Experimental Lite Mode (#265)

new code-size compiler flags: :lite-mode, :elide-to-string

:lite-mode emits updated variants of the original copy-on-write data
structures from 2011. Notably, in `:lite-mode` chunked-seqs are not
supported as they pull in a significant amount of code, which defeats
the purpose of the new flags. Also while the transient interfaces are
implemented on the copy-on-write data structures, they do not actually
provide a fast path. This is not by design, but simply a practical
scoping of effort as the work done here is already considerable.

The other serious code-size issue are `.toString` impls. There are
many interesting smaller programs that simply do not need the
convenient `.toString` invoking the ClojureScript recursive printing
machinery. By combining `:lite-mode true` with `:elide-to-string true`
the emitted code is cut in half or more for simpler programs.

For a rough idea of the space-savings, a hash-map literal is ~6K after
advanced compilation with the new flags and brotli
compression. ClojureScript 1.12.42 is ~17K brotli for the same program.

The following expression

(.log js/console
      (->> (map inc (range 10))
           (filter even?)
           (partition 2)
           (drop 1)
           (mapcat identity)
           into-array))

Is ~6K after advanced compilation + brotli compression. ClojureScript
is 1.12.42 is ~19K.

For users that want the full expressiveness of the Clojure APIs and
the interactivity of the REPL for smaller and/or simpler projects, but
don't need every last bit of performance, these two flags should lead
to compact artifacts.

Author: swannodette

.github/workflows/test.yaml

@@ -59,6 +59,63 @@ jobs:
           /System/Library/Frameworks/JavaScriptCore.framework/Versions/A/Helpers/jsc builds/out-adv/core-advanced-test.js | tee test-out.txt
           grep -qxF '0 failures, 0 errors.' test-out.txt
 
+  # Lite Tests
+  lite-test:
+    name: Lite Tests
+    runs-on: macos-14
+    steps:
+      - uses: actions/checkout@v2
+
+      - uses: actions/setup-java@v4
+        with:
+          distribution: 'temurin'
+          java-version: '21'
+
+      - uses: DeLaGuardo/[email protected]
+        with:
+          tools-deps: '1.10.1.763'
+
+      - name: Cache maven
+        uses: actions/cache@v4
+        env:
+          cache-name: cache-maven
+        with:
+          path: ~/.m2
+          key: ${{ runner.os }}-${{ env.cache-name }}-${{ hashFiles('**/deps.edn') }}
+          restore-keys: |
+            ${{ runner.os }}-${{ env.cache-name }}-
+
+      - name: Cache gitlibs
+        uses: actions/cache@v4
+        env:
+          cache-name: cache-gitlibs
+        with:
+          path: ~/.gitlibs
+          key: ${{ runner.os }}-${{ env.cache-name }}-${{ hashFiles('**/deps.edn') }}
+          restore-keys: |
+            ${{ runner.os }}-${{ env.cache-name }}-
+
+      # - name: Cache JSC
+      #   uses: actions/cache@v4
+      #   env:
+      #     cache-name: cache-jsc
+      #   with:
+      #     path: WebKit
+      #     key: ${{ runner.os }}-jsc
+      #     restore-keys: |
+      #       ${{ runner.os }}-jsc
+
+      - name: Build tests
+        run: clojure -M:lite.test.build
+
+      # - name: Install JSC
+      #   run: ./ci/install_jsc.sh
+
+      - name: Run tests
+        run: |
+          /System/Library/Frameworks/JavaScriptCore.framework/Versions/A/Helpers/jsc builds/out-lite/lite-test.js | tee test-out.txt
+          grep -qxF '0 failures, 0 errors.' test-out.txt
+
     # Runtime Tests
   runtime-windows-test:
     name: Runtime Windows Tests

deps.edn

@@ -19,6 +19,8 @@
                                       "-e" "(cljs.test-runner/-main)"]}
   :runtime.test.build    {:extra-paths ["src/test/cljs"]
                           :main-opts ["-m" "cljs.main" "-co" "resources/test.edn" "-c"]}
+  :lite.test.build       {:extra-paths ["src/test/cljs"]
+                          :main-opts ["-m" "cljs.main" "-co" "resources/lite_test.edn" "-c"]}
   :selfhost.test.build   {:extra-paths ["src/test/self"]
                           :main-opts ["-m" "cljs.main" "-co" "resources/self_host_test.edn" "-c"]}
   :selfparity.test.build {:extra-paths ["src/test/self"]

resources/lite_test.edn

@@ -0,0 +1,28 @@
+{:optimizations    :advanced
+ :main             lite-test-runner
+ :output-to        "builds/out-lite/lite-test.js"
+ :output-dir       "builds/out-lite"
+ :output-wrapper   true
+ :verbose          true
+ :compiler-stats   true
+ :parallel-build   true
+ :npm-deps         {:lodash "4.17.4"}
+ :closure-warnings {:non-standard-jsdoc :off :global-this :off}
+ :install-deps     true
+ :language-out     :es5
+ :foreign-libs
+ [{:file "src/test/cljs/calculator_global.js"
+   :provides ["calculator"]
+   :global-exports {calculator Calculator}}
+  {:file "src/test/cljs/es6_dep.js"
+   :module-type :es6
+   :provides ["es6_calc"]}
+  {:file "src/test/cljs/calculator.js"
+   :module-type :commonjs
+   :provides ["calculator"]}
+  {:file "src/test/cljs/es6_default_hello.js"
+   :provides ["es6_default_hello"]
+   :module-type :es6}]
+ :pseudo-names     true
+ :pretty-print     true
+ :lite-mode        true}

src/main/cljs/cljs/analyzer/passes/lite.cljc

@@ -0,0 +1,32 @@
+;; Copyright (c) Rich Hickey. All rights reserved.
+;; The use and distribution terms for this software are covered by the
+;; Eclipse Public License 1.0 (http://opensource.org/licenses/eclipse-1.0.php)
+;; which can be found in the file epl-v10.html at the root of this distribution.
+;; By using this software in any fashion, you are agreeing to be bound by
+;; the terms of this license.
+;; You must not remove this notice, or any other, from this software.
+
+(ns cljs.analyzer.passes.lite
+  (:refer-clojure :exclude [var?]))
+
+(defn var? [ast]
+  (= :var (:op ast)))
+
+(def ctor->simple-ctor
+  '{cljs.core/vector cljs.core/simple-vector
+    cljs.core/vec    cljs.core/simple-vec})
+
+(defn update-var [{:keys [name] :as ast}]
+  (let [replacement (get ctor->simple-ctor name)]
+    (-> ast
+      (assoc :name replacement)
+      (assoc-in [:info :name] replacement))))
+
+(defn replace-var? [ast]
+  (and (var? ast)
+       (contains? ctor->simple-ctor (:name ast))))
+
+(defn use-lite-types
+  [env ast _]
+  (cond-> ast
+    (replace-var? ast) update-var))