[Xamarin.Android.Build.Tasks] optimize $(AssemblySearchPaths)

[jonathanpeppers]

Context: https://docs.microsoft.com/en-us/visualstudio/msbuild/resolveassemblyreference-task?view=vs-2017
Context: https://twitter.com/KirillOsenkov/status/1043725067081764865

Kirill added a new analyzer to the Windows binlog viewer, noting
$(AssemblySearchPaths) that are unused during a build.

Looking at what we are using currently, it reports:

Unused AssemblySearchPaths locations
    {AssemblyFolders}
    {AssemblyFoldersFromConfig:C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\MSBuild\15.0\Bin\AssemblyFolders.config,v9.0}
    {CandidateAssemblyFiles}
    {Registry:Software\Microsoft\MonoAndroid,v9.0,AssemblyFoldersEx}
    bin\Debug\

Luckily, we already had MSBuild logic in place to remove {GAC} from
this list.

But we certainly are probing search paths that are nonsensical for
Xamarin.Android:

• {AssemblyFolders}: Specifies the task will use the Visual
  Studio.NET 2003 finding-assemblies-from-registry scheme.
• {AssemblyFoldersFromConfig:*}: this probes the system-wide config
  file in the MSBuild / Visual Studio install
• {CandidateAssemblyFiles}: The CandidateAssemblyFiles property on
  ResolveAssemblyReferences is blank. Docs say: Assemblies in this
  list will be considered when the SearchPaths parameter contains
  {CandidateAssemblyFiles} as one of the paths to consider.
• {Registry:*}: probe the Windows registry...
• bin\Debug or $(OutputPath): not needed, and a potential cause of
  confusion. This would be if there was an arbitrary extra assembly
  sitting in $(OutputPath).

And if we look further, we see odd things:

Primary reference "System, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e".
    Resolved file path is "C:\Users\myuser\Desktop\Git\xamarin-android\bin\Control\lib\xamarin.android\xbuild-frameworks\MonoAndroid\v1.0\System.dll".
    Reference found at search path location "{TargetFrameworkDirectory}".
    For SearchPath "{TargetFrameworkDirectory}".
        Considered "C:\Users\myuser\Desktop\Git\xamarin-android\bin\Control\lib\xamarin.android\xbuild-frameworks\MonoAndroid\v9.0\System.winmd", but it didn't exist.
        Considered "C:\Users\myuser\Desktop\Git\xamarin-android\bin\Control\lib\xamarin.android\xbuild-frameworks\MonoAndroid\v9.0\System.dll", but it didn't exist.
        Considered "C:\Users\myuser\Desktop\Git\xamarin-android\bin\Control\lib\xamarin.android\xbuild-frameworks\MonoAndroid\v9.0\System.exe", but it didn't exist.
        Considered "C:\Users\myuser\Desktop\Git\xamarin-android\bin\Control\lib\xamarin.android\xbuild-frameworks\MonoAndroid\v1.0\System.winmd", but it didn't exist.

Why are we looking for .winmd files? This is a UWP thing...

And then apparently, our build also will look for .pri files?
Another UWP thing...

AllowedRelatedFileExtensions
    .pdb
    .xml
    .pri
    .dll.config
    .exe.config
    .dll.mdb
    .exe.mdb

So the first fix is to trim $(AssemblySearchPaths) to precisely what
Xamarin.Android needs:

<AssemblySearchPaths>
    {HintPathFromItem};
    {TargetFrameworkDirectory};
    {RawFileName};
</AssemblySearchPaths>

We shouldn't be concerned about other system-wide places.

Next, we should only be looking at these file extensions:

<AllowedReferenceAssemblyFileExtensions>
    .dll;
</AllowedReferenceAssemblyFileExtensions>
<AllowedReferenceRelatedFileExtensions>
    .pdb;
    .xml;
    .dll.config;
    .dll.mdb;
</AllowedReferenceRelatedFileExtensions>

Removing .winmd from $(AllowedReferenceAssemblyFileExtensions) and
.pri from $(AllowedReferenceRelatedFileExtensions). We can also
ignore .exe* files, since Xamarin.Android projects only use .dll
files.

