Oh I see, I overlooked the -C option and and -Chelp. Thank you for these kind answers instead of RTFM.
This looks a little misleading:
Use +feature to enable a feature, or -feature to disable it.
For example, llc -mcpu=mycpu -mattr=+feature1,-feature2
But I guess it’s hard to fix since it comes from https://github.com/llvm/llvm-project/blob/release/17.x/llvm/lib/MC/MCSubtargetInfo.cpp#L123, right?
In any case, as expected I do not see any improvement compared to launching Julia without -Cnative,-fast-gather. I will wait for LLVM 17 and rerun the benchmark then.