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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. John Regehr‏ @johnregehr 1 Nov 2017

      anyone know offhand whether C99 or C11 explicitly justifies LLVM's optimization?

      8 replies 0 retweets 0 likes
      Show this thread
    2.  🎃 unsafe { mem::transmute(@erincandescent) }  🎃‏ @erincandescent 1 Nov 2017
      Replying to @johnregehr

      malloc is defined to return a pointer which does not alias any pre-existing object

      4 replies 0 retweets 1 like
    3. Rich Felker‏ @RichFelker 1 Nov 2017
      Replying to @erincandescent @oshepherd @johnregehr

      If nothing else, malloc returns a pointer to memory suitably aligned for any type, and 0xdeadbeef most definitely is not.

      4 replies 1 retweet 3 likes
    4. John Regehr‏ @johnregehr 1 Nov 2017
      Replying to @RichFelker @oshepherd

      good point but of course neither compiler cares about the actual value...

      1 reply 0 retweets 0 likes
    5. Rich Felker‏ @RichFelker 1 Nov 2017
      Replying to @johnregehr @oshepherd

      Indeed. At first it seems reasonable that a ptr converted from int could only meaningfully point to fixed hw or an obj whose addr leaked...

      1 reply 0 retweets 0 likes
    6. Rich Felker‏ @RichFelker 1 Nov 2017
      Replying to @RichFelker @johnregehr @oshepherd

      However pigeonhole principle makes things get nasty.

      2 replies 0 retweets 0 likes
    7. John Regehr‏ @johnregehr 1 Nov 2017
      Replying to @RichFelker @oshepherd

      yes an LLVM bug is filed about the case where you make a pointer (on a 32-bit machine) and try to guess it in a loop and it thinks you can't

      1 reply 0 retweets 0 likes
    8. Rich Felker‏ @RichFelker 1 Nov 2017
      Replying to @johnregehr @oshepherd

      If your guess involves any accesses, I think the bug is invalid; the compiler can infer UB from the access.

      2 replies 0 retweets 0 likes
    9. John Regehr‏ @johnregehr 1 Nov 2017
      Replying to @RichFelker @oshepherd

      https://bugs.llvm.org/show_bug.cgi?id=35102 …

      1 reply 0 retweets 0 likes
    10. Rich Felker‏ @RichFelker 1 Nov 2017
      Replying to @johnregehr @oshepherd

      Does it still happen if &i == (int*)cur is changed to (uintptr_t)i == cur?

      1 reply 0 retweets 0 likes
      Rich Felker‏ @RichFelker 1 Nov 2017
      Replying to @RichFelker @johnregehr @oshepherd

      There may be an argument that using an invalid pointer as an operand to == invokes UB, but ops on uintptr_t are all 100% well-defined.

      12:19 PM - 1 Nov 2017
      1 reply 0 retweets 0 likes
        1. New conversation
        2. John Regehr‏ @johnregehr 1 Nov 2017
          Replying to @RichFelker @oshepherd

          neither compiler cares about that distinction:https://godbolt.org/g/rJAetU 

          1 reply 0 retweets 1 like
        3. Rich Felker‏ @RichFelker 1 Nov 2017
          Replying to @johnregehr @oshepherd

          In that case they're probably both doing LOTS of wrong optimizations of stuff involving uintptr_t...

          2 replies 0 retweets 0 likes
        4. John Regehr‏ @johnregehr 1 Nov 2017
          Replying to @RichFelker @oshepherd

          yes we have test cases that both GCC and LLVM break

          0 replies 0 retweets 1 like
        5. End of conversation

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