readme update

README.md

@@ -1,4 +1,6 @@
 # popcnt emulator
+**Cyberpunk 2077 update**: see below!
+
 Pintool library for running Quantum Break on pre-SSE4.2 CPUs
 
 It's very raw and dirty, you'll probably have a very hard time building it and will probably have the game crashed eventually (when it gets to the non-patched popcnt). I'm "working" on it ("testing" it, you know ;) ).
@@ -22,3 +24,42 @@ Then running the game will look like:
 Note that you'll probably have to build both x86 and x64 versions of the tool and name them "instruction_hook64.dll" and "instruction_hook32.dll" respectively.
 
 If you wish to contact me (to ask for binaries, help or simply chat) please use the Issues tab.
+
+## Cyberpunk 2077
+**Good news**: it works. Not a 0% performance hit this time due to the different approach, but still quite playable.
+
+**Bad news**: _very_ version-bound stuff! Requires GOG version 1.03, probably won't work on other versions (but may though). You could get past the main menu with this, but crash later.
+
+**Not so bad news**: I will describe the way you could fix it to work with your version of the game, it is relatively easy.
+
+### Usage:
+Build only `popcnt_hotpatch` project! x64 Release mode. Adjust target paths to your preferred ones.
+
+It builds into `icudt.dll`. Don't change that name.
+
+Replace your game's `icudt.dll` library with this one (make a backup of the original!).
+
+Just start the game as usual!
+
+### For the tech-savvy fellows, here's how it was done:
+This time popcnt's are everywhere! They don't just pop up at the game loading stage and not used later, which rules out all PIN-based solutions (too slow).
+I've tried JIT, Trace mode and Probe. Probe could do the trick, but it refuses to hotwire code blocks less than 7 bytes long (popcnts are 4-5 bytes long).
+Manual patching is out of the picture too, as there are about 120 popcnts in the game! And they are used often.
+
+So, I did a different thing - built a DLL, which wraps a small ICU DLL packaged with the game (so the game could statically import it without patching).
+This DLL is installing a global exception handler, which intercepts "Illegal instruction" exceptions and emulates popcnt in the exception handler.
+Afterwards it is putting the result to the correct destination and jumps over popcnt.
+
+This solution is versatile and could be made even more optimized and version-independent, but still imposes some overhead.
+Also because the game starts throwing a lot of exceptions, attaching a debugger to it would really hit the brakes. But that's not a big issue since you probably wouldn't want to.
+
+### Adding new fixups to the tool
+If your game crashed with unhandled popcnt exception, the `hotfix.log` file will appear at the game folder with the instruction address.
+Disassemble this spot and add another `HOTFIX` line to the code at the bottom (there is a comment), using other lines as an example.
+
+Use `ctx` to access and modify registers and direct pointer access for memory (since we are inside the game's process, we don't have to `ReadProcessMemory` or something like that).
+
+### A note about the infamous AVX crash after Prologue
+This tool doesn't solve this crash because emulating the whole AVX feels a lot more tedious than just one simple instruction.
+
+But there are fixes for it already and they work. Basically they're just NOP'ing out AVX instructions, you could do it yourself.