fix(minifier): dce if statement should keep side effects and vars (#8433

) closes #7209 Note: Current output is sub-optimal.
oxc-project · Jan 11, 2025 · 3c93549 · 3c93549
1 parent 9a03bd2
commit 3c93549
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Showing 4 changed files with 40 additions and 16 deletions.
diff --git a/crates/oxc_minifier/src/ast_passes/peephole_remove_dead_code.rs b/crates/oxc_minifier/src/ast_passes/peephole_remove_dead_code.rs
@@ -204,22 +204,33 @@ impl<'a, 'b> PeepholeRemoveDeadCode {
             return Some(ctx.ast.statement_expression(if_stmt.span, expr));
         }
 
-        match ctx.get_boolean_value(&if_stmt.test) {
-            Some(true) => Some(ctx.ast.move_statement(&mut if_stmt.consequent)),
-            Some(false) => {
-                Some(if let Some(alternate) = &mut if_stmt.alternate {
-                    ctx.ast.move_statement(alternate)
+        if let Some(boolean) = ctx.get_side_free_boolean_value(&if_stmt.test) {
+            let mut keep_var = KeepVar::new(ctx.ast);
+            if boolean {
+                if let Some(alternate) = &if_stmt.alternate {
+                    keep_var.visit_statement(alternate);
+                }
+            } else {
+                keep_var.visit_statement(&if_stmt.consequent);
+            };
+            if let Some(var_stmt) = keep_var.get_variable_declaration_statement() {
+                if boolean {
+                    if_stmt.alternate = Some(var_stmt);
                 } else {
-                    // Keep hoisted `vars` from the consequent block.
-                    let mut keep_var = KeepVar::new(ctx.ast);
-                    keep_var.visit_statement(&if_stmt.consequent);
-                    keep_var
-                        .get_variable_declaration_statement()
-                        .unwrap_or_else(|| ctx.ast.statement_empty(SPAN))
-                })
+                    if_stmt.consequent = var_stmt;
+                }
+                return None;
             }
-            None => None,
+            return Some(if boolean {
+                ctx.ast.move_statement(&mut if_stmt.consequent)
+            } else {
+                if_stmt.alternate.as_mut().map_or_else(
+                    || ctx.ast.statement_empty(SPAN),
+                    |alternate| ctx.ast.move_statement(alternate),
+                )
+            });
         }
+        None
     }
 
     fn try_fold_for(

diff --git a/crates/oxc_minifier/tests/ast_passes/dead_code_elimination.rs b/crates/oxc_minifier/tests/ast_passes/dead_code_elimination.rs
@@ -58,7 +58,7 @@ fn dce_if_statement() {
     );
     test(
         "if (xxx) { foo } else if (false) { var a; var b; } else if (false) { var c; var d; }",
-        "if (xxx) foo; else var c, d;",
+        "if (xxx) foo; else if (false) var a, b; else if (false) var c, d;",
     );
 
     test("if (!false) { foo }", "foo");
@@ -229,7 +229,7 @@ fn dce_from_terser() {
         console.log(foo, bar, Baz);
         ",
         "
-        var qux;
+        if (0) var qux;
         console.log(foo, bar, Baz);
         ",
     );

diff --git a/crates/oxc_minifier/tests/ast_passes/mod.rs b/crates/oxc_minifier/tests/ast_passes/mod.rs
@@ -61,6 +61,19 @@ fn integration() {
     );
 
     test_same("a && (b && (c && (d && (e && (f && (g && (h && i && j && k && l && m && n && o && p && q && r && s && t && u && v && w && x && y && z)))))))");
+
+    test(
+        "if (((() => console.log('effect'))(), true)) {
+         } else {
+           var c = 1;
+           for (var c; unknownGlobal && true; unknownGlobal && true) var d;
+         }
+         console.log(c, d);
+        ",
+        "if ((() => console.log('effect'))(), !0) {} else for (var c = 1, c; unknownGlobal; unknownGlobal && !0) var d;
+        console.log(c, d);
+        ",
+    );
 }
 
 #[test] // https://github.com/oxc-project/oxc/issues/4341

diff --git a/tasks/minsize/minsize.snap b/tasks/minsize/minsize.snap
@@ -21,7 +21,7 @@ Original   | minified   | minified   | gzip       | gzip       | Fixture
 
 3.20 MB    | 1.01 MB    | 1.01 MB    | 331.79 kB  | 331.56 kB  | echarts.js
 
-6.69 MB    | 2.32 MB    | 2.31 MB    | 492.63 kB  | 488.28 kB  | antd.js   
+6.69 MB    | 2.32 MB    | 2.31 MB    | 492.64 kB  | 488.28 kB  | antd.js   
 
 10.95 MB   | 3.49 MB    | 3.49 MB    | 907.42 kB  | 915.50 kB  | typescript.js