VCD tracing is now parallelized using the same thread pool as the model.
We achieve this by breaking the top level trace functions into multiple
top level functions (as many as --threads), and after emitting the time
stamp to the VCD file on the main thread, we execute the tracing
functions in parallel on the same thread pool as the model (which we
pass to the trace file during registration), tracing into a secondary
per thread buffer. The main thread will then stitch (memcpy) the buffers
together into the output file.
This makes the `--trace-threads` option redundant with `--trace`, which
now only affects `--trace-fst`. FST tracing uses the previous offloading
scheme.
This obviously helps a lot in VCD tracing performance, and I have seen
better than Amdahl speedup, namely I get 3.9x on XiangShan 4T (2.7x on
OpenTitan 4T).
The --prof-threads option has been split into two independent options:
1. --prof-exec, for collecting verilator_gantt and other execution
related profiling data, and
2. --prof-pgo, for collecting data needed for PGO
The implementation of execution profiling is extricated from
VlThreadPool and is now a separate class VlExecutionProfiler. This means
--prof-exec can now be used for single-threaded models (though it does
not measure a lot of things just yet). For consistency VerilatedProfiler
is renamed VlPgoProfiler. Both VlExecutionProfiler and VlPgoProfiler are
in verilated_profiler.{h/cpp}, but can be used completely independently.
Also re-worked the execution profile format so it now only emits events
without holding onto any temporaries. This is in preparation for some
future optimizations that would be hindered by the introduction of function
locals via AstText.
Also removed the Barrier event. Clearing the profile buffers is not
notably more expensive as the profiling records are trivially
destructible.
* Add VL_ATTR_NO_SANITIZE_ALIGN macro to disable alignment check of ubsan
* Mark a function VL_ATTR_NO_SANITIZE_ALIGN because the function is intentionally using unaligned access for the sake of performance.
Co-authored-by: Wilson Snyder <wsnyder@wsnyder.org>
Use SIMD intrinsics to render VCD traces.
I have measured 10-40% single threaded performance increase with VCD
tracing on SweRV EH1 and lowRISC Ibex using SSE2 intrinsics to render
the trace. Also helps a tiny bit with FST, but now almost all of the FST
overhead is in the FST library.
I have reworked the tracing routines to use more precisely sized
arguments. The nice thing about this is that the performance without the
intrinsics is pretty much the same as it was before, as we do at most 2x
as much work as necessary, but in exchange there are no data dependent
branches at all.
