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fix(vm): correct BC17 DUP swap-mode operand decoding #340

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fix(vm): correct BC17 DUP swap-mode operand decoding #340

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6b0b33d
fix(vm): correct BC17 DUP swap-mode operand decoding
emiyl Jul 23, 2026
8a4574e
Fix PS2
Fancy2209 Jul 24, 2026
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37 changes: 25 additions & 12 deletions src/vm.c
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Original file line number Diff line number Diff line change
Expand Up @@ -57,6 +57,22 @@ static int gmlTypeNativeSize(uint8_t gmlType) {
}
}

#if IS_WAD17_OR_HIGHER_ENABLED
static const char* gmlTypeName(uint8_t gmlType) {
switch (gmlType) {
case GML_TYPE_DOUBLE: return "DOUBLE";
case GML_TYPE_FLOAT: return "FLOAT";
case GML_TYPE_INT32: return "INT32";
case GML_TYPE_INT64: return "INT64";
case GML_TYPE_BOOL: return "BOOL";
case GML_TYPE_VARIABLE: return "VARIABLE";
case GML_TYPE_STRING: return "STRING";
case GML_TYPE_INT16: return "INT16";
default: return "UNKNOWN";
}
}
#endif

static void stackPush(VMContext* ctx, RValue val) {
require(VM_STACK_SIZE > ctx->stack.top);
#ifdef ENABLE_VM_TRACING
Expand Down Expand Up @@ -1844,7 +1860,7 @@ static int32_t bytesToSlotCount(VMContext* ctx, int32_t nativeBytes, int32_t sta
return slots;
}

static void handleDup(VMContext* ctx, uint32_t instr) {
static void handleDup(VMContext* ctx, uint32_t instr, MAYBE_UNUSED uint32_t instrAddr) {
uint16_t operand = (uint16_t)(instr & 0xFFFF);
uint8_t type1 = instrType1(instr);
int32_t typeSize = gmlTypeNativeSize(type1);
Expand All @@ -1853,17 +1869,14 @@ static void handleDup(VMContext* ctx, uint32_t instr) {
// Swap mode (WAD17+): bit 15 of operand is set.
// The Dup instruction doubles as a stack rotation when bit 15 is set.
// It takes the top N items and moves them below the next M items.
// Bits 0-10: top group size (in native type units)
// Bits 11-14: bottom group size (in native type units)
// Bits 0-10: top group size (in stack slots)
// Bits 11-14: bottom group size (in stack slots)
if (IS_WAD17_OR_HIGHER(ctx) && (operand & 0x8000) != 0) {
int32_t topNativeCount = operand & 0x7FF;
int32_t bottomNativeCount = (operand >> 11) & 0xF;
int32_t topBytes = topNativeCount * typeSize;
int32_t bottomBytes = bottomNativeCount * typeSize;

// Convert byte counts to slot counts
int32_t topSlots = bytesToSlotCount(ctx, topBytes, ctx->stack.top);
int32_t bottomSlots = bytesToSlotCount(ctx, bottomBytes, ctx->stack.top - topSlots);
int32_t topSlots = operand & 0x7FF;
int32_t bottomSlots = (operand >> 11) & 0xF;
requireMessageFormatted(__FILE__, __LINE__, topSlots >= 0 && bottomSlots >= 0, "VM: Dup swap got negative group sizes in %s at offset %u [instr=0x%08X topSlots=%d bottomSlots=%d operand=0x%04X type1=0x%X(%s)]", ctx->currentCodeName, instrAddr, instr, topSlots, bottomSlots, operand, type1, gmlTypeName(type1));
requireMessageFormatted(__FILE__, __LINE__, ctx->stack.top >= topSlots + bottomSlots, "VM: Dup swap stack underflow in %s at offset %u [instr=0x%08X stackTop=%d topSlots=%d bottomSlots=%d operand=0x%04X type1=0x%X(%s)]", ctx->currentCodeName, instrAddr, instr, ctx->stack.top, topSlots, bottomSlots, operand, type1, gmlTypeName(type1));
requireMessageFormatted(__FILE__, __LINE__, topSlots <= 32, "VM: Dup swap temp buffer overflow risk in %s at offset %u [instr=0x%08X topSlots=%d operand=0x%04X type1=0x%X(%s)]", ctx->currentCodeName, instrAddr, instr, topSlots, operand, type1, gmlTypeName(type1));

int32_t totalSlots = topSlots + bottomSlots;
int32_t baseIdx = ctx->stack.top - totalSlots;
Expand Down Expand Up @@ -3224,7 +3237,7 @@ static RValue executeLoop(VMContext* ctx) {

// Duplicate
case OP_DUP:
handleDup(ctx, instr);
handleDup(ctx, instr, instrAddr);
break;

// Branches
Expand Down
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