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switch case liveness: default is always last
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@evanw
evanw committed Feb 7, 2025
1 parent 7aa47c3 commit f4e9d19
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Showing 3 changed files with 105 additions and 88 deletions.
8 changes: 6 additions & 2 deletions internal/bundler_tests/bundler_dce_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -1449,14 +1449,18 @@ func TestDeadCodeInsideUnusedCases(t *testing.T) {
}
// Check for "default"
switch (0) {
case 1: _ = require('./FAIL-default-1'); break
default: _ = require('./a'); break
}
switch (1) {
case 1: _ = require('./a'); break
default: _ = require('./FAIL-default'); break
}
switch (0) {
case 1: _ = require('./FAIL-default-1'); break
default: _ = require('./a'); break
case 0: _ = require('./FAIL-default-0'); break
default: _ = require('./FAIL-default'); break
case 0: _ = require('./a'); break
}
// Check for non-constant cases
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12 changes: 10 additions & 2 deletions internal/bundler_tests/snapshots/snapshots_dce.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -968,6 +968,14 @@ switch (1) {
_ = null;
break;
}
switch (0) {
case 1:
_ = null;
break;
default:
_ = require_a();
break;
}
switch (1) {
case 1:
_ = require_a();
Expand All @@ -981,10 +989,10 @@ switch (0) {
_ = null;
break;
default:
_ = require_a();
_ = null;
break;
case 0:
_ = null;
_ = require_a();
break;
}
switch (1) {
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173 changes: 89 additions & 84 deletions internal/js_parser/js_parser.go
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Original file line number Diff line number Diff line change
Expand Up @@ -59,7 +59,6 @@ type parser struct {
declaredSymbols []js_ast.DeclaredSymbol
globPatternImports []globPatternImport
runtimeImports map[string]ast.LocRef
deadCaseChecker deadCaseChecker
duplicateCaseChecker duplicateCaseChecker
unrepresentableIdentifiers map[string]bool
legacyOctalLiterals map[js_ast.E]logger.Range
Expand Down Expand Up @@ -748,84 +747,87 @@ type fnOnlyDataVisit struct {
silenceMessageAboutThisBeingUndefined bool
}

type livenessStatus uint8
type livenessStatus int8

const (
livenessUnknown livenessStatus = iota
alwaysDead
alwaysLive
alwaysDead livenessStatus = -1
livenessUnknown livenessStatus = 0
alwaysLive livenessStatus = 1
)

type deadCaseChecker struct {
test js_ast.E
earlierCaseWasMaybeTaken bool
furtherCasesAreDead bool
mayHaveFallenThrough bool
type switchCaseLiveness struct {
status livenessStatus
canFallThrough bool
}

func (dc *deadCaseChecker) reset(p *parser, test js_ast.E) {
*dc = deadCaseChecker{
test: test,
furtherCasesAreDead: p.isControlFlowDead,
}
}

func (dc *deadCaseChecker) checkCase(c js_ast.Case) (status livenessStatus) {
if dc.furtherCasesAreDead {
return alwaysDead
}
func analyzeSwitchCasesForLiveness(s *js_ast.SSwitch) []switchCaseLiveness {
cases := make([]switchCaseLiveness, 0, len(s.Cases))
defaultIndex := -1

// Determine the status of the individual cases independently
maxStatus := alwaysDead
for i, c := range s.Cases {
if c.ValueOrNil.Data == nil {
defaultIndex = i
}

// Check the value for strict equality
var status livenessStatus
if maxStatus == alwaysLive {
status = alwaysDead // Everything after an always-live case is always dead
} else if c.ValueOrNil.Data == nil {
status = alwaysDead // This is the default case, and will be filled in later
} else if isEqualToTest, ok := js_ast.CheckEqualityIfNoSideEffects(s.Test.Data, c.ValueOrNil.Data, js_ast.StrictEquality); ok {
if isEqualToTest {
status = alwaysLive // This branch will always be matched, and will be taken unless an earlier branch was taken
} else {
status = alwaysDead // This branch will never be matched, and will not be taken unless there was fall-through
}
} else {
status = livenessUnknown // This branch depends on run-time values and may or may not be matched
}
if maxStatus < status {
maxStatus = status
}

// Check for strict equality
var isEqualToTest bool
var isEqualityKnown bool
if c.ValueOrNil.Data != nil {
// Non-default case
isEqualToTest, isEqualityKnown = js_ast.CheckEqualityIfNoSideEffects(dc.test, c.ValueOrNil.Data, js_ast.StrictEquality)
} else {
// Default case
if !dc.earlierCaseWasMaybeTaken {
isEqualToTest = true
isEqualityKnown = true
// Check for potential fall-through by checking for a jump at the end of the body
canFallThrough := true
stmts := c.Body
for len(stmts) > 0 {
switch s := stmts[len(stmts)-1].Data.(type) {
case *js_ast.SBlock:
stmts = s.Stmts // If this ends with a block, check the block's body next
continue
case *js_ast.SBreak, *js_ast.SContinue, *js_ast.SReturn, *js_ast.SThrow:
canFallThrough = false
}
break
}

cases = append(cases, switchCaseLiveness{
status: status,
canFallThrough: canFallThrough,
})
}

