Bug 10866 - [PATCH] Linker does not remove unused method when implements unused interface method
Summary: [PATCH] Linker does not remove unused method when implements unused interface...
Alias: None
Product: iOS
Classification: Xamarin
Component: Tools ()
Version: 6.3.x
Hardware: PC Mac OS
: --- normal
Target Milestone: Untriaged
Assignee: Sebastien Pouliot
Depends on:
Reported: 2013-03-04 02:41 UTC by Marek Safar
Modified: 2013-03-05 11:06 UTC (History)
1 user (show)

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

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Description Marek Safar 2013-03-04 02:41:20 UTC
Sample which reproduces the problem

using System;

interface I
	void Foo ();

class A : I
	public void Foo ()

class B : I
	// This method is not used but it's not removed
	public void Foo ()

public class Test
    public static void Main ()
		new A ().Foo ();
		new B ();

Suggested fix, which helps reduce mscorlib size further (tested with the latest mscorlib)

index 804d9d6..b484b3d 100644
--- a/mcs/tools/linker/Mono.Linker.Steps/MarkStep.cs
+++ b/mcs/tools/linker/Mono.Linker.Steps/MarkStep.cs
@@ -843,6 +843,9 @@ namespace Mono.Linker.Steps {
                        foreach (MethodDefinition base_method in base_methods) {
+                               if (base_method.DeclaringType.IsInterface)
+                                       continue;
                                MarkMethod (base_method);
                                MarkBaseMethods (base_method);
Comment 1 Sebastien Pouliot 2013-03-04 17:08:34 UTC
Maybe I do not completely follow but:

1) Applying the above patch does not remove B::Foo;

2) It would be invalid. IOW if `Foo` is removed from `B` then it won't be implementing `I`;

3) Even if the patch worked (to remove B::Foo) it would be easy to break, e.g. add:

	static void F (I i)
		i.Foo ();

and call:

	F (new B ());

Scanning `F` will mark `I.Foo`, not B.Foo, so the code will be invalid.
Comment 2 Marek Safar 2013-03-04 17:12:40 UTC
I am not sure how you ran it or maybe you applied it incorrectly. The patch removes B::Foo and obviously it removes I:Foo too otherwise the code would be invalid. So the output binary has only A:Foo and empty I interface.
Comment 3 Marek Safar 2013-03-04 17:16:31 UTC
Forget to comment about 3.

That would not break the patch at all. It'd mark I:Foo because you are calling it from inside F so the linker see I:Foo used.
Comment 4 Sebastien Pouliot 2013-03-04 17:28:13 UTC
My bad, B::Foo was not removed because the unit test I wrote preserved the `F` method (1) and since I::Foo is not marked it means `I` is satisfied (2) and if it's marked by `F` (like my mistake) then `B:Foo` is kept (3) and valid.

Please commit and bump - I'll commit the unit tests (to linkall.app) afterward.
Comment 5 Marek Safar 2013-03-05 11:06:17 UTC
Fixed in master, with extra check for broken compilers