Are Vivado results repeatable for identical tool inputs?
For the most part the answer is yes, Vivado should generate identical results between runs involving identical:
This applies to all parts of the design flow from HDL synthesis through bitstream generation. For example a build script should generate identical results when run repeatedly on the same system or when run on a similarly configured machine.
Results between different Operating Systems are generally repeatable but not guaranteed, particularly for the case of Linux vs. Windows.
Note about determinism
The terms repeatable and deterministic are often used interchangeably which is incorrect. Vivado uses multi-threading for parallel execution which by definition makes it non-deterministic. However multiple threads within Vivado are programmed to be synchronized to make multi-threaded execution repeatable. Note that Vivado manages other factors besides multi-threading that cause non-determinism such as OS system calls and memory management.
Identifying repeatability issues
The most common symptoms of divergent runs are different post-route timing results. If you suspect a case of non-repeatability and have verified that tool inputs are identical, you can further verify divergence using checksums. Vivado reports a checksum in the log at each intermediate stage of each implementation command, a signature based on the design netlist and physical data. Checksums can be compared between different runs and checksum mismatches help identify where results diverge.
Example: The checksums below begin to diverge and thereafter never converge. If all inputs are identical then this is likely a repeatability issue.
Checkpoints vs. in-memory runs
Checkpoints should produce repeatable results compared to the equivalent in-memory design flow. Consider the following command sequences, one running the entire design flow in memory, and the other re-entering the flow at phys_opt_design using the placed checkpoint from the in-memory flow.
These two runs should give identical results. Although checksums might differ due to netlist sorting differences after the open_checkpoint command, checksums should converge after the first implementation command which is phys_opt_design:
Each implementation command automatically sorts the netlist before running to ensure netlist consistency, so that results do not diverge due to netlist differences.
If repeatability is absolutely critical to your design environment, the following can help maximize repeatability:
Running in single-threaded mode.
When using different number of CPUs between runs, simultaneous threads might execute operations in different orders and cause diverging results, even when run using the same machine and OS.
To run in single-threaded mode, use the following:
set_param general.maxThreads 1
This disables multithreading and eliminates associated repeatability issues.
Using the route_design -ultrathreads argument
The -ultrathreads argument was added as a way to reduce Router run time.
This reduced runtime comes at the expense of repeatability, and differences are expected.
Running on the same machine or running on machines using the same OS.
This reduces the chances of encountering non-repeatability due to the way core functions and system calls are implemented from one operating system to another, especially in the case of Windows vs. Linux.
The chances are further reduced by
1) Fine-tuning the environment to eliminate all of the following scenarios:
2) Eliminating differences in computing hardware:
If you encounter non-repeatability with identical inputs as described in the Solution introduction, please open a Service Request and provide a testcase that demonstrates the problem.
Without a testcase it might be difficult to pinpoint and fix any problems but it helps to supply as much information as possible.
Non-repeatability between Linux and Windows is expected to occur occasionally and the recommended solution is to restrict Vivado runs to Linux-only or Windows-only.