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BRIAN_____'s profile
Brian Smith
Brian Smith
Brian Smith
@BRIAN_____

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Brian Smith

@BRIAN_____

Code farmer. Security, crypto, performance, networking, usability. Rust, C++, C, Haskell, DSLs, etc. *ring*, webpki, crypto-bench, mozilla::pkix.

Honolulu & San Francisco
briansmith.org
Joined April 2008

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    Brian Smith‏ @BRIAN_____ May 21

    OK, the CPU does all that crazy stuff but we're still assuming it doesn't cheat at CMOV?

    2:57 PM - 21 May 2018
    • 2 Retweets
    • 13 Likes
    • Rustacean::from("") Tony Arcieri Geoffrey Sneddon 🏳️‍🌈 Peter J Jethro Beekman Pratyush Mishra Geoffroy Couprie Jack Lloyd Jed Davis 🏳️‍🌈
    3 replies 2 retweets 13 likes
      1. New conversation
      2. Vlad Krasnov‏ @thecomp1ler May 21
        Replying to @BRIAN_____

        It is hard to imagine how it would cheat. The move itself is essentially free either way, and the condition has to be resolved first.

        2 replies 0 retweets 0 likes
      3. Chandler Carruth‏ @chandlerc1024 May 22
        Replying to @thecomp1ler @BRIAN_____

        Super easy to cheat: is the branch predictor to predict the direction and try to speculate further. But we specifically asked about this and they confirm in their white paper that this works on today's processors.

        1 reply 0 retweets 1 like
      4. Vlad Krasnov‏ @thecomp1ler May 22
        Replying to @chandlerc1024 @BRIAN_____

        Possible, but what if it mispredicts? That would be a poor optimization. Also would need to have several CMOV ops with same condition to be worthwhile, and that is beyond the current uop-fusion Intel can do.

        1 reply 0 retweets 1 like
      5. Chandler Carruth‏ @chandlerc1024 May 22
        Replying to @thecomp1ler @BRIAN_____

        It mispredicting would be just as bad as the branch predictor mispredicting? And handled the same way? Predictors today have *extremely* high hit rates. It shouldn't matter how many cmov operations you have, it matters how much speculative execution predicting it unlocks.

        2 replies 0 retweets 0 likes
      6. Brian Smith‏ @BRIAN_____ May 22
        Replying to @chandlerc1024 @thecomp1ler

        It probably wouldn't make sense for Intel to use the branch predictor for CMOV itself b/c (in the past) they advised people to only use it for things that branch prediction is bad at. However I could see generalized dead store detection logic for instructions afterward cheating.

        1 reply 0 retweets 1 like
      7. Chandler Carruth‏ @chandlerc1024 May 22
        Replying to @BRIAN_____ @thecomp1ler

        Not *the* branch predictor, but *a* predictor similar to a branch predictor.

        0 replies 0 retweets 1 like
      8. End of conversation
      1. New conversation
      2. Chandler Carruth‏ @chandlerc1024 May 22
        Replying to @BRIAN_____

        They've confirmed that it doesn't cheat for the purpose of Spectre mitigation in their white paper for existing processors. They reserved the right to change that in the future, but IMO, they don't have that luxury. Ship sailed, will need PCMOV or something else to change.

        1 reply 0 retweets 2 likes
      3. Chandler Carruth‏ @chandlerc1024 May 22
        Replying to @chandlerc1024 @BRIAN_____

        And I've been very clear to Intel about my perspective here.

        1 reply 0 retweets 1 like
      4. Chandler Carruth‏ @chandlerc1024 May 22
        Replying to @chandlerc1024 @BRIAN_____

        We've also tested cmov pretty hard to verify that it works against Spectre. This isn't a proof, but strong evidence.

        0 replies 0 retweets 3 likes
      5. End of conversation
      1. Jarred Nicholls‏ @jarrednicholls May 21
        Replying to @BRIAN_____

        Your tweets as of late have made me think I’m coming into the middle of a conversation.

        0 replies 0 retweets 0 likes
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