Improving trace quality by tracking "fitness" and "exit quality"

Feature or enhancement

Proposal:

The performance of the JIT depends heavily on the quality of the traces that the front-end produces. If traces are too short, very long, overlap a lot, or drop into the interpreter too often, performance suffers.

To avoid those cases of poor performance, we should track two values during tracing:

• The fitness of the current trace
• The quality of the end of the trace as a point to stop the trace.

If fitness < exit_quality we should stop the trace.

Fitness:

Fitness should:

• start high
• be reduced by branches, how much depending on the bias of branch
• be reduced (by quite a lot) by backward edges
• be reduced a small amount by each instruction (to discourage excessively long traces)
• start lower for non-branch side exits to reduce code replication

Exit quality

Exit quality should be high for:

• The starting point of the trace (very high for this)
• ENTER_EXECUTOR instructions
• Merge points in the control flow graph (will need the bytecode compiler to generate this data)

Exit quality should be low for:

• Specializable instructions

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs

• gh-147966
• gh-148082
• gh-148089
• gh-148301
• gh-148840
• gh-148959

Start by reviewing the proposal's fitness and exit-quality criteria, then examine the linked PRs gh-147966, gh-148082, gh-148089, gh-148301, gh-148840, and gh-148959. The work is done when tracing stops when fitness falls below exit quality and the listed trace, branch, backward-edge, instruction, and merge-point behaviors are covered.