// Check for potential fall-through by checking for a jump at the end of the body
canFallThrough := true
stmts := c.Body
for len(stmts) > 0 {
switch s := stmts[len(stmts)-1].Data.(type) {
case *js_ast.SBlock:
stmts = s.Stmts // If this ends with a block, check the block's body next
continue
case *js_ast.SBreak, *js_ast.SContinue, *js_ast.SReturn, *js_ast.SThrow:
canFallThrough = false
}
break
// Set the liveness for the default case last based on the other cases
if defaultIndex != -1 {
// The negation here transposes "always live" with "always dead"
cases[defaultIndex].status = -maxStatus
}

// Update the state machine
if isEqualityKnown {
if isEqualToTest {
// This branch will always be matched, and will be taken unless an earlier branch was taken
if !dc.earlierCaseWasMaybeTaken {
status = alwaysLive
}
if !canFallThrough {
dc.furtherCasesAreDead = true
}
dc.earlierCaseWasMaybeTaken = true
} else {
// This branch will never be matched, and will not be taken unless there was fall-through
if !dc.mayHaveFallenThrough {
status = alwaysDead
// Then propagate fall-through information in linear fall-through order
for i, c := range cases {
// Propagate state forward if this isn't dead. Note that the "can fall
// through" flag does not imply "must fall through". The body may have
// an embedded "break" inside an if statement, for example.
if c.status != alwaysDead {
for j := i + 1; j < len(cases) && cases[j-1].canFallThrough; j++ {
cases[j].status = livenessUnknown
}
}
} else {
// This branch depends on run-time values and may or may not be matched
dc.earlierCaseWasMaybeTaken = true
}
dc.mayHaveFallenThrough = canFallThrough && status != alwaysDead
return
return cases
}

const bloomFilterSize = 251
Expand Down Expand Up @@ -10959,23 +10961,36 @@ func (p *parser) visitAndAppendStmt(stmts []js_ast.Stmt, stmt js_ast.Stmt) []js_
p.pushScopeForVisitPass(js_ast.ScopeBlock, s.BodyLoc)
oldIsInsideSwitch := p.fnOrArrowDataVisit.isInsideSwitch
p.fnOrArrowDataVisit.isInsideSwitch = true
p.deadCaseChecker.reset(p, s.Test.Data)
end := 0
for _, c := range s.Cases {
// Visit the value for non-default cases
var status livenessStatus

// Visit case values first
for i := range s.Cases {
c := &s.Cases[i]
if c.ValueOrNil.Data != nil {
c.ValueOrNil = p.visitExpr(c.ValueOrNil)
status = p.deadCaseChecker.checkCase(c)
p.warnAboutEqualityCheck("case", c.ValueOrNil, c.ValueOrNil.Loc)
p.warnAboutTypeofAndString(s.Test, c.ValueOrNil, onlyCheckOriginalOrder)
} else {
status = p.deadCaseChecker.checkCase(c)
}
}

// Check for duplicate case values
p.duplicateCaseChecker.reset()
for _, c := range s.Cases {
if c.ValueOrNil.Data != nil {
p.duplicateCaseChecker.check(p, c.ValueOrNil)
}
}

// Then analyze the cases to determine which ones are live and/or dead
cases := analyzeSwitchCasesForLiveness(s)

// Then visit case bodies, and potentially filter out dead cases
end := 0
for i, c := range s.Cases {
isAlwaysDead := cases[i].status == alwaysDead

// Potentially treat the case body as dead code
old := p.isControlFlowDead
if status == alwaysDead {
if isAlwaysDead {
p.isControlFlowDead = true
}
c.Body = p.visitStmts(c.Body, stmtsSwitch)
Expand All @@ -10986,29 +11001,19 @@ func (p *parser) visitAndAppendStmt(stmts []js_ast.Stmt, stmt js_ast.Stmt) []js_
// removed safely, so if the body isn't empty then that means it contains
// some statements that can't be removed safely (e.g. a hoisted "var").
// So don't remove this case if the body isn't empty.
if p.options.minifySyntax && status == alwaysDead && len(c.Body) == 0 {
if p.options.minifySyntax && isAlwaysDead && len(c.Body) == 0 {
continue
}

// Make sure the assignment to the body above is preserved
s.Cases[end] = c
end++
}

// Filter out all removed cases
s.Cases = s.Cases[:end]

p.fnOrArrowDataVisit.isInsideSwitch = oldIsInsideSwitch
p.popScope()

// Check for duplicate case values
p.duplicateCaseChecker.reset()
for _, c := range s.Cases {
if c.ValueOrNil.Data != nil {
p.duplicateCaseChecker.check(p, c.ValueOrNil)
}
}

// Unwrap switch statements in dead code
if p.options.minifySyntax && p.isControlFlowDead {
for _, c := range s.Cases {
Expand Down

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