perf(es/minifier): Make character frequency analysis parallel

.changeset/tall-flowers-remember.md

@@ -0,0 +1,6 @@
+---
+swc_core: minor
+swc_ecma_minifier: minor
+---
+
+perf(es/minifier): Make character frequency analysis parallel

crates/swc_ecma_minifier/src/util/base54.rs

@@ -8,6 +8,7 @@ use swc_common::{
 };
 use swc_ecma_ast::*;
 use swc_ecma_codegen::{text_writer::WriteJs, Emitter};
+use swc_ecma_utils::parallel::{cpu_count, Parallel, ParallelExt};
 use swc_ecma_visit::{noop_visit_type, visit_obj_and_computed, Visit, VisitWith};
 
 #[derive(Clone, Copy)]
@@ -226,31 +227,43 @@ impl CharFreq {
     }
 
     pub fn compute(p: &Program, preserved: &FxHashSet<Id>, unresolved_ctxt: SyntaxContext) -> Self {
-        let cm = Lrc::new(DummySourceMap);
+        let (mut a, b) = swc_parallel::join(
+            || {
+                let cm = Lrc::new(DummySourceMap);
+                let mut freq = Self::default();
+
+                {
+                    let mut emitter = Emitter {
+                        cfg: swc_ecma_codegen::Config::default()
+                            .with_target(EsVersion::latest())
+                            .with_minify(true),
+                        cm,
+                        comments: None,
+                        wr: &mut freq,
+                    };
+
+                    emitter.emit_program(p).unwrap();
+                }
 
-        let mut freq = Self::default();
+                freq
+            },
+            || {
+                let mut visitor = CharFreqAnalyzer {
+                    freq: Default::default(),
+                    preserved,
+                    unresolved_ctxt,
+                };
 
-        {
-            let mut emitter = Emitter {
-                cfg: swc_ecma_codegen::Config::default()
-                    .with_target(EsVersion::latest())
-                    .with_minify(true),
-                cm,
-                comments: None,
-                wr: &mut freq,
-            };
-
-            emitter.emit_program(p).unwrap();
-        }
+                // Subtract
+                p.visit_with(&mut visitor);
 
-        // Subtract
-        p.visit_with(&mut CharFreqAnalyzer {
-            freq: &mut freq,
-            preserved,
-            unresolved_ctxt,
-        });
+                visitor.freq
+            },
+        );
 
-        freq
+        a += b;
+
+        a
     }
 
     pub fn compile(self) -> Base54Chars {
@@ -290,11 +303,24 @@ impl CharFreq {
 }
 
 struct CharFreqAnalyzer<'a> {
-    freq: &'a mut CharFreq,
+    freq: CharFreq,
     preserved: &'a FxHashSet<Id>,
     unresolved_ctxt: SyntaxContext,
 }
 
+impl Parallel for CharFreqAnalyzer<'_> {
+    fn create(&self) -> Self {
+        Self {
+            freq: Default::default(),
+            ..*self
+        }
+    }
+
+    fn merge(&mut self, other: Self) {
+        self.freq += other.freq;
+    }
+}
+
 impl Visit for CharFreqAnalyzer<'_> {
     noop_visit_type!();
 
@@ -325,6 +351,42 @@ impl Visit for CharFreqAnalyzer<'_> {
 
     /// This is preserved anyway
     fn visit_module_export_name(&mut self, _: &ModuleExportName) {}
+
+    fn visit_expr_or_spreads(&mut self, n: &[ExprOrSpread]) {
+        self.maybe_par(cpu_count(), n, |v, n| {
+            n.visit_with(v);
+        });
+    }
+
+    fn visit_exprs(&mut self, exprs: &[Box<Expr>]) {
+        self.maybe_par(cpu_count(), exprs, |v, expr| {
+            expr.visit_with(v);
+        });
+    }
+
+    fn visit_module_items(&mut self, items: &[ModuleItem]) {
+        self.maybe_par(cpu_count(), items, |v, item| {
+            item.visit_with(v);
+        });
+    }
+
+    fn visit_opt_vec_expr_or_spreads(&mut self, n: &[Option<ExprOrSpread>]) {
+        self.maybe_par(cpu_count(), n, |v, n| {
+            n.visit_with(v);
+        });
+    }
+
+    fn visit_prop_or_spreads(&mut self, n: &[PropOrSpread]) {
+        self.maybe_par(cpu_count(), n, |v, n| {
+            n.visit_with(v);
+        });
+    }
+
+    fn visit_stmts(&mut self, stmts: &[Stmt]) {
+        self.maybe_par(cpu_count(), stmts, |v, stmt| {
+            stmt.visit_with(v);
+        });
+    }
 }
 
 impl AddAssign for CharFreq {