[NativeAOT] Generate optimized type mapping #9856

simonrozsival · 2025-02-27T13:36:27Z

This PR replaces the current managed type mapping dictionary, which needs to be fully build at startup by individually calling Add methods, with a pre-generated static array of class name hashes.

The array of hashes is sorted so we can use binary search to find the index corresponding to any class name. This idea is not new, it is exacatly how the native type maps work at the moment.

Once we know the index of a class, we can use a generated IL switch to look up the corresponding Type. I chose to generate this jumptable in code because this is what Native AOT understands best. Constructing a static array of metadata tokens is AFAIK not an option for Native AOT.

The generated IL is conceptually equivalent to the following C# code:

static class TypeMapping
{
    public static Type? GetType(string javaClassName)
    {
        ulong hash = Hash(javaClassName);
        int index = BinarySearch(Hashes, hash);
        return GetTypeByIndex(index);
    }

    private static ReadOnlySpan<ulong> Hashes
    {
        get
        {
            var ptr = Unsafe.AsPointer(ref '<PrivateImplementationDetails>'.'<Microsoft.Android.Runtime.TypeMapping>s_hashes');
            return new ReadOnlySpan<ulong>(ptr, 54);
        }
    }

    private static Type? GetTypeByIndex (int index)
        => index switch
        {
            0 => typeof(A),
            1 => typeof(B),
            // ...
            _ => null,
        };
}

The s_hashes field is stored as a byte buffer in the assembly and it is loaded from the static RVA field under <PrivateImplementationDetails>. The relevant generated code looks like this for a simple app (samples/NativeAOT.csproj):

//
// class TypeMapping
//

.field assembly static initonly valuetype Microsoft.Android.Runtime.TypeMapping/HashesArray s_hashes at I_00004A33
.data cil I_00004A33 = bytearray (
	c5 27 01 ad 86 95 90 00 2e b5 ff b6 eb ef 2c 04
	5d 20 71 9d c0 8f 78 09 24 db 2e 9c f9 cc 8d 0a
	bc d1 4e 4c de f1 d9 0b 83 c5 37 e8 d7 c3 b3 0d
	f9 b1 64 fe 72 d0 d1 0f 8e 99 32 40 ab c2 dd 10
	95 3d a9 89 b0 a9 81 12 da a1 1a e9 62 96 ed 1e
	7c 52 0b 4c 25 28 37 26 6c 89 44 40 86 46 7b 2a
	a5 c1 d6 0b 52 8e 84 37 e2 97 e3 1f 8e fa b2 47
	2a 9a c3 8b 8e d0 d8 47 37 b2 3a d6 7d 11 9e 49
	34 29 c1 56 77 21 7b 54 8e 7a 2a 5e 62 1c 2d 5b
	a0 a4 53 91 42 b7 18 60 51 0d 00 bc 15 cc 4e 60
	e7 00 00 75 1c cc 4b 68 fb 35 55 50 bf bc e7 68
	ee 7c 46 a9 1c c5 e8 68 e7 3c 4e 6d 2f b4 b7 6c
	d4 1c 3d 47 2d 2a c0 6c 67 31 a1 c1 cb 5c c2 70
	66 1f cb d6 59 50 76 73 8e 50 c2 47 46 19 8f 76
	0c a3 d9 c9 c1 49 bb 76 47 49 52 5a 0c 86 c7 76
	43 06 de 1a 9c 34 a4 77 07 5a 17 8d 74 58 2c 7c
	d0 05 a6 c5 b5 ff 97 7c f1 36 3a fc 31 bc 8f 86
	35 45 f1 6e 96 88 2a 87 60 b1 04 f9 af c9 ee 89
	12 08 0d 44 6c a0 32 8c 35 3a b0 71 3c 2c 44 91
	c8 25 9d d5 9e 5d 1b 93 09 94 5f 54 87 b1 96 a0
	59 32 69 4d 3f c4 a6 a0 5f 7d 08 b5 e6 2a 9c a8
	ce 20 91 5b 2a c5 8c af 5f 58 9d 1e 18 3f 65 bf
	57 90 af e1 71 13 35 c1 4b b8 2d 6e 9c 25 7a c5
	68 e8 b8 1d 2d 1d 94 d1 5b dc 7a b6 52 f4 cf d4
	56 48 36 d6 2b 3f 18 d7 d0 46 bc 99 55 15 bc da
	c2 5c aa 9f 40 3b 87 ed 68 5c 3c ea 95 3a f2 ef
	b3 c5 64 33 43 f6 0c f1 dc e9 8b df 19 2a 72 f9
)

.method private hidebysig specialname static 
	valuetype [System.Private.CoreLib]System.ReadOnlySpan`1<uint64> get_Hashes () cil managed 
{
	.custom instance void [System.Private.CoreLib]System.Runtime.CompilerServices.NullableContextAttribute::.ctor(uint8) = (
		01 00 00 00 00
	)
	// Method begins at RVA 0x42b4
	// Header size: 12
	// Code size: 21 (0x15)
	.maxstack 2
	.locals init (
		[0] valuetype [System.Private.CoreLib]System.ReadOnlySpan`1<uint64>
	)

	IL_0000: ldsflda valuetype Microsoft.Android.Runtime.TypeMapping/HashesArray Microsoft.Android.Runtime.TypeMapping::s_hashes
	IL_0005: call void* [System.Private.CoreLib]System.Runtime.CompilerServices.Unsafe::AsPointer<uint8>(!!0&)
	IL_000a: ldc.i4 54
	IL_000f: newobj instance void valuetype [System.Private.CoreLib]System.ReadOnlySpan`1<uint64>::.ctor(void*, int32)
	IL_0014: ret
} // end of method TypeMapping::get_Hashes

.method private hidebysig static 
	class [System.Private.CoreLib]System.Type GetTypeByIndex (
		int32 index
	) cil managed 
{
	.custom instance void [System.Private.CoreLib]System.Runtime.CompilerServices.NullableContextAttribute::.ctor(uint8) = (
		01 00 02 00 00
	)
	// Method begins at RVA 0x4328
	// Header size: 12
	// Code size: 823 (0x337)
	.maxstack 1

	IL_0000: ldarg.0
	IL_0001: switch (IL_00e3, IL_00ee, ..., IL_032a)

	IL_00de: br IL_0335

	IL_00e3: ldtoken [Mono.Android]Java.IO.File
	IL_00e8: call class [System.Private.CoreLib]System.Type [System.Private.CoreLib]System.Type::GetTypeFromHandle(valuetype [System.Private.CoreLib]System.RuntimeTypeHandle)
	IL_00ed: ret

	IL_00ee: ldtoken [Mono.Android]Android.Runtime.InputStreamAdapter
	IL_00f3: call class [System.Private.CoreLib]System.Type [System.Private.CoreLib]System.Type::GetTypeFromHandle(valuetype [System.Private.CoreLib]System.RuntimeTypeHandle)
	IL_00f8: ret

	// ...

	IL_032a: ldtoken [Mono.Android]Java.Lang.StackTraceElement
	IL_032f: call class [System.Private.CoreLib]System.Type [System.Private.CoreLib]System.Type::GetTypeFromHandle(valuetype [System.Private.CoreLib]System.RuntimeTypeHandle)
	IL_0334: ret

	IL_0335: ldnull
	IL_0336: ret
} // end of method TypeMapping::GetTypeByIndex

.class nested private explicit ansi HashesArray
	extends [System.Private.CoreLib]System.ValueType
{
	.pack 1
	.size 432

} // end of class HashesArray