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Determinism In Engine
The synchronised code in the Recoil Engine is deterministic. Therefore certain considerations must be observed when working in these areas of code.
There are a few programming practices that should be observed in the engine simulation code to help avoid introducing desyncs.
- Use the Recoil provided RNG function.
- Use only stable sorting algorithms i.e. a sorting algorithm that maintains the relative order of the items with equal sort keys.
- Avoid relying on sort of order of strings - different locales can have different sort orders.
- Do not call functions as arguments to other functions - the order of the calls are not guaranteed. Instead, call them before the function and store the return value in a local variable.
This example will risk a desync:
DoSomething(GetRNG(), GetRNG());
Instead do this:
int firstNumber = GetRNG();
int secondNumber = GetRNG();
DoSomething(firstNumber, secondNumber);
Multi-threading parts of the engine may offer the opportunity for better performance; however, there are several situations that can cause the engine to desync. So avoid the following:
- Do not trigger script call-backs during an MT section. Scripts, whether COB, BOS, Lua, or LUS, are not thread-safe. Collect the calls that need to be made and issue them in a proceeding single-threaded section.
- Synced parameters must not be modified in an MT section because they update the sync checksum on update. Store the new value in a separate parameter and apply the new value in a proceeding single-threaded section.
- When a parameter is updated when visited per job, the parameter needs to be expanded in an array with a number of entries equal to
ThreadPool::MAX_THREADS. This allows each thread to tracking visiting the parameter without interfering with other threads' visits by using anindex == thread number. - Pay careful attention to whether a job output can influence an input into another job. For example, don't move units if the jobs also check the unit's position.
- Beware of lists resizing and potentially reordering their elements as a result.
Do not use the reciprocal functions: Intel specified a maximum error guarantee, which allowed them to use make adjustments depending on the needs of each processor family, but that can result in different results at the bit level.
As a result, do not use the following SSE functions:
_mm_rcp_ps()
_mm_rcp_ss()
_mm_rsqrt_ps()
_mm_rsqrt_ss()
The engine uses the strepflop library to setup all FPU-related CPU-bound hardware to ensure that regardless of whether a compiler has used SSE or FPU, general floating point calculations will conform to IEEE754. This is important because most third party libraries are pre-compiled to use the FPU rather than SSE.
The engine also checks multiple times in each sim frame that the FPU options are correct.