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OpcodeDispatcher: Remove old bad assumption in INC/DEC

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Somewhere there was an assumption made that INC and DEC supported the
repeat prefix. This isn't actually the case, while the prefix can be
encoded, it is a nop and should only expect to be used for padding.

Adds a unittest to ensure that behaviour is as expected.
This commit is contained in:
Ryan Houdek 2024-08-15 10:22:31 -07:00
parent 933c65d805
commit 2eb7a9ff28
No known key found for this signature in database
2 changed files with 33 additions and 12 deletions
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12
FEXCore/Source/Interface/Core/OpcodeDispatcher.cpp
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@ -3055,12 +3055,6 @@ void OpDispatchBuilder::RDTSCOp(OpcodeArgs) {
}
void OpDispatchBuilder::INCOp(OpcodeArgs) {
if (Op->Flags & FEXCore::X86Tables::DecodeFlags::FLAG_REP_PREFIX) {
LogMan::Msg::EFmt("Can't handle REP on this");
DecodeFailure = true;
return;
}
Ref Dest;
Ref Result;
const auto Size = GetSrcBitSize(Op);
@ -3102,12 +3096,6 @@ void OpDispatchBuilder::INCOp(OpcodeArgs) {
}
void OpDispatchBuilder::DECOp(OpcodeArgs) {
if (Op->Flags & FEXCore::X86Tables::DecodeFlags::FLAG_REP_PREFIX) {
LogMan::Msg::EFmt("Can't handle REP on this");
DecodeFailure = true;
return;
}
Ref Dest;
Ref Result;
const auto Size = GetSrcBitSize(Op);

33
unittests/ASM/FEX_bugs/repeat_on_incdec.asm Normal file
View File

@ -0,0 +1,33 @@
%ifdef CONFIG
{
"RegData": {
"RBX": "0",
"RCX": "8",
"RDX": "0",
"RSP": "0"
}
}
%endif
; FEX-Emu had a bug where it thought repeat worked on increment and decrement instructions.
; While the prefix can be encoded on the instructions, it is ignored by the hardware implementation.
; This checks to ensure that inc/dec ignore the repeat prefix, and that rcx isn't ever changed from it.
mov rsp, 0
mov rcx, 8
lea rax, [rel .test]
rep inc rsp
rep inc byte [rax]
rep dec rsp
rep dec byte [rax]
mov rbx, [rel .test]
mov rdx, [rel .test + 8]
hlt
.test:
db 0, 0, 0, 0, 0, 0, 0, 0
db 0, 0, 0, 0, 0, 0, 0, 0