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Tree-shaking: drop library code unreachable from -main (lever 3/4)

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`jolt build --tree-shake` (or deps.edn :jolt/build {:tree-shake true}) does
reachability DCE over the re-emitted app + library namespaces: keep -main, every
side-effecting (non-def) top-level form, and every def reachable from those; drop
the rest. A macro (expanded at AOT, never called at runtime) is prunable too.

Sound: bails (keeps everything) if REACHABLE code resolves vars by name at runtime
(eval/resolve/ns-resolve/requiring-resolve/find-var/intern/load-string/...), which a
static call graph can't follow. Unreached eval-using library code is simply shaken
away and never triggers the bail. clojure.core and the compiler image stay baked
(prelude + image blobs), so only re-emitted namespaces are shaken for now.

The reachability machinery is in emit-image.ss (records: keep?/fqn/refs/str via
reduce-ir-children) + build.ss (BFS + bail check). build-smoke covers it (drops the
unreachable `twice` macro, output unchanged). Opt-in; default builds are untouched.
full make test green.

Scope note: this shakes the re-emitted app/lib code only. Measurement shows jolt's
compiled code is ~5.8MB of a ~9.8MB binary, dominated by the clojure.core prelude
(~1.5-2MB) and the compiler image (~0.8MB) — both baked blobs this pass doesn't
touch. Those (shake-core, drop-compiler-when-no-eval) are the larger footprint wins,
filed as follow-ups.
This commit is contained in:
Yogthos 2026-06-23 19:45:13 -04:00
parent 3e0fb805da
commit 75ac93689b
4 changed files with 139 additions and 10 deletions
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9
jolt-core/jolt/main.clj
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@ -170,10 +170,13 @@
natives (encode-natives (:natives resolved))
;; closed-world direct-linking is opt-in: the --direct-link flag or a
;; deps.edn :jolt/build {:direct-link true}. Off otherwise.
direct-link? (boolean (or (some #{"--direct-link"} more) (:direct-link build)))]
direct-link? (boolean (or (some #{"--direct-link"} more) (:direct-link build)))
;; tree-shaking (drop library code not reachable from -main): --tree-shake
;; or deps.edn :jolt/build {:tree-shake true}.
tree-shake? (boolean (or (some #{"--tree-shake"} more) (:tree-shake build)))]
;; embed-dirs (absolute) are walked + baked into the binary by the driver;
;; project-paths (relative) become runtime io/resource roots (ship-alongside).
(jolt.host/build-binary entry out mode natives embed-dirs project-paths direct-link?)))))
(jolt.host/build-binary entry out mode natives embed-dirs project-paths direct-link? tree-shake?)))))
(defn- nrepl [more]
;; resolve the project (deps on the roots, native libs loaded), then start the
@ -191,7 +194,7 @@
(println "usage: jolt <command> [args]")
(println " run -m NS [args] resolve deps.edn, load NS, call its -main")
(println " run FILE load a Clojure file")
(println " build -m NS [-o OUT] [--opt|--dev] [--direct-link] compile a standalone binary")
(println " build -m NS [-o OUT] [--opt|--dev] [--direct-link] [--tree-shake] compile a standalone binary")
(println " -M:alias [args] run the alias's :main-opts")
(println " -A:alias [args] add the alias's paths/deps")
(println " repl start a line REPL")