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RichFelker's profile
Rich Felker
Rich Felker
Rich Felker
@RichFelker

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Rich Felker

@RichFelker

Yeah, I do @musllibc, FOSS & infosec stuff. But now is not the time for a mostly-/only-tech Twitter feed.

musl-libc.org
Joined March 2014

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    1. friend void‏ @volatile_void 13 May 2016
      Replying to @ch3root @RichFelker

      I made the mistake, going in the other direction—tis-interpreter to GCC—of forgetting GCC doesn't have a duty to warn.

      3 replies 0 retweets 0 likes
    2. friend void‏ @volatile_void 13 May 2016
      Replying to @volatile_void @ch3root

      I agree w/@RichFelker GCC isn't doing wrong: it assumes malloc can only return 0 for this input, so the program always runs into UB

      1 reply 0 retweets 0 likes
    3. Alexander Cherepanov‏ @ch3root 13 May 2016
      Replying to @volatile_void @RichFelker

      No. Just replace SIZE_MAX by SIZE_MAX / 4 * 3 and add -m32 and you get a genuine compiler bug.

      2 replies 0 retweets 0 likes
    4. friend void‏ @volatile_void 13 May 2016
      Replying to @ch3root

      well 32-bit GCC still assumes that malloc has to return 0 for this input.

      2 replies 0 retweets 0 likes
    5. Alexander Cherepanov‏ @ch3root 13 May 2016
      Replying to @volatile_void

      Do you mean it as a philosophical POV or that gcc will optimize "if (p)" away based on this assumption?

      2 replies 0 retweets 0 likes
    6. friend void‏ @volatile_void 13 May 2016
      Replying to @ch3root

      I meant it philosophically, but now I have investigated and ugh https://godbolt.org/g/piu9ka  cc @RichFelker

      1 reply 0 retweets 0 likes
    7. friend void‏ @volatile_void 13 May 2016
      Replying to @volatile_void @ch3root @RichFelker

      I am pretty sure it's possible to merge these to programs into one that shows GCC is as wrong as Clang on this.

      1 reply 0 retweets 0 likes
    8. friend void‏ @volatile_void 13 May 2016
      Replying to @volatile_void @ch3root @RichFelker

      oops that was still Clang being wrong in that link, no sign of GCC being inconsistent so far.

      1 reply 0 retweets 0 likes
    9. Rich Felker‏ @RichFelker 13 May 2016
      Replying to @volatile_void @ch3root

      Indeed. The invalid optimization clang is making is assuming malloc doesn't fail. Unless GCC also does that, GCC is ok.

      1 reply 0 retweets 1 like
    10. Alexander Cherepanov‏ @ch3root 14 May 2016
      Replying to @RichFelker

      Why do you think this is an invalid optimization? Compilers routinely mess with libc functions, e.g. inline memcpy.

      2 replies 0 retweets 0 likes
      Rich Felker‏ @RichFelker 14 May 2016
      Replying to @ch3root

      Because the behavior is changed. A call which necessarily must fail falsely succeeds.

      8:35 AM - 14 May 2016
      2 replies 0 retweets 0 likes
        1. New conversation
        2. Alexander Cherepanov‏ @ch3root 14 May 2016
          Replying to @RichFelker

          The change of behavior is exactly the reason for optimizations.

          2 replies 0 retweets 1 like
        3. Rich Felker‏ @RichFelker 14 May 2016
          Replying to @ch3root

          All opt takes place by "as if" rule. Optimizations that violate that are not valid. Changing observable behavior is a bug

          2 replies 0 retweets 0 likes
        4. Alexander Cherepanov‏ @ch3root 14 May 2016
          Replying to @RichFelker

          Sure. Everything boils down to the question what "observable behavior" is.

          1 reply 0 retweets 0 likes
        5. Alexander Cherepanov‏ @ch3root 14 May 2016
          Replying to @ch3root @RichFelker

          The amount of used memory is not part of observable behavior. In the same way as the execution time is not.

          3 replies 0 retweets 0 likes
        6. friend void‏ @volatile_void 14 May 2016
          Replying to @ch3root

          friend void Retweeted friend void

          I submit this example to youhttps://twitter.com/spun_off/status/731563481007325187 …

          friend void added,

          friend void @volatile_void
          Replying to @jfbastien
          What UB? This program is defined and prints SIZE_MAX / sizeof(short) according to the C11 standard. https://godbolt.org/g/Qs36E0 
          1 reply 0 retweets 0 likes
        7. Alexander Cherepanov‏ @ch3root 14 May 2016
          Replying to @volatile_void

          There are two separate questions here. 1) Objects larger than PTRDIFF_MAX. That's what you discuss mostly. Complex question.

          1 reply 0 retweets 0 likes
        8. Alexander Cherepanov‏ @ch3root 14 May 2016
          Replying to @ch3root

          2) Optimizations of malloc <= PTRDIFF_MAX. AIUI @RichFelker's POV is that this opt is wrong no matter what the size. I disagree.

          1 reply 0 retweets 0 likes
        9. friend void‏ @volatile_void 14 May 2016
          Replying to @ch3root

          I have not seen Rich say that (though I did see you and JF Bastien argue as if he had)

          1 reply 0 retweets 0 likes
        10. 1 more reply
        1. New conversation
        2. Rich Felker‏ @RichFelker 14 May 2016
          Replying to @RichFelker @ch3root

          Aside from SIZE_MAX/2 there are all sorts of other reasons it might have to fail, like rlimits.

          1 reply 0 retweets 0 likes
        3. Alexander Cherepanov‏ @ch3root 14 May 2016
          Replying to @RichFelker

          Right, that was my next question:-) You are saying that no malloc could be optimized away bc it changes mem footprint?

          1 reply 0 retweets 0 likes
        4. Alexander Cherepanov‏ @ch3root 14 May 2016
          Replying to @ch3root @RichFelker

          What about stack? Suppose I have a tail recursion which (unoptimized) overflows the stack. Can it be optimized?

          0 replies 0 retweets 0 likes
        5. End of conversation

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