The determinism that ensures a game replay can use only initial conditions to recreate a whole sequence. There is tons of code relying on fp determinism.
Care must also be taken not to risk using the x87 80-bit registers differently, but these days this is much easier - just don’t use x87 at all and use SSE instead.
I write software that does finite element analysis and similar things and results are reproducible across machines and across versions of the software.
I’m surprised you say it’s common as I haven’t yet bumped into the problem and I have a large suite of complex FP tests (engineering calculations) that have reproduced to the last digit over 15 years and dozens of different test machines of all flavors.
Results calculated 2004 on P4’s still work out to the same results after millions of calculations on every cpu since then and dozens of versions of the software.
Edit: SSE actually do transcendentals at all so it’s possible I’m saved by stable software defined transcendentals in .net (effectively then the Windows x64 implementations)
My statement was about non-SSE scalar floating point operations. Using different optimizations ('Debug'/-O0 versus 'Release'/-O3 mode) will possibly produce different results, unless you are very careful. Using a different compiler (gcc versus clang versus visual studio) will likely produce different results.
If you want results to be reproducible across different CPU architectures (x86, arm64 etc) one option is using fixed-point arithmetic.
> Using different optimizations ('Debug'/-O0 versus 'Release'/-O3 mode) will possibly produce different results, unless you are very careful.
Yes. But .NET I think is much more predictable in that case as I don’t observe differences from optimization either. Having a spec and a decent memory model and no undefined behavior makes the compiler worse at optimizing things but better at consistency I guess. If the language spec strictly defines what math operators do and prevents reordering and similar, then there isn’t much outside transcendental functions that can go wrong. On 32bit .NET this did go wrong because whether or not something was in an 80bit x87 register or spilled to a 64 bit memory value seemed to depend on the moon phase. Those were bad times.
> If you want results to be reproducible across different CPU architectures (x86, arm64 etc) one option is using fixed-point arithmetic.
Luckily I never had that need. .NET (C#), x86-64, Windows. In that target, things look extremely stable now.
Step outside into 32bit or non-windows or C/C++ or even non x86 then all bets are off obviously.
You can absolutely have determinism across different architectures, if you stick with architectures that support IEEE 754-2008. Also if a compiler break determinism this is a serious bug (as long as you don't enable -ffast-math of course - otherwise you're asking for breakage).
Unfortunately this means no SIMD, also you need to bring your own transcendental functions (sin, cos, etc). Also you need to be wary of other forms of nondeterminism like threads.
There's a physics engine meant for games and robotics called Rapier [0] that has a feature enhanced-determinism that will enable cross-platform determinism (by disabling threads and simd). If you spot any form of nondeterminism, it's absolutely a bug (either on the compiler or on the library itself), just like it would be a bug if it used only integers (which, by the way, was how nphysics, the antecessor of Rapier, implemented determinism: it used fixed-point math with integers [1])
[0] https://rapier.rs/docs/user_guides/rust/determinism/ - also it has amazing docs
[1] https://rustsim.org/blog/2020/06/01/this-month-in-rustsim/#m...