Linker performance issue with GetCustomDebugInformation

[jeromelaban]

When linking a set of assemblies which contain Microsoft.SourceLink.GitHub with EmbedUntrackedSources enabled, the processing time will increase significantly.

Lots of the processing time is spent in GetCustomDebugInformation : https://github.com/jbevain/cecil/blob/5e37f44535e0753ff3e4dbdef4ef148ca8bb09e0/Mono.Cecil/AssemblyReader.cs#L3241

Here's a stack trace leading to this:

 	mscorlib.dll!System.IO.Stream.InternalCopyTo(System.IO.Stream destination, int bufferSize)	Unknown
>	Mono.Cecil.dll!Mono.Cecil.MetadataReader.GetCustomDebugInformation(Mono.Cecil.Cil.ICustomDebugInformationProvider provider) Line 3242	C#
 	Mono.Cecil.dll!Mono.Cecil.MetadataReader.GetDocument(uint rid) Line 2881	C#
 	Mono.Cecil.dll!Mono.Cecil.MetadataReader.ReadSequencePoints(Mono.Cecil.MethodDefinition method) Line 2870	C#
 	Mono.Cecil.dll!Mono.Cecil.Cil.PortablePdbReader.Read(Mono.Cecil.MethodDefinition method) Line 118	C#
 	Mono.Cecil.dll!Mono.Cecil.Cil.CodeReader.ReadMethodBody() Line 109	C#
 	Mono.Cecil.dll!Mono.Cecil.Cil.CodeReader.ReadMethodBody(Mono.Cecil.MethodDefinition method) Line 61	C#
 	Mono.Cecil.dll!Mono.Cecil.MethodDefinition.get_Body.AnonymousMethod__41_0(Mono.Cecil.MethodDefinition method, Mono.Cecil.MetadataReader reader) Line 154	C#
 	Mono.Cecil.dll!Mono.Cecil.ModuleDefinition.Read<System.__Canon, System.__Canon>(ref System.__Canon variable, System.__Canon item, System.Func<System.__Canon, Mono.Cecil.MetadataReader, System.__Canon> read) Line 989	C#
 	Mono.Cecil.dll!Mono.Cecil.MethodDefinition.Body.get() Line 156	C#
 	monolinker.exe!Mono.Linker.Steps.RemoveUnreachableBlocksStep.RewriteBody(Mono.Cecil.MethodDefinition method) Line 129	C#
 	monolinker.exe!Mono.Linker.Steps.RemoveUnreachableBlocksStep.RewriteBodies(Mono.Collections.Generic.Collection<Mono.Cecil.TypeDefinition> types) Line 119	C#
 	monolinker.exe!Mono.Linker.Steps.RemoveUnreachableBlocksStep.Process() Line 45	C#
 	monolinker.exe!Mono.Linker.Steps.BaseStep.Process(Mono.Linker.LinkContext context) Line 60	C#
 	monolinker.exe!Mono.Linker.Pipeline.ProcessStep(Mono.Linker.LinkContext context, Mono.Linker.Steps.IStep step) Line 134	C#
 	monolinker.exe!Mono.Linker.Pipeline.Process(Mono.Linker.LinkContext context) Line 127	C#
 	monolinker.exe!Mono.Linker.Driver.Run(Mono.Linker.ILogger customLogger) Line 609	C#
 	monolinker.exe!Mono.Linker.Driver.Execute(string[] args, Mono.Linker.ILogger customLogger) Line 67	C#
 	monolinker.exe!Mono.Linker.Driver.Main(string[] args) Line 51	C#

[marek-safar]

Will keep this open as tracking issue for Cecil issue

[jeromelaban]

This issue is getting problematic when using C# 9.0 source generators, as their symbols and source is getting embedded in the pdbs, making the linker step significantly longer.

(/cc @jaredpar who may be interested as this relates to source generation)

[jaredpar]

Source generators should observably be no different than just embedding sources during build as it's just embedding a subset of files.

[jeromelaban]

The origin of the files will not impact, indeed. I'm mentioning this issue as there may be more scenarios coming in the future involving source generation than ones with source link.

[jeromelaban]

Fixed in jbevain/cecil#684