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Remove uses of unsafeNulls from the JVM backend #26094

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34 changes: 16 additions & 18 deletions compiler/src/dotty/tools/backend/jvm/BCodeBodyBuilder.scala
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Expand Up @@ -2,7 +2,6 @@ package dotty.tools
package backend
package jvm

import scala.language.unsafeNulls
import scala.annotation.{switch, tailrec}
import scala.collection.mutable.SortedMap
import scala.tools.asm
Expand Down Expand Up @@ -202,7 +201,7 @@ trait BCodeBodyBuilder(val primitives: ScalaPrimitives) extends BCodeSkelBuilder

if (isArrayGet(code)) {
// load argument on stack
assert(args.length == 1, s"Too many arguments for array get operation: $tree");
assert(args.length == 1, s"Too many arguments for array get operation: $tree")
stack.push(k)
genLoad(args.head, INT)
stack.pop()
Expand Down Expand Up @@ -335,7 +334,7 @@ trait BCodeBodyBuilder(val primitives: ScalaPrimitives) extends BCodeSkelBuilder
bc.store(idx, tk)
val localVarStart = currProgramPoint()
if (!isSynth) { // there are case <synthetic> ValDef's emitted by patmat
varsInScope ::= (sym -> localVarStart)
varsInScope = (sym -> localVarStart) :: varsInScope.nn
}
generatedType = UNIT

Expand Down Expand Up @@ -547,7 +546,7 @@ trait BCodeBodyBuilder(val primitives: ScalaPrimitives) extends BCodeSkelBuilder
private def fieldOp(field: Symbol, isLoad: Boolean, specificReceiver: Symbol | Null)(using Context): Unit = {
val useSpecificReceiver = specificReceiver != null && !field.isScalaStatic

val owner = bTypeLoader.classBTypeFromSymbol(if (useSpecificReceiver) specificReceiver else field.owner).internalName
val owner = bTypeLoader.classBTypeFromSymbol(if (useSpecificReceiver) specificReceiver.nn else field.owner).internalName
val fieldJName = field.javaSimpleName
val fieldDescr = symInfoTK(field).descriptor
val isStatic = field.isStaticMember
Expand Down Expand Up @@ -653,7 +652,7 @@ trait BCodeBodyBuilder(val primitives: ScalaPrimitives) extends BCodeSkelBuilder
if (earlyReturnVar == null) {
earlyReturnVar = locals.makeLocal(returnType, "earlyReturnVar", expr.tpe, expr.span)
}
locals.store(earlyReturnVar)
locals.store(earlyReturnVar.nn)
}
bc.goTo(nextCleanup)
shouldEmitCleanup = true
Expand Down Expand Up @@ -986,7 +985,7 @@ trait BCodeBodyBuilder(val primitives: ScalaPrimitives) extends BCodeSkelBuilder

var flatKeys: List[Int] = Nil
var targets: List[asm.Label] = Nil
var default: asm.Label = null
var default: asm.Label | Null = null
var switchBlocks: List[(asm.Label, Tree)] = Nil

genLoad(selector, INT)
Expand Down Expand Up @@ -1020,7 +1019,7 @@ trait BCodeBodyBuilder(val primitives: ScalaPrimitives) extends BCodeSkelBuilder
if !hasDefault then
default = new asm.Label

bc.emitSWITCH(mkArrayReverse(flatKeys), mkArrayL(targets.reverse), default, MIN_SWITCH_DENSITY)
bc.emitSWITCH(mkArrayReverse(flatKeys), mkArrayL(targets.reverse), default.nn, MIN_SWITCH_DENSITY)

// emit switch-blocks.
for (sb <- switchBlocks.reverse) {
Expand All @@ -1030,7 +1029,7 @@ trait BCodeBodyBuilder(val primitives: ScalaPrimitives) extends BCodeSkelBuilder
}

if !hasDefault then
markProgramPoint(default)
markProgramPoint(default.nn)
emitThrowMatchError()
} else {

Expand All @@ -1042,13 +1041,12 @@ trait BCodeBodyBuilder(val primitives: ScalaPrimitives) extends BCodeSkelBuilder
* This mirrors the way that Java compiles `switch` on Strings.
*/

var default: asm.Label = null
var indirectBlocks: List[(asm.Label, Tree)] = Nil
var default: asm.Label | Null = null
var indirectBlocks: List[(asm.Label, Tree | Null)] = Nil


