Division isn't constant-time on Intel chips so the code was adding a
large multiple of md_size to try and force the operation to always take
the maximum amount of time.
I'm less convinced, these days, that compilers aren't going to get smart
enough to optimise that away so use Barrett reduction instead.
Change-Id: Ib8c514192682a2fcb4b1fb7e7c6dd1301d9888d0
Reviewed-on: https://boringssl-review.googlesource.com/6906
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <alangley@gmail.com>
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