feat(minifier): implement dce with var hoisting (#4160)

crates/oxc_minifier/src/ast_passes/remove_dead_code.rs

@@ -1,12 +1,17 @@
 use oxc_allocator::{Allocator, Vec};
-use oxc_ast::{ast::*, visit::walk_mut, AstBuilder, VisitMut};
-use oxc_span::SPAN;
+use oxc_ast::{
+    ast::*, syntax_directed_operations::BoundNames, visit::walk_mut, AstBuilder, Visit, VisitMut,
+};
+use oxc_span::{Atom, Span, SPAN};
+use oxc_syntax::scope::ScopeFlags;
 
 use crate::{compressor::ast_util::get_boolean_value, folder::Folder};
 
 /// Remove Dead Code from the AST.
 ///
 /// Terser option: `dead_code: true`.
+///
+/// See `KeepVar` at the end of this file for `var` hoisting logic.
 pub struct RemoveDeadCode<'a> {
     ast: AstBuilder<'a>,
     folder: Folder<'a>,
@@ -35,26 +40,41 @@ impl<'a> RemoveDeadCode<'a> {
 
     /// Removes dead code thats comes after `return` statements after inlining `if` statements
     fn dead_code_elimintation(&mut self, stmts: &mut Vec<'a, Statement<'a>>) {
-        let mut removed = true;
+        // Fold if statements
         for stmt in stmts.iter_mut() {
-            if self.fold_if_statement(stmt) {
-                removed = true;
+            if self.fold_if_statement(stmt) {}
+        }
+
+        // Remove code after `return` and `throw` statements
+        let mut index = None;
+        'outer: for (i, stmt) in stmts.iter().enumerate() {
+            if matches!(stmt, Statement::ReturnStatement(_) | Statement::ThrowStatement(_)) {
+                index.replace(i);
                 break;
             }
-        }
-
-        if !removed {
-            return;
-        }
-
-        let mut index = None;
-        for (i, stmt) in stmts.iter().enumerate() {
-            if matches!(stmt, Statement::ReturnStatement(_)) {
-                index.replace(i);
+            // Double check block statements folded by if statements above
+            if let Statement::BlockStatement(block_stmt) = stmt {
+                for stmt in &block_stmt.body {
+                    if matches!(stmt, Statement::ReturnStatement(_) | Statement::ThrowStatement(_))
+                    {
+                        index.replace(i);
+                        break 'outer;
+                    }
+                }
             }
         }
-        if let Some(index) = index {
-            stmts.drain(index + 1..);
+
+        let Some(index) = index else { return };
+
+        let mut keep_var = KeepVar::new(self.ast);
+
+        for stmt in stmts.iter().skip(index) {
+            keep_var.visit_statement(stmt);
+        }
+
+        stmts.drain(index + 1..);
+        if let Some(stmt) = keep_var.get_variable_declaration_statement() {
+            stmts.push(stmt);
         }
     }
 