// Cases grouped by their hashCode
val casesByHash = SortedMap.empty[Int, List[(String, Either[asm.Label, Tree])]]
var caseFallback: Tree = null

for (caze @ CaseDef(pat, guard, body) <- cases) {
assert(guard == tpd.EmptyTree, guard)
Expand All @@ -1062,7 +1060,7 @@ trait BCodeBodyBuilder(val primitives: ScalaPrimitives) extends BCodeSkelBuilder
case Ident(nme.WILDCARD) =>
assert(default == null, s"multiple default targets in a Match node, at ${tree.span}")
default = new asm.Label
indirectBlocks ::= (default, body)
indirectBlocks ::= (default.nn, body)
case Alternative(alts) =>
// We need an extra basic block since multiple strings can lead to this code
val indirectCaseGroupLabel = new asm.Label
Expand Down Expand Up @@ -1097,7 +1095,7 @@ trait BCodeBodyBuilder(val primitives: ScalaPrimitives) extends BCodeSkelBuilder
val hasDefault = default != null
if !hasDefault then
default = new asm.Label
indirectBlocks ::= (default, null)
indirectBlocks ::= (default.nn, null)

// Push the hashCode of the string (or `0` it is `null`) onto the stack and switch on it
genLoadIfTo(
Expand All @@ -1109,7 +1107,7 @@ trait BCodeBodyBuilder(val primitives: ScalaPrimitives) extends BCodeSkelBuilder
INT,
LoadDestination.FallThrough
)
bc.emitSWITCH(mkArrayReverse(flatKeys), mkArrayL(targets.reverse), default, MIN_SWITCH_DENSITY)
bc.emitSWITCH(mkArrayReverse(flatKeys), mkArrayL(targets.reverse), default.nn, MIN_SWITCH_DENSITY)

// emit blocks for each hash case
for ((hashLabel, caseAlternatives) <- hashBlocks.reverse) {
Expand All @@ -1130,7 +1128,7 @@ trait BCodeBodyBuilder(val primitives: ScalaPrimitives) extends BCodeSkelBuilder
}
markProgramPoint(keepGoing)
}
bc.goTo(default)
bc.goTo(default.nn)
}

// emit blocks for common patterns
Expand Down Expand Up @@ -1165,7 +1163,7 @@ trait BCodeBodyBuilder(val primitives: ScalaPrimitives) extends BCodeSkelBuilder
def emitLocalVarScopes(): Unit =
if (BackendUtils.emitVars) {
val end = currProgramPoint()
for ((sym, start) <- varsInScope.reverse) {
for ((sym, start) <- varsInScope.nn.reverse) {
emitLocalVarScope(sym, start, end)
}
}
Expand Down Expand Up @@ -1445,7 +1443,7 @@ trait BCodeBodyBuilder(val primitives: ScalaPrimitives) extends BCodeSkelBuilder
assert(style.isVirtual || style.isSuper || specificReceiver == methodOwner, s"specificReceiver can only be specified for virtual and super calls. $method - $specificReceiver")

val useSpecificReceiver = specificReceiver != null && !defn.isBottomClass(specificReceiver) && !method.isScalaStatic
val receiver = if (useSpecificReceiver) specificReceiver else methodOwner
val receiver: Symbol = if (useSpecificReceiver) specificReceiver.nn else methodOwner

// TODO this JVM bug was resolved a very long time ago, workaround could be removed?
// workaround for a JVM bug: https://bugs.openjdk.java.net/browse/JDK-8154587
Expand Down Expand Up @@ -1603,7 +1601,7 @@ trait BCodeBodyBuilder(val primitives: ScalaPrimitives) extends BCodeSkelBuilder
case Literal(Constant(null)) => true
case _ => false
}
def ifOneIsNull(l: Tree, r: Tree): Tree = if (isNull(l)) r else if (isNull(r)) l else null
def ifOneIsNull(l: Tree, r: Tree): Tree | Null = if (isNull(l)) r else if (isNull(r)) l else null
val nonNullSide = if (ScalaPrimitivesOps.isReferenceEqualityOp(code)) ifOneIsNull(l, r) else null
if (nonNullSide != null) {
// special-case reference (in)equality test for null (null eq x, x eq null)
Expand Down
5 changes: 2 additions & 3 deletions compiler/src/dotty/tools/backend/jvm/BCodeHelpers.scala
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Expand Up @@ -2,7 +2,6 @@ package dotty.tools
package backend
package jvm

import scala.language.unsafeNulls
import scala.tools.asm
import scala.tools.asm.{AnnotationVisitor, ClassWriter, Opcodes}
import scala.collection.mutable
Expand Down Expand Up @@ -205,7 +204,7 @@ trait BCodeHelpers(val bTypeLoader: BTypeLoader, val bTypes: WellKnownBTypes) ex
}

