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Only generate generic signature if it differs from descriptor #25880

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3e48e57
Only generate generic signature if it differs from descriptor
SolalPirelli Apr 20, 2026
2b6e294
fix test
SolalPirelli Apr 21, 2026
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69 changes: 29 additions & 40 deletions compiler/src/dotty/tools/backend/jvm/BCodeHelpers.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -308,11 +308,12 @@ trait BCodeHelpers(val backendUtils: BackendUtils)(using ctx: Context) extends B
*
* @param sym The symbol for which to generate a signature.
* @param owner The owner of `sym`.
* @param descriptor The descriptor of the symbol; the signature is unnecessary if they are equal.
* @return The generic signature of `sym` before erasure, as specified in the Java Virtual
* Machine Specification, §4.3.4, or `null` if `sym` doesn't need a generic signature.
* @see https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.3.4
*/
def getGenericSignature(sym: Symbol, owner: Symbol): String | Null = {
def getGenericSignature(sym: Symbol, owner: Symbol, descriptor: String | Null): String | Null = {
atPhase(erasurePhase) {
// Finding the member's type is nontrivial because of erasure and how it interacts with other phases.
def computeMemberType(): Type = {
Expand Down Expand Up @@ -345,7 +346,11 @@ trait BCodeHelpers(val backendUtils: BackendUtils)(using ctx: Context) extends B
owner.denot.thisType.memberInfo(sym)
}

getGenericSignatureHelper(sym, owner, computeMemberType()).orNull
if ctx.base.settings.XnoGenericSig.value then null
else
val genSig = getGenericSignatureHelper(sym, owner, computeMemberType()).orNull
if genSig == descriptor then null
else genSig
}
}

Expand Down Expand Up @@ -375,12 +380,12 @@ trait BCodeHelpers(val backendUtils: BackendUtils)(using ctx: Context) extends B
)

// TODO needed? for(ann <- m.annotations) { ann.symbol.initialize }
val jgensig = getStaticForwarderGenericSignature(m, module)
val jReturnType = ts.toTypeKind(methodInfo.resultType)
val mdesc = MethodBType(paramJavaTypes, jReturnType).descriptor
val jgensig = getStaticForwarderGenericSignature(m, module, mdesc)
val (throws, others) = m.annotations.partition(_.symbol eq defn.ThrowsAnnot)
val thrownExceptions: List[String] = getExceptions(throws)

val jReturnType = ts.toTypeKind(methodInfo.resultType)
val mdesc = MethodBType(paramJavaTypes, jReturnType).descriptor
val mirrorMethodName = m.javaSimpleName
val lengthOk = if jgensig ne null then BCodeUtils.checkConstantStringLength(jgensig)
else BCodeUtils.checkConstantStringLength(mirrorMethodName, mdesc)
Expand Down Expand Up @@ -639,22 +644,17 @@ trait BCodeHelpers(val backendUtils: BackendUtils)(using ctx: Context) extends B
} // end of trait JAndroidBuilder

private def getGenericSignatureHelper(sym: Symbol, owner: Symbol, memberTpe: Type)(using Context): Option[String] = {
if (needsGenericSignature(sym)) {
val erasedTypeSym = TypeErasure.fullErasure(sym.denot.info).typeSymbol
if (erasedTypeSym.isPrimitiveValueClass) {
// Suppress signatures for symbols whose types erase in the end to primitive
// value types. This is needed to fix #7416.
None
} else {
val jsOpt = GenericSignatures.javaSig(sym, memberTpe)
if (ctx.settings.XverifySignatures.value) {
jsOpt.foreach(verifySignature(sym, _))
}

jsOpt
}
} else {
val erasedTypeSym = TypeErasure.fullErasure(sym.denot.info).typeSymbol
if (erasedTypeSym.isPrimitiveValueClass) {
// Suppress signatures for symbols whose types erase in the end to primitive
// value types. This is needed to fix #7416.
None
} else {
val jsOpt = GenericSignatures.javaSig(sym, memberTpe)
if (ctx.settings.XverifySignatures.value) {
jsOpt.foreach(verifySignature(sym, _))
}
jsOpt
}
}

Expand Down Expand Up @@ -686,32 +686,21 @@ trait BCodeHelpers(val backendUtils: BackendUtils)(using ctx: Context) extends B
}
}

