Add identity method to reduce JIT-ing #880
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@@ Coverage Diff @@
## master #880 +/- ##
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Coverage 92.38% 92.39%
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Files 110 111 +1
Lines 3441 3443 +2
Branches 1020 1021 +1
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+ Hits 3179 3181 +2
Misses 200 200
Partials 62 62
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I don't see the value in adding this because it's not improving anything. It's less ergonomic/succinct (or more verbose) than just a lambda (x => x) and it's not removing duplication of any “methods” per the PR subject line. What's more, it doesn't work with anonymous types.
Selected Method: public static IEnumerable<T> OrderedMerge<T, TKey>(
this IEnumerable<T> first,
IEnumerable<T> second,
Func<T, TKey> keySelector)
{
return OrderedMerge(first, second, keySelector, Identity<T>, Identity<T>, (a, _) => a, null);
}Decompile of [Extension]
public static IEnumerable<T> OrderedMerge<[System.Runtime.CompilerServices.Nullable(2)] T, [System.Runtime.CompilerServices.Nullable(2)] TKey>(IEnumerable<T> first, IEnumerable<T> second, Func<T, TKey> keySelector)
{
return OrderedMerge(first, second, keySelector, <>c__156<T, TKey>.<>9__156_0 ?? (<>c__156<T, TKey>.<>9__156_0 = new Func<T, T>(<>c__156<T, TKey>.<>9.<OrderedMerge>b__156_0)), <>c__156<T, TKey>.<>9__156_1 ?? (<>c__156<T, TKey>.<>9__156_1 = new Func<T, T>(<>c__156<T, TKey>.<>9.<OrderedMerge>b__156_1)), <>c__156<T, TKey>.<>9__156_2 ?? (<>c__156<T, TKey>.<>9__156_2 = new Func<T, T, T>(<>c__156<T, TKey>.<>9.<OrderedMerge>b__156_2)), null);
}Decompile of [Extension]
public static IEnumerable<T> OrderedMerge<[System.Runtime.CompilerServices.Nullable(2)] T, [System.Runtime.CompilerServices.Nullable(2)] TKey>(IEnumerable<T> first, IEnumerable<T> second, Func<T, TKey> keySelector)
{
return OrderedMerge(first, second, keySelector, <OrderedMerge>O__157_0<T, TKey>.<0>__Identity ?? (<OrderedMerge>O__157_0<T, TKey>.<0>__Identity = new Func<T, T>(Identity)), <OrderedMerge>O__157_0<T, TKey>.<0>__Identity ?? (<OrderedMerge>O__157_0<T, TKey>.<0>__Identity = new Func<T, T>(Identity)), <>c__157<T, TKey>.<>9__157_0 ?? (<>c__157<T, TKey>.<>9__157_0 = new Func<T, T, T>(<>c__157<T, TKey>.<>9.<OrderedMerge>b__157_0)), null);
}In the original version, the two identity lambdas are compiled separately ( This improves JIT significantly:
Until .net7.0 + tiered PGO, these methods will not get inlined because they are accessed via delegate. This means they also take additional space in memory due to the translated code. Also, it works fine with anonymous types - see LinqPAD attachment here: |
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@viceroypenguin Thanks for the detailed explanation. I understand now the savings you're trying to make with this and it would have been great to have the justification in the initial description. I'll come back to you on this, hopefully before the week is over. |
My main concern was as a method group within a query where you're expecting to use var map =
Enumerable.Range(1, 10)
.Select(x => new { X = x, Y = x * 2 })
.ToDictionary(x => x.X, Identity);I was somewhat misled during my initial review by the explicit type annotations like |
Ah, right. Totally missed that. Amazing how powerful the typing system is sometimes. :) |
Co-authored-by: Atif Aziz <code@raboof.com>
Currently, the C# compiler does not de-dupe methods, including identity methods, such as
x => x. This means each instance ofx => xis compiled as a separate method; and each method must be JIT to asm separately, and must be done so for each type separately.Adding an
Identitymethod improves JIT significantly:Until .net7.0 + tiered PGO, these methods will not get inlined because they are accessed via delegate. This means they also take additional space in memory due to the translated code.