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feat: drop unused local variables #2360

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feat: drop unused local variables #2360

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2 changes: 1 addition & 1 deletion internal/bundler/bundler_dce_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -2777,7 +2777,7 @@ func TestConstValueInliningNoBundle(t *testing.T) {
`,
"/exprs-before.js": `
function nested() {
const x = [, '', {}, 0n, /./, function() {}, () => {}]
const x_REMOVE = [, '', {}, 0n, /./, function() {}, () => {}]
const y_REMOVE = 1
function foo() {
return y_REMOVE
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3 changes: 0 additions & 3 deletions internal/bundler/snapshots/snapshots_dce.txt
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Expand Up @@ -143,9 +143,6 @@ x = [1, 1];

---------- /out/exprs-before.js ----------
function nested() {
const x = [, "", {}, 0n, /./, function() {
}, () => {
}];
function foo() {
return 1;
}
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45 changes: 45 additions & 0 deletions internal/js_parser/js_parser.go
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Expand Up @@ -7702,6 +7702,36 @@ func (p *parser) mangleStmts(stmts []js_ast.Stmt, kind stmtsKind) []js_ast.Stmt
}

case *js_ast.SLocal:
// Drop unused local variables
//
// The declaration must not be exported. We can't just check for the
// "export" keyword because something might do "export {id};" later on.
// Instead we just ignore all top-level declarations for now. That means
// this optimization currently only applies in nested scopes.
//
// Ignore declarations if the scope is shadowed by a direct "eval" call.
// The eval'd code may indirectly reference this symbol and the actual
// use count may be greater than 0.
if p.currentScope != p.moduleScope && !p.currentScope.ContainsDirectEval {
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If this costs too much, I think this condition could be p.currentScope.Parent == p.moduleScope && !p.currentScope.ContainsDirectEval instead.

// Ignore "var" declarations unless we are running in function body.
// Because we may not have visited all of their uses yet by this point.
if s.Kind != js_ast.LocalVar || kind == stmtsFnBody {
newDecls := make([]js_ast.Decl, 0)
for _, decl := range s.Decls {
if id, ok := decl.Binding.Data.(*js_ast.BIdentifier); ok {
if !p.isSymbolUsed(id.Ref, s.Kind == js_ast.LocalVar) && (decl.ValueOrNil.Data == nil || p.exprCanBeRemovedIfUnused(decl.ValueOrNil)) {
continue
}
}
newDecls = append(newDecls, decl)
}
if len(newDecls) == 0 {
continue
}
s.Decls = newDecls
}
}

// Merge adjacent local statements
if len(result) > 0 {
prevStmt := result[len(result)-1]
Expand Down Expand Up @@ -8090,6 +8120,21 @@ func (p *parser) mangleStmts(stmts []js_ast.Stmt, kind stmtsKind) []js_ast.Stmt
return result
}

func (p *parser) isSymbolUsed(ref js_ast.Ref, checkMerged bool) bool {
for {
symbol := p.symbols[ref.InnerIndex]
if symbol.UseCountEstimate > 0 {
return true
}
if !checkMerged || symbol.Link == js_ast.InvalidRef {
break
}

ref = symbol.Link
}
return false
}

func (p *parser) substituteSingleUseSymbolInStmt(stmt js_ast.Stmt, ref js_ast.Ref, replacement js_ast.Expr) bool {
var expr *js_ast.Expr

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28 changes: 24 additions & 4 deletions internal/js_parser/js_parser_test.go
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Expand Up @@ -2964,7 +2964,7 @@ func TestMangleIndex(t *testing.T) {
func TestMangleBlock(t *testing.T) {
expectPrintedMangle(t, "while(1) { while (1) {} }", "for (; ; )\n for (; ; )\n ;\n")
expectPrintedMangle(t, "while(1) { const x = y; }", "for (; ; ) {\n const x = y;\n}\n")
expectPrintedMangle(t, "while(1) { let x; }", "for (; ; ) {\n let x;\n}\n")
expectPrintedMangle(t, "while(1) { let x; }", "for (; ; )\n ;\n")
expectPrintedMangle(t, "while(1) { var x; }", "for (; ; )\n var x;\n")
expectPrintedMangle(t, "while(1) { class X {} }", "for (; ; ) {\n class X {\n }\n}\n")
expectPrintedMangle(t, "while(1) { function x() {} }", "for (; ; )\n var x = function() {\n };\n")
Expand Down Expand Up @@ -4187,7 +4187,7 @@ func TestMangleInlineLocals(t *testing.T) {
check("let x = arg0; return y ? x : z;", "let x = arg0;\nreturn y ? x : z;")
check("let x = arg0; return y ? z : x;", "let x = arg0;\nreturn y ? z : x;")
check("let x = arg0; return (arg1 ? 1 : 2) ? x : 3;", "return arg0;")
check("let x = arg0; return (arg1 ? 1 : 2) ? 3 : x;", "let x = arg0;\nreturn 3;")
check("let x = arg0; return (arg1 ? 1 : 2) ? 3 : x;", "return 3;")
check("let x = arg0; return (arg1 ? y : 1) ? x : 2;", "let x = arg0;\nreturn !arg1 || y ? x : 2;")
check("let x = arg0; return (arg1 ? 1 : y) ? x : 2;", "let x = arg0;\nreturn arg1 || y ? x : 2;")
check("let x = arg0; return (arg1 ? y : 1) ? 2 : x;", "let x = arg0;\nreturn !arg1 || y ? 2 : x;")
Expand Down Expand Up @@ -4344,8 +4344,8 @@ func TestTrimCodeInDeadControlFlow(t *testing.T) {
expectPrintedMangle(t, "if (1) a(); else { debugger }", "a();\n")

