I came across 
. Previously, when microbenchmarking Rust code that used count_ones()
in an inner loop (can't recall what code this was), I noticed 4x
runtime speed when compiling for target_cpu=nehalem and running on a
much later CPU.

I'm wondering:

Have we done benchmark comparisons with libxul compiled for the
newly-defined x86_64 levels?

How feasible would it be, considering CI cost, to compile for multiple
x86_64 levels and make the Windows installer / updater pick the right
one and to use the new glibc-hwcaps mechanism on Linux?

For macOS and Android, do we actively track the baseline CPU age that
Firefox-compatible OS versions run on and adjust the compiler options
accordingly when we drop compatibility for older OS versions?

Henri Sivonen
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