Bug 3814 - Calling value type over generic type parameter is slow
Summary: Calling value type over generic type parameter is slow
Alias: None
Product: Runtime
Classification: Mono
Component: JIT ()
Version: unspecified
Hardware: PC Linux
: --- normal
Target Milestone: ---
Assignee: Bugzilla
Depends on:
Reported: 2012-03-08 12:28 UTC by Marek Safar
Modified: 2012-03-12 20:11 UTC (History)
3 users (show)

Is this bug a regression?: ---
Last known good build:

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Description Marek Safar 2012-03-08 12:28:24 UTC
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication5
    interface IA
        void Method();

    struct S : IA
        public void Method()

    struct SG
        public void Test<T>(ref T t) where T : IA

	class Program
		static void Main (string[] args)
			S s;
            var sw = new Stopwatch();
            for (int i = 0; i < 100000000; ++i)
                s = new S ();
                new SG().Test(ref s);
			Console.WriteLine (s);

on amd64

Mono: 620
.NET: 52
Comment 1 Zoltan Varga 2012-03-10 13:42:46 UTC
This has nothing to do with calling valuetypes, the mono JIT inlines everything into a tight loop, while the MS JIT realizes that the loop doesn't do anything and eliminates it altogether.
Comment 2 Marek Safar 2012-03-12 08:24:22 UTC
That's not the case. Increasing loop number count by 10 fold increases the time by 10 fold for .NET

from 52 to 551
Comment 3 Zoltan Varga 2012-03-12 11:06:30 UTC
The loop doesn't do anything, MS is just better at removing this nothingness than we are.
Comment 4 Zoltan Varga 2012-03-12 20:11:05 UTC
-O=all now reduces this to 71 from 462, and running with the LLVM backend reduces it to 1.