Desync on all replays and games with Windows 11 Arm on M1 Max Apple

[malu5531]

From the first link these two function calls are important to ensure deterministic floating point calculations across different systems (such as Intel and AMD):

_controlfp(_PC_24, _MCW_PC)
_controlfp(_RC_NEAR, _MCW_RC)

The first is Precision Control, the second is Rounding Control. According to the microsoft documentation linked for controlfp - setting the Precision Control to 24 bits is not supported on ARM. Making this call on ARM will simply not change anything. The Rounding Control to NEAR seems to be ok.

(Precision Control is not supported for x64 either, but supreme commander FA is 32-bit application I think)

[malu5531]

On a second thought, might also be this assert that fails the first tick:

gpAssert( (_controlfp(0, 0) & _MCW_PC) == _PC_24 );

Since setting the _PC_24 flag does not do anything on ARM, when checking the flags, that will be unchanged. First thought - could we disable one of these asserts with a patch? Or somehow pretend the flags are always set as expected on Apple Silicon with _PC_24 set (or redefine _MCW_PC)?

However, two more interesting bits of information I found, comparing Intel and Apple Silicon, which seems to hint at calculations using binary64 (double) match but binary32 (single float / 32 bit (24 bit precision)) don't match:

https://stackoverflow.com/questions/71441137/np-float32-floating-point-differences-between-intel-macbook-and-m1

https://eclecticlight.co/2021/04/22/can-you-trust-floating-point-arithmetic-on-apple-silicon/

If so, that would mean the proper fix would be to replace all single floats in the game with double floats.