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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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    1. Brian Smith‏ @BRIAN_____ May 21

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

      3 replies 2 retweets 13 likes
    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. Vlad Krasnov‏ @thecomp1ler May 22
      Replying to @chandlerc1024 @BRIAN_____

      In cryptography it would in theory be unpredictable.

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

      For crypto code, yes. But despite the common use of cmov in crypto code there are *tons* of cmovs generated by compilers all over the place in "normal" code that would likely be valuable to predict in many cases. Again, I'm just trying to outline a hypothetical reason for this.

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

      More simply, once you have branch prediction and if you can implement instructions in terms of other instructions, it's simple to implement cmov as a test + conditional jump, and it's more work to make a non-predicting constant-time cmov.

      11:37 AM - 22 May 2018
      2 replies 0 retweets 0 likes
        1. Vlad Krasnov‏ @thecomp1ler May 22
          Replying to @BRIAN_____ @chandlerc1024

          I am afraid that argument valid for and/or with masks as well. And with -1 can be implemented as test + jmp, same as or with 0.

          0 replies 0 retweets 0 likes
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        1. JF Bastien‏ @jfbastien May 22
          Replying to @BRIAN_____ @chandlerc1024 @thecomp1ler

          Say Intel wanted to pull a Transmeta… CMOV would be special indeed.

          0 replies 0 retweets 0 likes
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