1507432 - IonBuilder: Prevent inlining when the caller block is cold.

Status: RESOLVED FIXED

(Core :: JavaScript Engine: JIT, enhancement, P5)

Description

[Nicolas B. Pierron [:nbp]]

When running IonBuilder, with Type Inference we are capable of figuring out the function pointer and inlining it even in blocks which have a 0 use-count.

Today our inlining heuristics are based only on the callee use-count, which might miss-lead us into inlining extra code, at the cost of extra time spent in the compiler on the main thread and off-thread.

We can improve over this state by adding a minimum caller threshold before inlining.

Using a threshold at 0, would ensure that we do not inline in block which were never reached, and are likely to be removed by branch pruning. Using a larger threshold might cause issues later on with Scalar Replacement. Non-zero-hit branches are not removed by branch pruning, and these function might "escape" some object/array/scope-chain which could have been removed with Scalar Replacement.

Comment 1

[Nicolas B. Pierron [:nbp]]

Ok, this is harder than I initially thought as we do not increment the code-coverage counters when we are in Ion, which implies that after a recompilation, we might still not inline the function, even if the code got visited multiple time since.

An alternative would be to increment the coverage-counters before the call-site, and only once per block, if we ever fail to inline due to caller counters which are not hot enough.  Currently, we de increment the code-coverage counter after bailout, so the inconsistency of code-coverage counters might not cause much issues.

Comment 2

[Nicolas B. Pierron [:nbp]]

Inlining is disable in the first Ion stage, and the second Ion stage can be considered as being disabled.
So, this should not matter anymore.

Comment 3

[Tom S [:evilpie]]

I think this is not a problem with trial-inlining.