// @M don't generate java generics sigs for (members of) implementation
// classes, as they are monomorphic (TODO: ok?)
private final def needsGenericSignature(sym: Symbol): Boolean = !(
// pp: this condition used to include sym.hasexpandedname, but this leads
// to the total loss of generic information if a private member is
// accessed from a closure: both the field and the accessor were generated
// without it. This is particularly bad because the availability of
// generic information could disappear as a consequence of a seemingly
// unrelated change.
ctx.base.settings.XnoGenericSig.value
|| sym.is(Artifact)
|| sym.isAllOf(LiftedMethod)
|| sym.is(Bridge)
)

private def getStaticForwarderGenericSignature(sym: Symbol, moduleClass: Symbol): String = {
private def getStaticForwarderGenericSignature(sym: Symbol, moduleClass: Symbol, descriptor: String | Null): String | Null = {
// scala/bug#3452 Static forwarder generation uses the same erased signature as the method if forwards to.
// By rights, it should use the signature as-seen-from the module class, and add suitable
// primitive and value-class boxing/unboxing.
// But for now, just like we did in mixin, we just avoid writing a wrong generic signature
// (one that doesn't erase to the actual signature). See run/t3452b for a test case.

val memberTpe = atPhase(erasurePhase) { moduleClass.denot.thisType.memberInfo(sym) }
val erasedMemberType = ElimErasedValueType.elimEVT(TypeErasure.transformInfo(sym, memberTpe))
if (erasedMemberType =:= sym.denot.info)
getGenericSignatureHelper(sym, moduleClass, memberTpe).orNull
if !ctx.base.settings.XnoGenericSig.value then
val memberTpe = atPhase(erasurePhase) { moduleClass.denot.thisType.memberInfo(sym) }
val erasedMemberType = ElimErasedValueType.elimEVT(TypeErasure.transformInfo(sym, memberTpe))
if (erasedMemberType =:= sym.denot.info)
val gensig = getGenericSignatureHelper(sym, moduleClass, memberTpe).orNull
if gensig == descriptor then null
else gensig
else null
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@SolalPirelli SolalPirelli Apr 20, 2026

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Not pretty, but this method belongs in the lowest circle of Hell anyway and should disappear if I ever have the courage to clean up mirror generic signature generation.

else null
}

Expand Down
11 changes: 6 additions & 5 deletions compiler/src/dotty/tools/backend/jvm/BCodeSkelBuilder.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -336,7 +336,7 @@ trait BCodeSkelBuilder(using ctx: Context) extends BCodeHelpers {

val flags = BCodeUtils.javaFlags(claszSymbol)

val thisSignature = getGenericSignature(claszSymbol, claszSymbol.owner)
val thisSignature = getGenericSignature(claszSymbol, claszSymbol.owner, null)
val lengthOk = if thisSignature ne null then BCodeUtils.checkConstantStringLength(thisSignature)
else BCodeUtils.checkConstantStringLength(thisName)
if !lengthOk then
Expand Down Expand Up @@ -423,7 +423,8 @@ trait BCodeSkelBuilder(using ctx: Context) extends BCodeHelpers {
* No code is needed for this module symbol.
*/
for (f <- claszSymbol.info.decls.filter(p => p.isTerm && !p.is(Method))) {
val javagensig = getGenericSignature(f, claszSymbol)
val descriptor = symInfoTK(f).descriptor
val javagensig = getGenericSignature(f, claszSymbol, descriptor)
val flags = javaFieldFlags(f)

assert(!f.isStaticMember || !claszSymbol.is(Trait) || !f.is(Mutable),
Expand All @@ -432,7 +433,7 @@ trait BCodeSkelBuilder(using ctx: Context) extends BCodeHelpers {
val jfield = new asm.tree.FieldNode(
flags,
f.javaSimpleName,
symInfoTK(f).descriptor,
descriptor,
javagensig,
null // no initial value
)
Expand Down Expand Up @@ -736,15 +737,15 @@ trait BCodeSkelBuilder(using ctx: Context) extends BCodeHelpers {
*/
private def initJMethod(flags: Int, params: List[Symbol]): Unit = {

val jgensig = getGenericSignature(methSymbol, claszSymbol)
val mdesc = ts.asmMethodType(methSymbol).descriptor
val jgensig = getGenericSignature(methSymbol, claszSymbol, mdesc)
val (excs, others) = methSymbol.annotations.partition(_.symbol eq defn.ThrowsAnnot)
val thrownExceptions: List[String] = getExceptions(excs)

val bytecodeName =
if (isMethSymStaticCtor) CLASS_CONSTRUCTOR_NAME
else jMethodName

val mdesc = ts.asmMethodType(methSymbol).descriptor
val lengthOk = if jgensig ne null then BCodeUtils.checkConstantStringLength(jgensig)
else BCodeUtils.checkConstantStringLength(bytecodeName, mdesc)
if !lengthOk then
Expand Down
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