Split type t into Imm values and Sym hash-consed nodes

[filipeom]

This commit fundamentally changes the representation of expressions (t) by lifting constant values out of the hash-consing machinery.

Changes:

• t is now a variant: Imm of Value.t | Sym of expr Hc.hash_consed.
• Removed Val and Loc constructors from the underlying expr type.
• Constants are no longer hash-consed; they are stored directly as Imm.

Potential Performance Improvements:

• Creating constant expressions no longer requires a hash-cons table lookup or allocation of a hash-cons node.
• The global hash-cons table is no longer polluted with ephemeral constant values.

BREAKING CHANGE: Previously, because all expressions were hash-consed, structural equality implied physical equality (phys_equal was sufficient for all comparisons).

This invariant no longer holds for Imm values.

[redianthus]

What is the impact on performances for ECMA-SL on your hash-consing heavy program you mentioned? (The one where switching from weak to strong reduced the overhead from 40% to 20%)

[filipeom]

What is the impact on performances for ECMA-SL on your hash-consing heavy program you mentioned? (The one where switching from weak to strong reduced the overhead from 40% to 20%)

It's around 14% with this PR. So the issue may actually be in ECMA-SL. I think we might be generating very hashconsing unfriendly expressions. I'm now trying to get tracing into ECMA-SL and smtml to see if I can identify what types of formualas/expressions cause the slowdown.

In owi I really didn't see any change between the three branches 😅

[redianthus]

Interesting. Have you tried using landmarks while running this particular program?

[filipeom]

Interesting. Have you tried using landmarks while running this particular program?

Not yet! But I was very interested in what you did in Owi (I think this was the PR OCamlPro/owi#871?). I think we will experiment doing the same thing in ECMA-SL. It should be easier since we're not multicore 😅

[redianthus]

Yes, there is nothing you should do to get it working. Simply add:

 (instrumentation
  (backend landmarks --auto)))

to your library/executable in dune.

Then :

OCAML_LANDMARKS=on dune exec --instrument-with landmarks --profile release -- owi run test/run/binary_loop.wasm

[hra687261]

Interesting. Have you tried using landmarks while running this particular program?

Not yet! But I was very interested in what you did in Owi (I think this was the PR OCamlPro/owi#871?). I think we will experiment doing the same thing in ECMA-SL. It should be easier since we're not multicore 😅

(This is the right PR: OCamlPro/owi#872 that one is outdated :) )