I've implemented a minimal test compute shader in order to get a feel for the performance of Metal atomic functions, specifically atomic_fetch_add_explicit and atomic_compare_exchange_weak_explicit using atomic_uint in the device address space.
Running a compute kernel which runs atomic_fetch_add_explicit once, 307200 times, takes around 50 µs on an iPhone 12 Pro. Running atomic_fetch_add_explicit 10 times per thread instead takes about 650 µs, which makes sense. However, I'm confused by the Xcode shader profiler's line-by-line performance metrics:
Blue, red and yellow mean arithmetic, synchronisation and control flow, respectively. According to Apple's documentation, this result means the atomic function calls each take zero percent of the function's total elapsed time:
The statistics for lines in the function body indicate the time as a percent of the function's total elapsed time.
However clearly that's not the case, as my time expenditure is multiplied roughly ten-fold as I go from one to ten calls per thread.
Is this an Xcode bug, or am I missing something here? Answers appreciated, answers with sources doubly appreciated. :)