@@ -70,7 +90,14 @@ impl<'a> RemoveDeadCode<'a> {
                 *stmt = if let Some(alternate) = &mut if_stmt.alternate {
                     self.ast.move_statement(alternate)
                 } else {
-                    self.ast.statement_empty(SPAN)
+                    // Keep hoisted `vars` from the consequent block.
+                    let mut keep_var = KeepVar::new(self.ast);
+                    keep_var.visit_statement(&if_stmt.consequent);
+                    if let Some(stmt) = keep_var.get_variable_declaration_statement() {
+                        stmt
+                    } else {
+                        self.ast.statement_empty(SPAN)
+                    }
                 };
                 true
             }
@@ -98,3 +125,50 @@ impl<'a> RemoveDeadCode<'a> {
         }
     }
 }
+
+struct KeepVar<'a> {
+    ast: AstBuilder<'a>,
+    vars: std::vec::Vec<(Atom<'a>, Span)>,
+}
+
+impl<'a> Visit<'a> for KeepVar<'a> {
+    fn visit_variable_declarator(&mut self, decl: &VariableDeclarator<'a>) {
+        if decl.kind.is_var() {
+            decl.id.bound_names(&mut |ident| {
+                self.vars.push((ident.name.clone(), ident.span));
+            });
+        }
+    }
+
+    fn visit_function(&mut self, _it: &Function<'a>, _flags: Option<ScopeFlags>) {
+        /* skip functions */
+    }
+
+    fn visit_class(&mut self, _it: &Class<'a>) {
+        /* skip classes */
+    }
+}
+
+impl<'a> KeepVar<'a> {
+    fn new(ast: AstBuilder<'a>) -> Self {
+        Self { ast, vars: std::vec![] }
+    }
+
+    fn get_variable_declaration_statement(self) -> Option<Statement<'a>> {
+        if self.vars.is_empty() {
+            return None;
+        }
+
+        let kind = VariableDeclarationKind::Var;
+        let decls = self.ast.vec_from_iter(self.vars.into_iter().map(|(name, span)| {
+            let binding_kind = self.ast.binding_pattern_kind_binding_identifier(span, name);
+            let id =
+                self.ast.binding_pattern::<Option<TSTypeAnnotation>>(binding_kind, None, false);
+            self.ast.variable_declarator(span, kind, id, None, false)
+        }));
+
+        let decl = self.ast.variable_declaration(SPAN, kind, decls, false);
+        let stmt = self.ast.statement_declaration(self.ast.declaration_from_variable(decl));
+        Some(stmt)
+    }
+}

crates/oxc_minifier/tests/oxc/remove_dead_code.rs

@@ -1,5 +1,5 @@
 use oxc_allocator::Allocator;
-use oxc_codegen::WhitespaceRemover;
+use oxc_codegen::CodeGenerator;
 use oxc_minifier::RemoveDeadCode;
 use oxc_parser::Parser;
 use oxc_span::SourceType;
@@ -12,7 +12,7 @@ fn print(source_text: &str, remove_dead_code: bool) -> String {
     if remove_dead_code {
         RemoveDeadCode::new(&allocator).build(program);
     }
-    WhitespaceRemover::new().build(program).source_text
+    CodeGenerator::new().build(program).source_text
 }
 
 pub(crate) fn test(source_text: &str, expected: &str) {
@@ -53,6 +53,18 @@ fn remove_dead_code() {
         "function foo(undefined) { if (!undefined) { } }",
         "function foo(undefined) { if (!undefined) { } }",
     );
+
+    test("if (true) { foo; } if (true) { foo; }", "{ foo; } { foo; }");
+
+    test(
+        "
+        if (true) { foo; return }
+        foo;
+        if (true) { bar; return }
+        bar;
+        ",
+        "{foo; return }",
+    );
 }
 
 // https://github.com/terser/terser/blob/master/test/compress/dead-code.js
@@ -68,12 +80,54 @@ fn remove_dead_code_from_terser() {
                 y();
             }
         }",
-        "
-          function f() {
+        "function f() {
             a();
             b();
             x = 10;
             return;
         }",
     );
+
+    // NOTE: `if (x)` is changed to `if (true)` because const inlining is not implemented yet.
+    test(
+        r#"function f() {
+            g();
+            x = 10;
+            throw new Error("foo");
+            if (true) {
+                y();
+                var x;
+                function g(){};
+                (function(){
+                    var q;
+                    function y(){};
+                })();
+            }
+        }
+        f();
+        "#,
+        r#"function f() {
+            g();
+            x = 10;
+            throw new Error("foo");
+            var x;
+        }
+        f();
+        "#,
+    );
+
+    test(
+        "if (0) {
+            let foo = 6;
+            const bar = 12;
+            class Baz {};
+            var qux;
+        }
+        console.log(foo, bar, Baz);
+        ",
+        "
+        var qux;
+        console.log(foo, bar, Baz);
+        ",
+    );
 }