perf(es/minifier): Make the first run of DCE more efficient (#9868)

**Description:**

We can drop constant branches from precompress pass to make the pass after it faster.

Author: kdy1

.changeset/stale-apricots-dream.md

@@ -0,0 +1,6 @@
+---
+swc_core: minor
+swc_ecma_minifier: minor
+---
+
+perf(es/minifier): Make the first run of DCE more efficient

crates/swc_ecma_minifier/src/lib.rs

@@ -44,6 +44,7 @@ use swc_common::{comments::Comments, pass::Repeated, sync::Lrc, SourceMap, Synta
 use swc_ecma_ast::*;
 use swc_ecma_transforms_optimization::debug_assert_valid;
 use swc_ecma_usage_analyzer::marks::Marks;
+use swc_ecma_utils::ExprCtx;
 use swc_ecma_visit::VisitMutWith;
 use swc_timer::timer;
 
@@ -126,7 +127,11 @@ pub fn optimize(
     if let Some(_options) = &options.compress {
         let _timer = timer!("precompress");
 
-        n.visit_mut_with(&mut precompress_optimizer());
+        n.visit_mut_with(&mut precompress_optimizer(ExprCtx {
+            unresolved_ctxt: SyntaxContext::empty().apply_mark(marks.unresolved_mark),
+            is_unresolved_ref_safe: false,
+            in_strict: false,
+        }));
         debug_assert_valid(&n);
     }
 

crates/swc_ecma_minifier/src/pass/precompress.rs

@@ -1,26 +1,50 @@
 use std::vec::Vec;
 
+use swc_common::util::take::Take;
 use swc_ecma_ast::*;
 use swc_ecma_transforms_base::perf::{Parallel, ParallelExt};
+use swc_ecma_utils::{ExprCtx, ExprExt, Value::Known};
 use swc_ecma_visit::{noop_visit_mut_type, VisitMut, VisitMutWith};
 
-use crate::HEAVY_TASK_PARALLELS;
+use crate::CPU_COUNT;
 
 /// Optimizer invoked before invoking compressor.
 ///
 /// - Remove parens.
 ///
 /// TODO: remove completely after #8333
-pub(crate) fn precompress_optimizer<'a>() -> impl 'a + VisitMut {
-    PrecompressOptimizer {}
+pub(crate) fn precompress_optimizer<'a>(expr_ctx: ExprCtx) -> impl 'a + VisitMut {
+    PrecompressOptimizer { expr_ctx }
 }
 
 #[derive(Debug)]
-pub(crate) struct PrecompressOptimizer {}
+pub(crate) struct PrecompressOptimizer {
+    expr_ctx: ExprCtx,
+}
+
+impl PrecompressOptimizer {
+    /// Drops RHS from `null && foo`
+    fn optimize_bin_expr(&mut self, n: &mut Expr) {
+        let Expr::Bin(b) = n else {
+            return;
+        };
+
+        if b.op == op!("&&") && b.left.as_pure_bool(&self.expr_ctx) == Known(false) {
+            *n = *b.left.take();
+            return;
+        }
+
+        if b.op == op!("||") && b.left.as_pure_bool(&self.expr_ctx) == Known(true) {
+            *n = *b.left.take();
+        }
+    }
+}
 
 impl Parallel for PrecompressOptimizer {
     fn create(&self) -> Self {
-        Self {}
+        Self {
+            expr_ctx: self.expr_ctx.clone(),
+        }
     }
 
     fn merge(&mut self, _: Self) {}
@@ -29,38 +53,44 @@ impl Parallel for PrecompressOptimizer {
 impl VisitMut for PrecompressOptimizer {
     noop_visit_mut_type!();
 
+    fn visit_mut_expr(&mut self, n: &mut Expr) {
+        n.visit_mut_children_with(self);
+
+        self.optimize_bin_expr(n);
+    }
+
     fn visit_mut_stmts(&mut self, n: &mut Vec<Stmt>) {
-        self.maybe_par(*HEAVY_TASK_PARALLELS, n, |v, n| {
+        self.maybe_par(*CPU_COUNT, n, |v, n| {
             n.visit_mut_with(v);
         });
     }
 
     fn visit_mut_module_items(&mut self, n: &mut Vec<ModuleItem>) {
-        self.maybe_par(*HEAVY_TASK_PARALLELS, n, |v, n| {
+        self.maybe_par(*CPU_COUNT, n, |v, n| {
             n.visit_mut_with(v);
         });
     }
 
     fn visit_mut_exprs(&mut self, n: &mut Vec<Box<Expr>>) {
-        self.maybe_par(*HEAVY_TASK_PARALLELS, n, |v, n| {
+        self.maybe_par(*CPU_COUNT, n, |v, n| {
             n.visit_mut_with(v);
         });
     }
 
     fn visit_mut_opt_vec_expr_or_spreads(&mut self, n: &mut Vec<Option<ExprOrSpread>>) {
-        self.maybe_par(*HEAVY_TASK_PARALLELS, n, |v, n| {
+        self.maybe_par(*CPU_COUNT, n, |v, n| {
             n.visit_mut_with(v);
         });
     }
 
     fn visit_mut_expr_or_spreads(&mut self, n: &mut Vec<ExprOrSpread>) {
-        self.maybe_par(*HEAVY_TASK_PARALLELS, n, |v, n| {
+        self.maybe_par(*CPU_COUNT, n, |v, n| {
             n.visit_mut_with(v);
         });
     }
 
     fn visit_mut_var_declarators(&mut self, n: &mut Vec<VarDeclarator>) {
-        self.maybe_par(*HEAVY_TASK_PARALLELS, n, |v, n| {
+        self.maybe_par(*CPU_COUNT, n, |v, n| {
             n.visit_mut_with(v);
         });
     }