JIT should track single-BB locals and multi-BB locals separately

[jakobbotsch]

If my measurements are correct then over libraries.pmi, 55% of tracked locals are wholly defined and then used within a single BB:

Total number of non-parameter tracked locals: 1784135
Total number of non-parameter tracked locals wholly defined and only used in a single BB: 990613 (55.5%)

Unless I'm missing something these locals can be tracked separately and per-BB instead of globally in the varsets and should not contribute to the complexity of our dataflow analyses. Thus, we should consider always tracking such locals and not having them count towards the tracked locals limit.

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Issue Details

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If my measurements are correct then over libraries.pmi, 55% of tracked locals are wholly defined and then used within a single BB:

Total number of non-parameter tracked locals: 1784135
Total number of non-parameter tracked locals wholly defined and only used in a single BB: 990613 (55.5%)

Unless I'm missing something these locals can be tracked separately and per-BB instead of globally in the varsets and should not contribute to the complexity of our dataflow analyses. Thus, we should consider always tracking such locals and not having them count towards the tracked locals limit.

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

[jakobbotsch]

cc @dotnet/jit-contrib

[AndyAyersMS]

These single-block locals also don't really need to be in SSA, there can be at most one reaching def for each use (modulo some kind of conditional select operator).

The challenge in creating different classes of locals is usually how much of it shows through elsewhere. In my experience it is hard to hide something like this, so many producers / consumers may need to be modified.

There are already vestiges of something like this in the jit -- lvaGrabTemp has a shortLifetime parameter that perhaps once meant that this temp would only be used within a block.