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Fix Java 6 bytecode verification error for complex Brainf*ck programs #21

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Jul 11, 2025
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Fix Java 6 bytecode verification error for complex Brainf*ck programs #21

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116 changes: 92 additions & 24 deletions src/main/java/am/ik/bf/codegen/JvmByteCode6Generator.java
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Original file line number Diff line number Diff line change
@@ -1,5 +1,6 @@
package am.ik.bf.codegen;

import java.io.ByteArrayOutputStream;
import java.io.OutputStream;
import java.nio.ByteBuffer;
import java.util.ArrayList;
Expand Down Expand Up @@ -31,6 +32,8 @@ public class JvmByteCode6Generator implements CodeGenerator {

private final ClassConstant targetClass;

private final List<Integer> branchTargets = new ArrayList<>();

private final ClassConstant javaLangSystemClass = constantPool.addClass(constantPool.addUtf8("java/lang/System"));

private final FieldrefConstant systemOutFieldRef = constantPool.addFieldref(javaLangSystemClass,
Expand Down Expand Up @@ -59,16 +62,26 @@ public void begin() {
Opcode.NEWARRAY, ArrayType.T_INT, Opcode.ASTORE_1, // memory
Opcode.ICONST_0, Opcode.ISTORE_2 // pointer
));

// Clear any previous data when reusing the generator
this.branchTargets.clear();
}

@Override
public void end() {
this.code.add(Opcode.RETURN);
generateStackMapTableAndWriteClass();
}

private void generateStackMapTableAndWriteClass() {
final ClassConstant javaLangObjectClass = constantPool.addClass(constantPool.addUtf8("java/lang/Object"));
final Utf8Constant mainUt8 = constantPool.addUtf8("main");
final Utf8Constant javaLangStringArrayType = constantPool.addUtf8("([Ljava/lang/String;)V");
final Utf8Constant codeUtf8 = constantPool.addUtf8("Code");
final Utf8Constant stackMapTableUtf8 = constantPool.addUtf8("StackMapTable");

final ClassConstant intArrayClass = constantPool.addClass(constantPool.addUtf8("[I"));
final ByteArrayOutputStream stackMapTableData = generateMinimalStackMapTable(intArrayClass);

new ByteCodeWriter(this.out) //
.write(0xca, 0xfe, 0xba, 0xbe) // cafebabe
Expand All @@ -81,38 +94,92 @@ public void end() {
})
.writeMethods(methods -> methods.add(AccessFlag.ACC_PUBLIC + AccessFlag.ACC_STATIC, mainUt8,
javaLangStringArrayType,
method -> method.writeAttributes(attributes -> attributes.add(codeUtf8,
attribute -> attribute.writeU2(4) // max_stack
.writeU2(3) // max_locals
.writeCode((Object[]) code.toArray(code.toArray(new Integer[0])))
.writeU2(0) // exception_table_length
.writeU2(0) // attributes_count
)))) //
method -> method.writeAttributes(attributes -> attributes.add(codeUtf8, attribute -> {
attribute.writeU2(4) // max_stack
.writeU2(3) // max_locals
.writeCode((Object[]) code.toArray(code.toArray(new Integer[0])))
.writeU2(0) // exception_table_length
.writeAttributes(attrs -> attrs.add(stackMapTableUtf8, attr -> {
attr.write(stackMapTableData.toByteArray());
}));
})))) //
.writeAttributes(attributesDef -> {
});
}

private ByteArrayOutputStream generateMinimalStackMapTable(ClassConstant intArrayClass) {
ByteArrayOutputStream stackMapTableData = new ByteArrayOutputStream();
ByteCodeWriter stackMapWriter = new ByteCodeWriter(stackMapTableData);

// Simple StackMapTable: initial frame + branch targets
var sortedTargets = branchTargets.stream()
.distinct()
.filter(target -> target > 8) // Only targets after initial setup
.sorted()
.limit(99) // Limit to prevent excessive frames
.toList();

// Write the number of frames (initial frame + branch targets)
stackMapWriter.writeU2(sortedTargets.size() + 1);

