From 2eb7a9ff286dea8bc049b21be5e1e19e1b6cb99a Mon Sep 17 00:00:00 2001
From: Ryan Houdek <Sonicadvance1@gmail.com>
Date: Thu, 15 Aug 2024 10:22:31 -0700
Subject: [PATCH] OpcodeDispatcher: Remove old bad assumption in INC/DEC

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.
---
 .../Interface/Core/OpcodeDispatcher.cpp       | 12 -------
 unittests/ASM/FEX_bugs/repeat_on_incdec.asm   | 33 +++++++++++++++++++
 2 files changed, 33 insertions(+), 12 deletions(-)
 create mode 100644 unittests/ASM/FEX_bugs/repeat_on_incdec.asm

diff --git a/FEXCore/Source/Interface/Core/OpcodeDispatcher.cpp b/FEXCore/Source/Interface/Core/OpcodeDispatcher.cpp
index 6c8102388..f73c8f363 100644
--- a/FEXCore/Source/Interface/Core/OpcodeDispatcher.cpp
+++ b/FEXCore/Source/Interface/Core/OpcodeDispatcher.cpp
@@ -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);
diff --git a/unittests/ASM/FEX_bugs/repeat_on_incdec.asm b/unittests/ASM/FEX_bugs/repeat_on_incdec.asm
new file mode 100644
index 000000000..86eb46956
--- /dev/null
+++ b/unittests/ASM/FEX_bugs/repeat_on_incdec.asm
@@ -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