This makes the log probing for System.dll much cleaner:

Primary reference "System, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e".
    Resolved file path is "C:\Users\myuser\Desktop\Git\xamarin-android\bin\Release\lib\xamarin.android\xbuild-frameworks\MonoAndroid\v1.0\System.dll".
    Reference found at search path location "{TargetFrameworkDirectory}".
    For SearchPath "{TargetFrameworkDirectory}".
        Considered "C:\Users\myuser\Desktop\Git\xamarin-android\bin\Release\lib\xamarin.android\xbuild-frameworks\MonoAndroid\v9.0\System.dll", but it didn't exist.

Here is the log of changes made to these properties here at MSBuild
evaluation time:

Property reassignment: $(AssemblySearchPaths)="
        {HintPathFromItem};
        {TargetFrameworkDirectory};
        {RawFileName};
    " (previous value: "
    {CandidateAssemblyFiles};
    ;
    {HintPathFromItem};
    {TargetFrameworkDirectory};
    {AssemblyFoldersFromConfig:C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\MSBuild\15.0\Bin\AssemblyFolders.config,v9.0};
    {Registry:Software\Microsoft\MonoAndroid,v9.0,AssemblyFoldersEx};
    {AssemblyFolders};
    {GAC};
    {RawFileName};
    bin\Debug\
    ") at xamarin-android\bin\Release\lib\xamarin.android\xbuild\Xamarin\Android\Xamarin.Android.Common.targets (338,2)

Property reassignment: $(AllowedReferenceAssemblyFileExtensions)="
        .dll;
    " (previous value: "
    .winmd;
    .dll;
    .exe
    ") at xamarin-android\bin\Release\lib\xamarin.android\xbuild\Xamarin\Android\Xamarin.Android.Common.targets (343,2)

Property reassignment: $(AllowedReferenceRelatedFileExtensions)="
        .pdb;
        .xml;
        .dll.config;
        .dll.mdb;
    " (previous value: "
    .pdb;
    .xml;
    .pri;
    .dll.config;
    .exe.config
    ") at xamarin-android\bin\Release\lib\xamarin.android\xbuild\Xamarin\Android\Xamarin.Android.Common.targets (347,2)

Results

I timed the Xamarin.Forms Control Gallery project, since I felt this
change would improve larger projects the most.

Project here:
https://github.com/jonathanpeppers/Xamarin.Forms/tree/msbuild-timing

Before:

 1106 ms  ResolveAssemblies                          2 calls
 1401 ms  ResolveAssemblyReference                  18 calls

After:

  909 ms  ResolveAssemblies                          2 calls
 1088 ms  ResolveAssemblyReference                  18 calls

This seems to directly save ~500ms to these targets in incremental
builds for this project. This project references ~7 other Xamarin.Android
projects.

Upstream

On MacOS, I also see probing for .winmd files!

Considered "xamarin-android/bin/Debug/lib/xamarin.android/xbuild-frameworks/MonoAndroid/v9.0/System.winmd", but it didn't exist.

Perhaps we need some upstream changes to MSBuild as well...

[jonathanpeppers]

Log files: AssemblySearchPaths.zip

[dellis1972]

not sure we need .exe for XA. Since none of our apps or projects use it.

[jonathanpeppers]

Yeah, I think we can drop .exe. This might improve build times a bit further.

[jonathanpeppers]

FYI I didn't see a significant performance difference in also removing .exe, .exe.config, .exe.mdb, but surely would help a little.

I think the biggest win was removing these:

    {AssemblyFolders}
    {AssemblyFoldersFromConfig:C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\MSBuild\15.0\Bin\AssemblyFolders.config,v9.0}
    {CandidateAssemblyFiles}
    {Registry:Software\Microsoft\MonoAndroid,v9.0,AssemblyFoldersEx}
    bin\Debug\