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FEXCore/Frontend: Ensure multiple prefix bytes work

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Only the last prefix byte is retained when multiple are set. We were
accidentally generating a mask.

Additionally with 64-bit code, the legacy segment prefixes don't
overwrite if FS or GS have been set. So no weird behaviour where FS/GS
is set, a legacy prefix is used for padding, and then it "ignores" a bad
prefix by ignoring only the latest one.
This commit is contained in:
Ryan Houdek committed 2025-07-30 12:30:13 -07:00
1 parent 6724673482
commit d4dcbfa90b
4 files changed
+22 -23
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FEXCore/Source/Interface/Core/OpcodeDispatcher.cpp
+6 -5
View File
@@ -3916,17 +3916,17 @@ uint32_t OpDispatchBuilder::GetDstBitSize(X86Tables::DecodedOp Op) const {
Ref OpDispatchBuilder::GetSegment(uint32_t Flags, uint32_t DefaultPrefix, bool Override) {
const auto GPRSize = GetGPROpSize();
uint32_t Prefix = Flags & FEXCore::X86Tables::DecodeFlags::FLAG_SEGMENTS;
if (Is64BitMode) {
if (Flags & FEXCore::X86Tables::DecodeFlags::FLAG_FS_PREFIX) {
if (Prefix == FEXCore::X86Tables::DecodeFlags::FLAG_FS_PREFIX) {
return _LoadContext(GPRSize, GPRClass, offsetof(FEXCore::Core::CPUState, fs_cached));
} else if (Flags & FEXCore::X86Tables::DecodeFlags::FLAG_GS_PREFIX) {
} else if (Prefix == FEXCore::X86Tables::DecodeFlags::FLAG_GS_PREFIX) {
return _LoadContext(GPRSize, GPRClass, offsetof(FEXCore::Core::CPUState, gs_cached));
}
// If there was any other segment in 64bit then it is ignored
} else {
uint32_t Prefix = Flags & FEXCore::X86Tables::DecodeFlags::FLAG_SEGMENTS;
if (!Prefix || Override) {
if (Prefix == FEXCore::X86Tables::DecodeFlags::FLAG_NO_PREFIX || Override) {
// If there was no prefix then use the default one if available
// Or the argument only uses a specific prefix (with override set)
Prefix = DefaultPrefix;
@@ -3934,6 +3934,7 @@ Ref OpDispatchBuilder::GetSegment(uint32_t Flags, uint32_t DefaultPrefix, bool O
// With the segment register optimization we store the GDT bases directly in the segment register to remove indexed loads
Ref SegmentResult {};
switch (Prefix) {
[[likely]] case FEXCore::X86Tables::DecodeFlags::FLAG_NO_PREFIX: return nullptr;
case FEXCore::X86Tables::DecodeFlags::FLAG_ES_PREFIX:
SegmentResult = _LoadContext(GPRSize, GPRClass, offsetof(FEXCore::Core::CPUState, es_cached));
break;
@@ -3952,7 +3953,7 @@ Ref OpDispatchBuilder::GetSegment(uint32_t Flags, uint32_t DefaultPrefix, bool O
case FEXCore::X86Tables::DecodeFlags::FLAG_GS_PREFIX:
SegmentResult = _LoadContext(GPRSize, GPRClass, offsetof(FEXCore::Core::CPUState, gs_cached));
break;
default: return nullptr;
default: FEX_UNREACHABLE;
}
CheckLegacySegmentRead(SegmentResult, Prefix);