Version Bubble implementation flaw: some methods are forced to rejit

[y-yamshchikov]

The runtime does not load precompiled R2R code for methods like System.Collections.Concurrent.ConcurrentDictionary<int32,System.__Canon>.TryAddInternal in case where assembly with entrypoint does not contain in it's manifest metadata a direct mentioning of System.Collections.Concurrent assembly. In case where the method is used from assembly with entrypoint things work fine.

I have explored this issue on x64 and armel machines.
All R2R NI assemblies were built using crossgen2 with "-O --compilebubblegenerics --inputbubble" options.
In my minimal reproducing case the Manifest Metadata of entrypoint assembly contains mentioning of the second assembly, which in turn uses System.Collections.Concurrent stuff. The second assembly's Manifest Metadata contains mentioning of System.Collections.Concurrent assembly.

The runtime makes it's final decision to rejit existing method with this code:

runtime/src/coreclr/vm/prestub.cpp

Lines 591 to 597 in 5d2490e

	pModule = SystemDomain::System()->DefaultDomain()->GetRootAssembly()->GetManifestModule();
	_ASSERT(pModule != NULL);
	if (pModule->IsReadyToRun() && pModule->IsInSameVersionBubble(GetModule()))
	{
	pCode = pModule->GetReadyToRunInfo()->GetEntryPoint(this, pConfig, TRUE /* fFixups */);
	}

IsInSameVersionBubble returns false for the ("clear" entrypoint) : (System.Collections.Concurrent) pair and code goes to rejit when it returns true for (entrypoint using Concurrent stuff) : (System.Collections.Concurrent) pair, the runtime takes some valuable pCode and things work fine.

Should we consider such a behavior a bug?

@jkotas @alpencolt @gbalykov

[Dotnet-GitSync-Bot]

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[jkotas]

cc @dotnet/crossgen-contrib

[mangod9]

moving to 7 since this is not critical for 6.