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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 @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
    2. 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
    3. 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
    4. 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.

      2 replies 1 retweet 6 likes
    5. 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
    6. 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
    7. Joe Groff‏ @jckarter 15 Apr 2019
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      Replying to @filpizlo @pcwalton and

      I think John's point is that there isn't a well defined list of these things for LLVM, so you end up having to shoot them as you find/are made aware of them.

      4 replies 0 retweets 2 likes
    8. Joe Groff‏ @jckarter 15 Apr 2019
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      Replying to @jckarter @filpizlo @pcwalton

      I bring up the recursive call thing because, yeah you can try to pattern-match and guard against that one in obvious cases, but it reaches Halting Problem territory when it can creep up through other optimizations

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

      That one is best fixed in llvm. (Context for this thread: my claim that it’s better to start with clang/llvm and refactor to make a no-UB compiler than to start from scratch.)

      1 reply 0 retweets 1 like
    10. Joe Groff‏ @jckarter 15 Apr 2019
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      Replying to @filpizlo @pcwalton

      huuuge +1 to that

      1 reply 0 retweets 0 likes
      Patrick Walton‏ @pcwalton 15 Apr 2019
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      Replying to @jckarter @filpizlo

      Well, it depends on how much you want LLVM’s optimizations. Cranelift is mostly an alternative nowadays if you want reasonable code with fast compile speed.

      8:27 PM - 15 Apr 2019
      • 3 Likes
      • rare liquids Mark Malström Joe Groff
      3 replies 0 retweets 3 likes
        1. Patrick Walton‏ @pcwalton 15 Apr 2019
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          Replying to @pcwalton @jckarter @filpizlo

          It’s pretty much LLVM or GCC if you want really fast code though.

          0 replies 0 retweets 0 likes
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        2. Joe Groff‏ @jckarter 15 Apr 2019
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          Replying to @pcwalton @filpizlo

          LLVM has managed to make concessions in its UB exploitation in the past; it doesn’t fall over into the next function when you end in unreachable anymore after all

          2 replies 0 retweets 3 likes
        3. Joe Groff‏ @jckarter 15 Apr 2019
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          Replying to @jckarter @pcwalton @filpizlo

          For things like recursion without side effects, seems like you could allow code motion and other passes to ignore nontermination while still not letting the recursion evaporate away to undef but leave a trap behind, without hurting code quality too much

          1 reply 0 retweets 0 likes
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        1. Filip Jerzy Pizło‏ @filpizlo 16 Apr 2019
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          Replying to @pcwalton @jckarter

          Or you could use a real compiler like B3.

          0 replies 0 retweets 0 likes
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