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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. Filip Jerzy Pizło‏ @filpizlo 15 Apr 2019
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      Replying to @vyodaiken @johnregehr

      Didn't know about it... btw for the purpose of creating a compiler with well-defined behavior, I would start with clang+llvm and then just refactor the UB out of llvm.

      0 replies 0 retweets 1 like
    2. Filip Jerzy Pizło‏ @filpizlo 15 Apr 2019
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      Replying to @vyodaiken @johnregehr

      Nah! There are two kinds of UB: UB in the IR that the compiler uses and UB in the language. LLVM has the former. That doesn't imply UB in the latter. It happens to imply that in llvm and gcc because of policy decisions.

      1 reply 0 retweets 2 likes
    3. Filip Jerzy Pizło‏ @filpizlo 15 Apr 2019
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      Replying to @filpizlo @vyodaiken @johnregehr

      For example: introduce a pass that runs before llvm opt pipeline that removes all TBAA, changes all geps to int math, remove all nsw/nuw flags from int math, replace all undef's with 0, and 0-initialize all alloca's. That gets you very close to no UB.

      1 reply 0 retweets 1 like
    4. John Regehr‏ @johnregehr 15 Apr 2019
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      Replying to @filpizlo @vyodaiken

      not really but it would reduce the amount of UB exploitation. there's a long tail scattered around many passes that you can't find so easily, requires a fine-toothed comb.

      1 reply 0 retweets 8 likes
    5. Filip Jerzy Pizło‏ @filpizlo 15 Apr 2019
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      Replying to @johnregehr @vyodaiken

      Not sure that’s really true. WebKit’s LLVM-based FTL JIT encountered no such problems to my knowledge. High probability we would have known. We even ran tests with the full -O3 pipeline. Maybe there are bugs, but I wouldn’t conflate that with UB.

      2 replies 0 retweets 0 likes
      Patrick Walton‏ @pcwalton 15 Apr 2019
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      Replying to @filpizlo @johnregehr @vyodaiken

      To name one off the top of my head: You have to do something to sanitize float-to-int casts or else they become undefs if out of range. For a long time “array[x as usize]” with x: f64 could cause UB in safe Rust for this reason.

      7:51 PM - 15 Apr 2019
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      • You and 52 others Brian who survived a powerline Steve Canon iximeow John Regehr Perry Lorier
      2 replies 1 retweet 6 likes
        1. New conversation
        2. Joe Groff‏ @jckarter 15 Apr 2019
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          Replying to @pcwalton @filpizlo and

          There's also the perpetually annoying 'a function calling itself in an infinite loop is UB'

          3 replies 0 retweets 4 likes
        3. Filip Jerzy Pizło‏ @filpizlo 15 Apr 2019
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          Replying to @jckarter @pcwalton and

          I feel like y’all are listing things that can be fixed through the vigorous application of software engineering.

          1 reply 0 retweets 1 like
        4. 6 more replies
        1. Filip Jerzy Pizło‏ @filpizlo 15 Apr 2019
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          Replying to @pcwalton @johnregehr @vyodaiken

          So add branches around that. No big deal.

          0 replies 0 retweets 0 likes
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