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Frontend: Disallow cross-page branches in multiblock #4270

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Frontend: Disallow cross-page branches in multiblock #4270

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16 changes: 9 additions & 7 deletions FEXCore/Source/Interface/Core/Frontend.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -929,6 +929,7 @@ void Decoder::BranchTargetInMultiblockRange() {
uint64_t TargetRIP = 0;
const auto GPRSize = CTX->GetGPROpSize();
bool Conditional = true;
const auto InstEnd = DecodeInst->PC + DecodeInst->InstSize;

switch (DecodeInst->OP) {
case 0x70 ... 0x7F: // Conditional JUMP
Expand All @@ -937,17 +938,17 @@ void Decoder::BranchTargetInMultiblockRange() {
// auto RIPOffset = LoadSource(Op, Op->Src[0], Op->Flags);
// auto RIPTargetConst = _Constant(Op->PC + Op->InstSize);
// Target offset is PC + InstSize + Literal
TargetRIP = DecodeInst->PC + DecodeInst->InstSize + DecodeInst->Src[0].Literal();
TargetRIP = InstEnd + DecodeInst->Src[0].Literal();
break;
}
case 0xE9:
case 0xEB: // Both are unconditional JMP instructions
TargetRIP = DecodeInst->PC + DecodeInst->InstSize + DecodeInst->Src[0].Literal();
TargetRIP = InstEnd + DecodeInst->Src[0].Literal();
Conditional = false;
break;
case 0xE8: // Call - Immediate target, We don't want to inline calls
if (ExternalBranches) {
ExternalBranches->insert(DecodeInst->PC + DecodeInst->InstSize);
ExternalBranches->insert(InstEnd);
}
[[fallthrough]];
case 0xC2: // RET imm
Expand All @@ -961,7 +962,9 @@ void Decoder::BranchTargetInMultiblockRange() {
}

// If the target RIP is x86 code within the symbol ranges then we are golden
bool ValidMultiblockMember = TargetRIP >= SymbolMinAddress && TargetRIP < SymbolMaxAddress;
// Forbid cross-page branches to both avoid massive (range-wise) code blocks in highly fragmented code and trying to decode unmapped branch targets
bool ValidMultiblockMember =
TargetRIP >= SymbolMinAddress && TargetRIP < std::min(FEXCore::AlignUp(InstEnd, FEXCore::Utils::FEX_PAGE_SIZE), SymbolMaxAddress);

#ifdef _M_ARM_64EC
ValidMultiblockMember = ValidMultiblockMember && !RtlIsEcCode(TargetRIP);
Expand All @@ -974,9 +977,8 @@ void Decoder::BranchTargetInMultiblockRange() {
MaxCondBranchBackwards = std::min(MaxCondBranchBackwards, TargetRIP);

// If we are conditional then a target can be the instruction past the conditional instruction
uint64_t FallthroughRIP = DecodeInst->PC + DecodeInst->InstSize;
if (!HasBlocks.contains(FallthroughRIP)) {
CurrentBlockTargets.insert(FallthroughRIP);
if (!HasBlocks.contains(InstEnd)) {
CurrentBlockTargets.insert(InstEnd);
}
}

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