Do more aggressive lambda lifting

[OlivierNicole]

With @vouillon we realized that the Lambda_lifting_simple pass that is performed by double translation makes some programs significantly faster. We measured roughly a 1.45 speedup on a large (proprietary) Bonsai benchmark. Presumably V8 is much faster with more toplevel functions and less nested closures.

Making this a draft PR for now because:

• It should probably be behind a flag: it makes the generated Javascript further from the source code structure;
• It theoretically results in a quadratic code size.

We probably need to do some measurements before merging. (And maybe with other engines.)

[OlivierNicole]

This seems to result in a modest speedup on average, including on partial_render_table. Is this the version of the benchmark that has a memory leak?

[vouillon]

This seems to result in a modest speedup on average, including on partial_render_table. Is this the version of the benchmark that has a memory leak?

No, I'm using a workaround.

[OlivierNicole]

Should I guard this by an --enable lambda-lift-all option?

[vouillon]

Yes, you should add a flag.

[OlivierNicole]

I re-ran the CI a few times to double-check whether this new pass increases the compilation time. The compilation time seems to oscillate around ±2 %, so I would say it’s in the noise.

[hhugo]

You should probably rebase and rerun the bench now that #1935 is merged to check that this PR still improves perfs

[hhugo]

@OlivierNicole, there might be some improvement and the size increase seems small. Do you want to rebase the PR one more time and have the feature under a flag ? That way, we can merge instead of letting the PR rot.

[OlivierNicole]

Yes, I intend to do this; I just haven’t found the time to squeeze it between my other tasks yet…

[OlivierNicole]

The test failures seem unrelated to this PR, as they also show up on master.

[OlivierNicole]

Sorry for the delay in coming back to this. I have put the feature behind a flag.

[hhugo]

Please a small test for this, probably in compiler/tests-compiler/

[OlivierNicole]

Done. The CI failure seems unrelated.