-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 27/09/12 07:59, Vincent Diepeveen wrote:
> Linus posted something like: "there is no excuse to not generate > CMOV instructions now that also intel core2 can execute them > fast". No, what he actually said was: http://ondioline.org/mail/cmov-a-bad-idea-on-out-of-order-cpus # - on a P4, branch mispredictions are expensive, but so is cmov, # so all the above is to some degree exaggerated. On nicer # microarchitectures (the Intel Core 2 in particular is something # I have to say is very nice indeed), the difference will be a lot # less noticeable. The loss from cmov isn't very big (it's not as # sucky as P4), but neither is the win (branch misprediction isn't # that expensive either). [...] [example showing CMOV version slower than non-CMOV on Core2 elided] [...] # So don't use cmov. Except for non-performance-critical code, or # if you really care about code-size, and it helps (which is actually # fairly rare: quite often cmov isn't even smaller than a conditional # jump and a regular move, partly because a regular move can take # arguments that a cmov cannot: move to memory, move from an immediate # etc etc, so depending on what you're moving, cmov simply isn't good # even if it's _just_ a move). cheers, Chris - -- Christopher Samuel Senior Systems Administrator VLSCI - Victorian Life Sciences Computation Initiative Email: [email protected] Phone: +61 (0)3 903 55545 http://www.vlsci.org.au/ http://twitter.com/vlsci -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.11 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://www.enigmail.net/ iEYEARECAAYFAlBjoucACgkQO2KABBYQAh8YDwCdHFfOo/V3FvUO6JVUvz7iAHrD ap8AnR6tuNJpStCWn9X+AGGVg4GdFkU5 =S7jk -----END PGP SIGNATURE----- _______________________________________________ Beowulf mailing list, [email protected] sponsored by Penguin Computing To change your subscription (digest mode or unsubscribe) visit http://www.beowulf.org/mailman/listinfo/beowulf
