JIT: Allow hot/cold splitting between a BBJ_COND block and its false target

[amanasifkhalid]

Next step for #93020. When doing hot/cold splitting, if the first cold block succeeds a BBJ_COND block (meaning the false target is the first cold block), we previously needed to insert a BBJ_ALWAYS block at the end of the hot section to unconditionally jump to the cold section. Since we will need to conditionally generate a jump to the false target depending on its location once bbFalseTarget can diverge from bbNext, this seemed like a nice opportunity to add that logic in, and instead generate a jump to the cold section by checking if a jump is needed to the false target, rather than by appending a BBJ_ALWAYS block to the hot section.

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Issue Details

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Next step for #93020. When doing hot/cold splitting, if the first cold block succeeds a BBJ_COND block (meaning the false target is the first cold block), we previously needed to insert a BBJ_ALWAYS block at the end of the hot section to unconditionally jump to the cold section. Since we will need to conditionally generate a jump to the false target depending on its location once bbFalseTarget can diverge from bbNext, this seemed like a nice opportunity to add that logic in, and instead generate a jump to the cold section by checking if a jump is needed to the false target, rather than by appending a BBJ_ALWAYS block to the hot section.

Author:	amanasifkhalid
Assignees:	amanasifkhalid
Labels:	area-CodeGen-coreclr
Milestone:	-

[amanasifkhalid]

/azp run runtime-coreclr crossgen2

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[amanasifkhalid]

/azp run runtime-coreclr crossgen2

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[amanasifkhalid]

cc @dotnet/jit-contrib. No diffs, but a slight TP impact. SPMI failures don't seem to be related. Hot/Cold splitting tests passed in runtime-coreclr crossgen2.

[BruceForstall]

Regarding no diffs: I don't believe we have any hot/cold splitting in any of the SPMI collections. We probably should.

[amanasifkhalid]

Regarding no diffs: I don't believe we have any hot/cold splitting in any of the SPMI collections. We probably should.

Would that just entail enabling "fake" hot/cold splitting (e.g. DOTNET_JitFakeProcedureSplitting=1) for some methods?

[BruceForstall]

Would that just entail enabling "fake" hot/cold splitting (e.g. DOTNET_JitFakeProcedureSplitting=1) for some methods?

That might be sufficient. Or maybe some crossgen2 + real splitting for some assemblies?

[amanasifkhalid]

Or maybe some crossgen2 + real splitting for some assemblies?

Ah, I just remembered that fake splitting also "fakes" the unwind info by treating the whole method as hot, so turning on real splitting for some crossgen'd assemblies might be more interesting/useful.