Well, Os is certainly smaller. One thing to look out for is that
microbenchmarks for kernels are usually the _worst_ things to test with
That's since a large part of the premise of the -Os speed advantage is
that it is better for icache (usually not an issue for microbenchmarks)
and that it is better for load/startup times (generally not a huge issue
for kernels, since the real startup costs of kernels tend to be entirely
So I suspect -Os tends to be more appropriate for user-mode code, and
especially code with low repeat rates. Possibly the "low repeat rate"
thing ends up being true of certain kernel subsystems too.
Think of it this way: if you win 10% in size, you're likely to map and
load 10% less code pages at run-time. Which is not a big issue for
traditional data-centric loads, but can be a _huge_ deal for things like
GUI programs etc where there is often more code than data.
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