// First frame: APPEND frame to establish local variables
stackMapWriter.write(253); // frame_type = append_frame (2 locals)
stackMapWriter.writeU2(6); // offset_delta = 7-1 (at istore_2 instruction)
stackMapWriter.write(7); // ITEM_Object for int[]
stackMapWriter.writeU2(intArrayClass.index());
stackMapWriter.write(1); // ITEM_Integer for pointer

// Add frames for all branch targets
int previousOffset = 6;
for (Integer target : sortedTargets) {
int offsetDelta = target - previousOffset - 1;
if (offsetDelta >= 0 && offsetDelta <= 65535) {
stackMapWriter.write(251); // frame_type = same_frame_extended
stackMapWriter.writeU2(offsetDelta);
previousOffset = target;
}
}

return stackMapTableData;
}

@Override
public void generateLoopStatement(LoopStatement statement) {
// Use GOTO_W from the start to handle large offsets
final int loopStart = this.code.size();

this.code.addAll(List.of( //
Opcode.ALOAD_1, // memory
Opcode.ILOAD_2, // pointer
Opcode.IALOAD, // memory[pointer]
Opcode.IFEQ //
Opcode.IFNE, 0, 8 // jump 8 bytes to skip the GOTO_W if non-zero
));
this.writeBytes(toU2(0)); // replace later
final int indexToReplace = this.code.size() - 2;
final int beforeSize = this.code.size();

// Add GOTO_W to exit the loop when value is zero
this.code.add(Opcode.GOTO_W);
final int gotoWExitPosition = this.code.size() - 1;
this.writeBytes(toU4(0)); // Placeholder for exit offset

// Generate loop body
statement.statements().forEach(s -> s.generate(this));
final int codeLen = this.code.size() - beforeSize;
// ifeq + branchbyte1 + branchbyte2 + goto + branchbyte1 + branchbyte2 = 6
final int indexIfeq = codeLen + 6;
final byte[] ifeq = toU2(indexIfeq);
for (int i = 0; i < ifeq.length; i++) {
this.code.set(indexToReplace + i, (int) ifeq[i]);
}
this.code.add(Opcode.GOTO);
this.writeBytes(toU2(-indexIfeq));

// Add GOTO_W back to loop start
this.code.add(Opcode.GOTO_W);
final int gotoWBackPosition = this.code.size() - 1;
final int gotoWBackOffset = loopStart - gotoWBackPosition;
this.writeBytes(toU4(gotoWBackOffset));

// Fix the GOTO_W exit offset
final int endPosition = this.code.size();
final int gotoWExitOffset = endPosition - gotoWExitPosition;
final byte[] exitBytes = toU4(gotoWExitOffset);
this.code.set(gotoWExitPosition + 1, (int) exitBytes[0]);
this.code.set(gotoWExitPosition + 2, (int) exitBytes[1]);
this.code.set(gotoWExitPosition + 3, (int) exitBytes[2]);
this.code.set(gotoWExitPosition + 4, (int) exitBytes[3]);

// Record branch target for StackMapTable
this.branchTargets.add(endPosition);
}

@Override
Expand Down Expand Up @@ -180,8 +247,9 @@ public void generateIncrementPointerExpression(IncrementPointerExpression expres
public void generateSetValueToZeroExpression(ResetValueExpression expression) {
this.code.addAll(List.of( //
Opcode.ALOAD_1, // memory
Opcode.ILOAD_2, // pointer,
Opcode.IASTORE, Opcode.ICONST_0));
Opcode.ILOAD_2, // pointer
Opcode.ICONST_0, // value 0
Opcode.IASTORE)); // memory[pointer] = 0
}

private void writeBytes(byte[] bytes) {
Expand All @@ -190,8 +258,8 @@ private void writeBytes(byte[] bytes) {
}
}

private static byte[] toU2(int i) {
return ByteBuffer.allocate(2).putShort((short) i).array();
private static byte[] toU4(int i) {
return ByteBuffer.allocate(4).putInt(i).array();
}

}
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