the data into two static ReadOnlySpans (one for keys, one for values) would probably be the most efficient, space-wise, and codegen-wise. Then just do a for loop, indexing into both spans and adding into dictionary.

Nice idea, wonder if it's possible to hide those two spans under the hood of the source gen then and let the user to specify any type of such immutable dictionaries

[MichalStrehovsky]

Ah, I just realized that the static ReadOnlySpan trick won't work because these are ints (the static ReadOnlySpan currently only works for bytes due to endianness issues with anything bigger than that). Might as well just make them arrays. The storage for these will be similarly optimal but we'll have a useless allocation and a memcpy operation into the array.

[stephentoub]

Yeah, we need #24961, if anyone wants to tackle that in the runtime and compiler. :-)

[jkotas]

You can have just one Span of bytes with the data and do the endianness conversion yourself.

         ReadOnlySpan<byte> data = ...

         var d = new Dictionary<long, char>(data.Length / (sizeof(long) + sizeof(char)));
         while (!data.IsEmpty)
         {
              d[BinaryPrimitives.ReadInt64LittleEndian(data)] = (char)BinaryPrimitives.ReadInt16LittleEndian(data.Slice(sizeof(long)));
              data = data.Slice(sizeof(long) + sizeof(char));
         }

[jkotas]

Or if you wanted to optimize the heck out of this, you can create a perfect or nearly perfect hash function for this set and use the ReadOnlySpan<byte> directly without copying it into Dictionary. The lookup function would be then just:

         ReadOnlySpan<byte> data = ...

         long key = ToUInt64Key(entity);
         int index = (sizeof(long) + sizeof(char)) * PerfectHash(key);
         if (BinaryPrimitives.ReadInt64LittleEndian(data.Slice(index)) == key) return BinaryPrimitives.ReadInt64LittleEndian(data.Slice(index + sizeof(long)));

[danmoseley]

I think Jan was suggesting not using Dictionary at all, and instead a perfect hash over a buffer.

If you use what you're doing here, looks like you can remove the byte in the 8th column as it's always zero.

[EgorBo]

Well, this code is only executed once, my goal was to decrease the native size (from 17605 bytes to only 277 bytes!) using the first Jan's suggestion. If you insist on the second more performant one I can change!

[danmoseley]

No I think this is fine, I was secretly hoping you would do it only because it would be interesting 😁

[danmoseley]

And kudos for the huge improvemnet. Are there any more of these big static dictionaries I wonder?

[danmoseley]

Found another, but likely not on a hot path:

runtime/src/libraries/System.ComponentModel.TypeConverter/src/System/ComponentModel/CultureInfoConverter.cs

Line 274 in 0e8efce

var result = new Dictionary<string, string>(Count)

[EgorBo]

Will take a look, thanks!