Mjolnir_IV: GTA IV CPU Performance Fix

A runtime patch that fixes critical CPU performance issues in GTA IV: The Complete Edition (Steam).

First known fix for GTA IV's CPU Core 0 pinning bug.

What It Does

GTA IV was ported from Xbox 360 in 2008. Several design decisions from that era cause severe performance issues on modern hardware:

Fix	Problem	Solution	Impact
CPU Affinity	Main thread pinned to Core 0 via SetThreadAffinityMask(thread, 1). One core at 70 to 90% while 15 sit idle	NOP the affinity call, let the OS scheduler distribute work	Core 0: 72% to 29%
Busy Wait Yield	Inner engine loop spins cmp/je with zero yield, burns 100% of a core doing nothing	Inject SwitchToThread() via code cave	Eliminates screen tearing, smoother frametime
Auto Adjuster	Quality auto adjuster uses broken VRAM estimation from console era, can force quality caps despite 12GB+ VRAM	Disable the adjuster flag, user keeps manual control	Full quality settings unlocked

A/B Test Results (per core CPU, 60 second samples)

Metric	Baseline	With Fix	Delta
Core 0 Usage	72.4%	28.6%	-43.8 pp
Total Process CPU	17.2%	17.6%	noise

Load is redistributed by the OS scheduler. Total work identical, just no longer bottlenecked on one core.

Installation

With FusionFix (recommended)

Most GTA IV players use FusionFix. FusionFix ships its own dinput8.dll (Ultimate ASI Loader), so use the .asi method:

1. Download Mjolnir_IV.asi from the Releases page
2. Drop it into your plugins folder alongside the FusionFix .asi
3. Launch the game normally

Grand Theft Auto IV/GTAIV/
├── GTAIV.exe
├── dinput8.dll              ← FusionFix's ASI Loader (do not replace)
├── d3d9.dll                 ← DXVK (ships with FusionFix v4.0+)
├── mjolnir_cpu_fix.log      ← created on launch
└── plugins/
    ├── GTAIV.EFLC.FusionFix.asi
    ├── GTAIV.EFLC.FusionFix.ini
    └── Mjolnir_IV.asi       ← drop here

Vanilla (no other mods)

1. Download dinput8.dll from the Releases page
2. Drop it next to GTAIV.exe in your game directory
3. Launch the game normally

Grand Theft Auto IV/GTAIV/
├── GTAIV.exe
├── dinput8.dll          ← Mjolnir (drop here)
└── mjolnir_cpu_fix.log  ← created on launch

Uninstall

Delete Mjolnir_IV.asi from plugins (or dinput8.dll from the game directory). That's it.

The DLL creates mjolnir_cpu_fix.log in the game directory on each launch. Check it to verify all fixes applied successfully.

FusionFix Compatibility

Mjolnir works alongside FusionFix with zero patch overlap. FusionFix handles graphics and rendering fixes; Mjolnir handles CPU and threading fixes.

Audited: FusionFix's fixes.ixx (1042 lines) contains zero thread, affinity, or streaming fixes. No overlapping patches.

Building from Source

Requires CMake 3.10+ and MSVC (Visual Studio Build Tools).

cd fix_template
cmake -B build -A Win32
cmake --build build --config Release

Output: fix_template/build/Release/dinput8.dll

Important: The .def file must be in CMakeLists.txt SOURCES or exports get stdcall decoration (_DirectInput8Create@20 instead of DirectInput8Create) and the game will crash silently.

How It Works

The DLL loads as a dinput8.dll proxy when used standalone, or as an .asi plugin when loaded by FusionFix's ASI loader. It:

1. Patches memory in DLL_PROCESS_ATTACH before the game's main thread runs
2. Forwards all real DirectInput8 calls to C:\Windows\System32\dinput8.dll (proxy mode only)
3. Handles ASLR: computes runtime base delta, all addresses go through R(addr) macro
4. Verifies expected opcodes before patching, won't corrupt a different exe version

Fix 1: CPU Affinity (NOP patch)

At 0x59D3A1, the game does push 1; call GetCurrentThread; push eax; call SetThreadAffinityMask, force pinning the main thread to CPU 0. We NOP 15 bytes around the mov esi, ecx instruction to remove the pin while preserving the register save.

Fix 2: Busy Wait Yield (code cave)

At 0xD9737D, the engine spins cmp byte [esi+0xC34], 0; je loop_top with zero yield. We replace the 9 byte cmp+je with a JMP to a VirtualAlloc'd code cave that performs the original comparison, calls SwitchToThread() via the IAT, and jumps back.

Fix 7: Auto Adjuster Disable (DWORD write)

The quality auto adjuster at 0x0110E6BC (flag: 0=active, nonzero=off) uses a VRAM estimation formula from the Xbox 360 era. Despite detecting 12GB correctly, the estimation math produces a ratio of -33.68, which can trigger quality downgrades on settings changes. Setting the flag to 1 gives users full manual control.

License

MIT

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Part of Project Mjolnir: RAGE Engine Analysis & Fix Suite. Framework originally built for Saints Row 2, dedicated to Mike "IdolNinja" Watson (1971-2021).