From 3f8f886f65fe90bcbce09dbe839bd014b759d235 Mon Sep 17 00:00:00 2001
From: "Pierre-Loup A. Griffais" <git@plagman.net>
Date: Mon, 14 Sep 2026 21:39:18 -0700
Subject: [PATCH] Frontend: patch cmp imm8 for cache

Usually imm8 seem more likely to be a function of the code around them, but
this one seems used by Mono for varying data over execution of same code.
Maybe an ID of some kind.
---
 FEXCore/Source/Interface/Core/CodeCache.cpp |  7 ++++--
 FEXCore/Source/Interface/Core/Frontend.cpp  | 24 ++++++++++++++++++++-
 2 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/FEXCore/Source/Interface/Core/CodeCache.cpp b/FEXCore/Source/Interface/Core/CodeCache.cpp
index a35fc674d..229d6e9bf 100644
--- a/FEXCore/Source/Interface/Core/CodeCache.cpp
+++ b/FEXCore/Source/Interface/Core/CodeCache.cpp
@@ -545,8 +545,11 @@ static inline int64_t ReadLiveGuestData(uint64_t SiteAddress, uint8_t ValueSize)
   uint64_t Raw = 0;
   memcpy(&Raw, reinterpret_cast<const void*>(SiteAddress), ValueSize);
   // manual sign-extension from guest live bytes
-  // 1/2 sizes not permitted in DetectDataMasks currently
-  if (ValueSize == 4) {
+  if (ValueSize == 1) {
+    return (int8_t)Raw;
+  } else if (ValueSize == 2) {
+    return (int16_t)Raw;
+  } else if (ValueSize == 4) {
     return (int32_t)Raw;
   } else {
     return (int64_t)Raw;
diff --git a/FEXCore/Source/Interface/Core/Frontend.cpp b/FEXCore/Source/Interface/Core/Frontend.cpp
index 9d1a97746..ac58961b6 100644
--- a/FEXCore/Source/Interface/Core/Frontend.cpp
+++ b/FEXCore/Source/Interface/Core/Frontend.cpp
@@ -1395,11 +1395,33 @@ bool Decoder::CheckIfCacheable(FEXCore::Core::InternalThreadState& Thread, const
 }
 
 void Decoder::DetectDataMasks(uint64_t OpAddress, DecodedBlocks& Block) {
+  FEXCore::X86Tables::DecodedOperand* LiteralToPatch = nullptr;
+
+  // cmp *, imm8 - seen varying in mono jitted code
+  {
+    FEXCore::X86Tables::ModRMDecoded ModRM;
+    ModRM.Hex = DecodeInst->ModRM;
+    if ((DecodeInst->OPRaw == 0x80 || DecodeInst->OPRaw == 0x83) && ModRM.reg == 7 && LastFieldReadSize == 1) {
+      for (auto& Src : DecodeInst->Src) {
+        if (Src.IsLiteral()) {
+          LiteralToPatch = &Src;
+          break;
+        }
+      }
+      if (LiteralToPatch && LiteralToPatch->Literal() != 0) {
+        Block.DataMasks.push_back({OpAddress + LastFieldReadOffset, DataMaskType::MOV, LastFieldReadSize});
+
+        LiteralToPatch->Type = X86Tables::DecodedOperand::OpType::LiteralPatchable;
+        LiteralToPatch->Data.LiteralPatchable.FieldOffset = LastFieldReadOffset;
+        LiteralToPatch->Data.LiteralPatchable.Width = LastFieldReadSize;
+      }
+    }
+  }
+
   if (LastFieldReadSize < 4) {
     return;
   }
 
-  FEXCore::X86Tables::DecodedOperand* LiteralToPatch = nullptr;
   DataMaskType Type = DataMaskType::NONE;
 
   // imm32 or imm64 at the end type instructions