expectPrintedMangle(t, "if (0) {let a = 1} else a()", "a();\n")
expectPrintedMangle(t, "if (1) {let a = 1} else a()", "{\n let a = 1;\n}\n")
expectPrintedMangle(t, "if (0) a(); else {let a = 1}", "{\n let a = 1;\n}\n")
expectPrintedMangle(t, "if (1) {let a = 1} else a()", "")
expectPrintedMangle(t, "if (0) a(); else {let a = 1}", "")
expectPrintedMangle(t, "if (1) a(); else {let a = 1}", "a();\n")

expectPrintedMangle(t, "if (1) a(); else { var a = b }", "if (1)\n a();\nelse\n var a;\n")
Expand Down Expand Up @@ -5285,3 +5285,23 @@ func TestAutoPureForWeakMap(t *testing.T) {
expectPrinted(t, "new WeakMap([x, []])", "new WeakMap([x, []]);\n")
expectPrinted(t, "new WeakMap([[], x])", "new WeakMap([[], x]);\n")
}

func TestDropLocalUnusedVariable(t *testing.T) {
expectPrintedMangle(t, "var v = 0", "var v = 0;\n")
expectPrintedMangle(t, "let v = 0", "let v = 0;\n")
expectPrintedMangle(t, "const v = 0", "const v = 0;\n")
expectPrintedMangle(t, "{ var v = 0 }", "var v = 0;\n")
expectPrintedMangle(t, "{ let v = 0 }", "")
expectPrintedMangle(t, "{ let v = 3; let v2 = v + 5 }", "{\n let v2 = 3 + 5;\n}\n") // does not support multi-level
expectPrintedMangle(t, "{ const v = 3; const v2 = v + 5 }", "")
expectPrintedMangle(t, "{ const v = 0 }", "")
expectPrintedMangle(t, "{ const v = (() => 'foo')() }", "{\n const v = (() => \"foo\")();\n}\n")
expectPrintedMangle(t, "{ const v = /* #__PURE__ */ (() => 'foo')() }", "")
expectPrintedMangle(t, "{ const v = 0; console.log(v) }", "console.log(0);\n")
// expectPrintedMangle(t, "function f() { const v = 0; eval('v') }", "function f() {\n const v = 0;\n eval(\"v\");\n}\n")
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@sapphi-red sapphi-red Jun 30, 2022 •
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This test fails without this PR.
const v = 0; is dropped at here.

esbuild/internal/js_parser/js_parser.go

Lines 9312 to 9317 in 698898f

// Only keep this declaration if it's top-level or exported (which
// could be in a nested TypeScript namespace), otherwise erase it
if p.currentScope.Parent == nil || s.IsExport {
s.Decls[end] = d
end++
}

Created an issue: #2361

expectPrintedMangle(t, "function f() { var v }", "function f() {\n}\n")
expectPrintedMangle(t, "function f() { let v }", "function f() {\n}\n")
expectPrintedMangle(t, "function f() { var v; console.log(v); }", "function f() {\n var v;\n console.log(v);\n}\n")
expectPrintedMangle(t, "function f() { var v; function g(){ v = 2 } }", "function f() {\n var v;\n function g() {\n v = 2;\n }\n}\n")
expectPrintedMangle(t, "function f() { function g(){ v = 2 }; var v; }", "function f() {\n function g() {\n v = 2;\n }\n var v;\n}\n")
}
1 change: 1 addition & 0 deletions scripts/uglify-tests.js
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Expand Up @@ -255,6 +255,7 @@ async function test_case(esbuild, test, basename) {
'let.js: issue_4229',
'let.js: issue_4245',
'let.js: use_before_init_3',
'let.js: issue_4276_1',
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This test is related to TDZ.
https://github.com/mishoo/UglifyJS/blob/4c227cc6bd795278c085a6b6dfd40a960ec35698/test/compress/let.js#L1486-L1492


// Error difference
'dead-code.js: dead_code_2_should_warn',
Expand Down