JIT: More precise side effect propagation in loop hoisting

[amanasifkhalid]

Follow-up to #118463. As we walk a loop's blocks in RPO, propagate the side-effecting nature of each block to its successors, rather than flipping a single flag once side-effecting code is encountered.

[Copilot]

Pull Request Overview

This PR improves the precision of side effect propagation during loop hoisting optimization in the JIT compiler. The change replaces a simple boolean flag with a more sophisticated bit vector approach to track which blocks have encountered side effects.

• Replaces the m_beforeSideEffect boolean field with a m_sideEffectBlocks bit vector
• Tracks side effect state per block rather than globally across the entire loop
• Propagates side effect state to successor blocks when side effects are encountered

[dotnet-policy-service]

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.

[amanasifkhalid]

cc @dotnet/jit-contrib, @AndyAyersMS PTAL. Diffs show PerfScore improvements outnumbering regressions in most collections, except for libraries_tests_no_tiered_compilation. Thanks!

[AndyAyersMS]

Can you call out some specific benchmarks where you expect to see perf improvements?

Can you verify any of these with local runs? Or via Egorbot?

[AndyAyersMS]

Approving but would still like to see some examples where this will resolve current perf issues.

[amanasifkhalid]

@EgorBot --arm --filter ByteMark.BenchLUDecomp

[amanasifkhalid]

@AndyAyersMS I'm not seeing as much impact in the microbenchmarks as I hoped for, mainly because this seems to impact the OSR variants of various benchmarks instead of their Tier 1 variants. Since this change looks riskier than #118463, I'm ok with holding off on this.

[AndyAyersMS]

@AndyAyersMS I'm not seeing as much impact in the microbenchmarks as I hoped for, mainly because this seems to impact the OSR variants of various benchmarks instead of their Tier 1 variants. Since this change looks riskier than #118463, I'm ok with holding off on this.

Sure, makes sense. Thanks for checking.

[JulieLeeMSFT]

@AndyAyersMS, is this ready to merge?

[AndyAyersMS]

We backed out a previous change so I'd say no. We need to look at that one and this carefully.