private def emitArgument(av: AnnotationVisitor,
name: String,
name: String | Null,
arg: Tree)(using Context): Unit = {
val narg = normalizeArgument(arg)
// Transformation phases are not run on annotation trees, so we need to run
Expand Down Expand Up @@ -649,7 +648,7 @@ trait BCodeHelpers(val bTypeLoader: BTypeLoader, val bTypes: WellKnownBTypes) ex
val erasedMemberType = ElimErasedValueType.elimEVT(TypeErasure.transformInfo(sym, memberTpe))
if (erasedMemberType =:= sym.denot.info)
val gensig = getGenericSignatureHelper(sym, moduleClass, memberTpe)
if gensig == null || descriptor.contentEquals(gensig) then null
if gensig == null || (descriptor != null && descriptor.contentEquals(gensig)) then null
else gensig.toString
else null
else null
Expand Down
1 change: 0 additions & 1 deletion compiler/src/dotty/tools/backend/jvm/BCodeIdiomatic.scala
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Expand Up @@ -2,7 +2,6 @@ package dotty.tools
package backend
package jvm

import scala.language.unsafeNulls
import scala.tools.asm
import scala.annotation.switch
import scala.tools.asm.tree.MethodInsnNode
Expand Down
47 changes: 24 additions & 23 deletions compiler/src/dotty/tools/backend/jvm/BCodeSkelBuilder.scala
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Expand Up @@ -2,7 +2,6 @@ package dotty.tools
package backend
package jvm

import scala.language.unsafeNulls
import scala.annotation.tailrec
import scala.collection.{immutable, mutable}
import scala.tools.asm
Expand All @@ -23,6 +22,8 @@ import SymbolUtils.given
import dotty.tools.dotc.core.NameOps.isStaticConstructorName
import tpd.*

import scala.compiletime.uninitialized

/*
*
* @author Miguel Garcia, http://lamp.epfl.ch/~magarcia/ScalaCompilerCornerReloaded/
Expand Down Expand Up @@ -132,14 +133,14 @@ trait BCodeSkelBuilder extends BCodeHelpers {
with BCJGenSigGen {

// Strangely I can't find this in the asm code 255, but reserving 1 for "this"
inline val MaximumJvmParameters = 254
private inline val MaximumJvmParameters = 254

// current class
var cnode: ClassNode1 = null
var thisName: String = null // the internal name of the class being emitted
private var cnode: ClassNode1 = uninitialized
private var thisName: String = uninitialized // the internal name of the class being emitted
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@SolalPirelli SolalPirelli May 19, 2026

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unfortunately given the code shape removing the need for uninitialized would require a fair amount of work. But at least we can make those private so nobody accidentally observes them when they shouldn't.


var claszSymbol: Symbol = null
var isCZStaticModule = false
protected var claszSymbol: Symbol = uninitialized
private var isCZStaticModule = false

// keep track of interfaces that are used in super calls, as they need to be directly inherited even if they are also indirectly inherited
val superCallTargets = mutable.LinkedHashSet[ClassBType]()
Expand All @@ -160,7 +161,7 @@ trait BCodeSkelBuilder extends BCodeHelpers {

/* ---------------- helper utils for generating classes and fields ---------------- */

def genPlainClass(cd0: TypeDef)(using Context) = (cd0: @unchecked) match {
def genPlainClass(cd0: TypeDef)(using Context): ClassNode1 = (cd0: @unchecked) match {
case TypeDef(_, impl: Template) =>
assert(cnode == null, "GenBCode detected nested methods.")

Expand Down Expand Up @@ -288,7 +289,7 @@ trait BCodeSkelBuilder extends BCodeHelpers {
TraceUtils.traceClassIfRequested(cnode)

assert(cd.symbol == claszSymbol, "Someone messed up BCodePhase.claszSymbol during genPlainClass().")

cnode
} // end of method genPlainClass()

/*
Expand Down Expand Up @@ -386,7 +387,7 @@ trait BCodeSkelBuilder extends BCodeHelpers {
.addFlagIf(!sym.is(Mutable), ACC_FINAL)
}

def addClassField(f: Symbol)(using Context): Unit = {
private def addClassField(f: Symbol)(using Context): Unit = {
val descriptor = symInfoTK(f).descriptor
val javagensig = getGenericSignature(f, claszSymbol, descriptor)
val flags = javaFieldFlags(f)
Expand All @@ -405,7 +406,7 @@ trait BCodeSkelBuilder extends BCodeHelpers {
emitAnnotations(jfield, f.annotations)
}

def addClassFields()(using Context): Unit =
private def addClassFields()(using Context): Unit =
/* Non-method term members are fields, except for module members. Module
* members can only happen on .NET (no flatten) for inner traits. There,
* a module symbol is generated (transformInfo in mixin) which is used
Expand All @@ -416,18 +417,18 @@ trait BCodeSkelBuilder extends BCodeHelpers {
claszSymbol.info.decls.filter(p => p.isTerm && !p.is(Method)).foreach(addClassField)

