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[SelectionDAG][RFC] Allow the user to specify a memeq function (v5).
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Authored by courbet on Fri, Jan 11, 4:45 AM.
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Right now, when we encounter a string equality check,
e.g. if (memcmp(a, b, s) == 0), we try to expand to a comparison if s is a
small compile-time constant, and fall back on calling memcmp() else.

This is sub-optimal because memcmp has to compute much more than

This patch replaces memcmp(a, b, s) == 0 by bcmp(a, b, s) == 0 on platforms
that support bcmp.

bcmp can be made much more efficient than memcmp because equality
compare is trivially parallel while lexicographic ordering has a chain

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