Performance issue with GetCustomDebugInformation

[marek-safar]

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 :

cecil/Mono.Cecil/AssemblyReader.cs

Line 3241 in 5e37f44

using (var deflate_stream = new DeflateStream (compressed_stream, CompressionMode.Decompress, leaveOpen: true))

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#

Originally reported as dotnet/linker#1409

[jbevain]

I see. Yeah that makes sense. We probably shouldn't even decompress/compress unless the user asked to access the uncompressed content, that should speed things up.