// current method
var mnode: MethodNode1 = null
var jMethodName: String = null
var isMethSymStaticCtor = false
var returnType: BType = null
var methSymbol: Symbol = null
var mnode: MethodNode1 = uninitialized
var jMethodName: String = uninitialized
private var isMethSymStaticCtor = false
var returnType: BType = uninitialized
var methSymbol: Symbol = uninitialized
// used by genLoadTry() and genSynchronized()
var earlyReturnVar: Symbol = null
var earlyReturnVar: Symbol | Null = null
var shouldEmitCleanup = false
// stack tracking
val stack = new BTypesStack
// line numbers
var lastEmittedLineNr = -1
private var lastEmittedLineNr = -1

object bc extends JCodeMethodN {
override def jmethod = PlainSkelBuilder.this.mnode
Expand All @@ -440,7 +441,7 @@ trait BCodeSkelBuilder extends BCodeHelpers {
* The `jumpDest` map is used to find the `LoadDestination` at the end of the `Labeled` block, as well as the
* corresponding expected type. The `LoadDestination` can never be `FallThrough` here.
*/
var jumpDest: immutable.Map[ /* Labeled */ Symbol, (BType, LoadDestination) ] = null
var jumpDest: immutable.Map[ /* Labeled */ Symbol, (BType, LoadDestination) ] = immutable.Map.empty
def registerJumpDest(labelSym: Symbol, expectedType: BType, dest: LoadDestination)(using Context): Unit = {
assert(labelSym.is(Label), s"trying to register a jump-dest for a non-label symbol, at: ${labelSym.span}")
assert(dest != LoadDestination.FallThrough, s"trying to register a FallThrough dest for label, at: ${labelSym.span}")
Expand Down Expand Up @@ -484,10 +485,10 @@ trait BCodeSkelBuilder extends BCodeHelpers {
* emitted for that purpose as described in `genLoadTry()` and `genSynchronized()`.
*/
var cleanups: List[asm.Label] = Nil
def registerCleanup(finCleanup: asm.Label): Unit = {
def registerCleanup(finCleanup: asm.Label | Null): Unit = {
if (finCleanup != null) { cleanups = finCleanup :: cleanups }
}
def unregisterCleanup(finCleanup: asm.Label): Unit = {
def unregisterCleanup(finCleanup: asm.Label | Null): Unit = {
if (finCleanup != null) {
assert(cleanups.head eq finCleanup,
s"Bad nesting of cleanup operations: $cleanups trying to unregister: $finCleanup")
Expand Down Expand Up @@ -585,7 +586,7 @@ trait BCodeSkelBuilder extends BCodeHelpers {
/* ---------------- Part 2 of program points, ie Labels in the ASM world ---------------- */

// bookkeeping the scopes of non-synthetic local vars, to emit debug info (`emitVars`).
var varsInScope: List[(Symbol, asm.Label)] = null // (local-var-sym -> start-of-scope)
var varsInScope: List[(Symbol, asm.Label)] | Null = null // (local-var-sym -> start-of-scope)

// helpers around program-points.
def lastInsn: asm.tree.AbstractInsnNode = mnode.instructions.getLast
Expand Down Expand Up @@ -806,7 +807,7 @@ trait BCodeSkelBuilder extends BCodeHelpers {
.withAttachment(BCodeHelpers.UseInvokeSpecial, ())
})

def genDefDef(dd: DefDef)(using Context): Unit = {
private def genDefDef(dd: DefDef)(using Context): Unit = {
val rhs = dd.rhs
val vparamss = dd.termParamss
// the only method whose implementation is not emitted: getClass()
Expand Down Expand Up @@ -924,7 +925,7 @@ trait BCodeSkelBuilder extends BCodeHelpers {

TraceUtils.traceMethodIfRequested(mnode)

mnode = null
mnode = null.asInstanceOf[MethodNode1] // for GC
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@SolalPirelli SolalPirelli May 19, 2026

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(I'm assuming)

} // end of method genDefDef()

def emitLocalVarScope(sym: Symbol, start: asm.Label, end: asm.Label, force: Boolean = false): Unit = {
Expand Down
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