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Restructure [link_and_optimize] as a [link] producer followed by a list of transformer passes ([dce], [optimize]), threaded through an explicit [step_io] (wasm file + optional sourcemap). A small runner sends every pass but the last to a fresh temp file, and the last straight to [output_file] / [opt_sourcemap_file]. Pure refactoring: the same passes run in the same order for every profile, so the output is unchanged. The point is that whichever pass ends the pipeline now writes the final file itself, so a later change that drops [optimize] at --opt 1 needs no copy to land the link's or DCE's output at the right path.
At --opt 1, drop the [optimize] (wasm-opt) pass from the pipeline, both in whole-program linking (remove it from the transformer list) and in separate compilation (write the unit's wasm directly instead of routing it through [Binaryen.optimize]). Our own passes handle what wasm-opt would have done; at --opt 2/3 wasm-opt still runs.
Add a [Var_coalescing] pass: a linear-scan variable coalescing pass for the Wasm backend, mirroring [Js_variable_coalescing]. When --opt 1 skips Binaryen we need our own pass to reuse locals; at --opt 2/3 it is off and Binaryen still handles coalescing. Runs in [post_process_function_body]; gated on [O1] and the new [wasm-var-coalescing] flag.
Add a [Local_sink] pass that rewrites
local.set x e
...
local.get x
into
...
local.tee x e
when the sink is sound: we must not cross another write to [x], and
moving [e] past intermediate code must not reorder observable effects.
Because [Local_sink] always introduces [local.tee] (never inlines [e]
itself), some of the tees it produces are immediately dead: the variable
is never read again. Extend [Var_coalescing] to detect such dead tees
using the liveness it already computes — a [LocalTee x _] Def node whose
successors do not list [x] in their [live_in] is a dead store. Replace it
with its inner expression, and drop the local from the function's
[locals] list when its last reference was the tee we just erased.
Runs first in [post_process_function_body], before [Var_coalescing];
gated on [O1] and the new [wasm-local-sink] flag.
Wasm encodes local indices as LEB128 u32 (1 byte for < 128, 2 above). With [wasm-opt] skipped at --opt 1, hot locals routinely cross the 128 boundary and pay an extra byte per access. Add a [Reorder_locals] pass that sorts non-parameter locals into a numeric block followed by a reference block, with same-type runs ordered by descending total use count and individual locals inside a run by descending per-local count. Indices are derived from list position, so reordering [locals] alone suffices. Runs at the end of [post_process_function_body], after [Initialize_locals.f]; gated on [O1] and the new [wasm-reorder-locals] flag.
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We skip calling wasm-opt with --opt 1. Expected benefits:
Instead, we perform a few optimizations to reduce the code size and avoid having too many variables per function:
local.setThe wasm_of_ocaml compilation time for the test suite is reduced by about 60%. On the CI,
dune build @runtest-wasmfrom 2m 19s down to 1m 40s (-28%).Speedup on ocaml and PRT: