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pcwalton's profile
Patrick Walton
Patrick Walton
Patrick Walton
@pcwalton

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Patrick Walton

@pcwalton

Research engineer at Mozilla

San Francisco, CA
pcwalton.github.io
Joined November 2009

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    1. C++ Delenda Est‏ @CodaFi_ 4 Nov 2019
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      Replying to @cutculus

      I think this is LLVM’s doing. LoopOpts are still pretty aggressive about assuming infinite loops make forward progress.

      1 reply 0 retweets 1 like
    2. Varun Gandhi‏ @cutculus 4 Nov 2019
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      Replying to @CodaFi_

      Sure, but I would've guessed they'd selectively disable optimizations which lead to such UB. It is quite harmless in practice though 🤷‍♂️

      2 replies 0 retweets 0 likes
    3. Joe Groff‏ @jckarter 4 Nov 2019
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      Replying to @cutculus @CodaFi_

      It's not that easy. You can't predict where infinite loops will show up in the face of the halting problem. LLVM needs to be fixed not to exploit this UB when it sees it. Even for C and C++, they disagree whether this is language-level UB

      3 replies 0 retweets 4 likes
    4. John Regehr‏ @johnregehr 4 Nov 2019
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      Replying to @jckarter @cutculus @CodaFi_

      I'm often curious what's the subset of programs that gets faster via this UB and how much of that gain can be gotten back by beefing up some loop analyses a bit

      1 reply 0 retweets 4 likes
    5. John Regehr‏ @johnregehr 4 Nov 2019
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      Replying to @johnregehr @jckarter and

      anyway this behavior as the LLVM-level default obviously needs to die, it's a crappy thing to have inherited from being a dedicated C/C++ IR

      1 reply 0 retweets 3 likes
    6. C++ Delenda Est‏ @CodaFi_ 4 Nov 2019
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      Replying to @johnregehr @jckarter @cutculus

      It doesn’t even make sense from that legacy’s standpoint. The standards are pretty clear about what forward progress means, so the frontends can mark infinite loops themselves.

      1 reply 0 retweets 1 like
    7. John Regehr‏ @johnregehr 4 Nov 2019
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      Replying to @CodaFi_ @jckarter @cutculus

      the issue isn't infinite loops but potentially infinite loops. a frontend, lacking deep static analysis, would need to mark basically every loop as potentially infinite.

      2 replies 0 retweets 2 likes
    8. Joe Groff‏ @jckarter 4 Nov 2019
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      Replying to @johnregehr @CodaFi_ @cutculus

      Would you get most of the benefit by saying that infinite loops without side effects may only trap or run forever? Seems like you could let a compiler optimize down to traps without letting it go all the way to UB

      1 reply 0 retweets 0 likes
    9. John Regehr‏ @johnregehr 4 Nov 2019
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      Replying to @jckarter @CodaFi_ @cutculus

      this is the best explanation I know of: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1528.htm … I don't think traps help us here. it really just comes down to who gets hurt by switching the default, and if we care about that.

      2 replies 0 retweets 1 like
    10. John Regehr‏ @johnregehr 4 Nov 2019
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      Replying to @johnregehr @jckarter and

      my opinion is that right now, today we switch the default and hide a few optimiziations behind a "provably terminates " attribute and anyone who has a problem with this gets to write loop analyses that add this attribute

      1 reply 0 retweets 2 likes
      Patrick Walton‏ @pcwalton 4 Nov 2019
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      Replying to @johnregehr @jckarter and

      Don’t we have this discussion like every year and the consensus is “yeah, let’s fix this behavior in LLVM” and then nobody does

      5:55 PM - 4 Nov 2019
      • 1 Retweet
      • 14 Likes
      • ryan montgomery MBL Bruce isiah meadows 🧢 Rhaladriel Manoj Ayyanawar ❄ NoChristInChristmasDaniel 🎄 BrendanEich Paul Crowley stepancheg
      2 replies 1 retweet 14 likes
        1. New conversation
        2. Joe Groff‏ @jckarter 4 Nov 2019
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          Replying to @pcwalton @johnregehr and

          “Let’s fix the bug in LLVM…by rewriting everything to use Cranelift”

          2 replies 0 retweets 8 likes
        3. Patrick Walton‏ @pcwalton 4 Nov 2019
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          Replying to @jckarter @johnregehr and

          I’m still a bit unhappy about how our noalias bug fix patch got nitpicked by Daniel Berlin and then stalled out. :\

          1 reply 0 retweets 3 likes
        4. 1 more reply
        1. John Regehr‏ @johnregehr 4 Nov 2019
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          Replying to @pcwalton @jckarter and

          I am merely shouting at passersby from the ivory tower

          0 replies 1 retweet 14 likes
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