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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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    Rich Felker‏ @RichFelker Sep 14

    Anyone want to guess what gcc generates for a==b?0:a-b ? Any ideas why?

    8:56 PM - 14 Sep 2018
    • 4 Likes
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    3 replies 0 retweets 4 likes
      1. New conversation
      2. Rich Felker‏ @RichFelker Sep 14

        The expression a==b?0:a-b is not stupid, btw. It's essentially the only correct way to write a-b when a and b are pointers and both being null is a possibility you want to allow for.

        3 replies 3 retweets 6 likes
        Show this thread
      3. Rich Felker‏ @RichFelker Sep 14

        I can't get clang to collapse the branch out either.

        1 reply 0 retweets 1 like
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      4. Rich Felker‏ @RichFelker Sep 14

        A similar case, also needed to deal safely with null pointers, gets optimized correctly: writing a<b as a!=b && a<b.

        1 reply 0 retweets 3 likes
        Show this thread
      5. Rich Felker‏ @RichFelker Sep 14

        Even more surprisingly, gcc fails to collapse the branch out of this: ptrdiff_t diff(char *a, char *b) { return a!=b ? a-b : (intptr_t)a-(intptr_t)b; } clang gets it right, though.

        3 replies 1 retweet 1 like
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      6. Rich Felker‏ @RichFelker Sep 14

        Oh, fun. gcc <= 7 gets it right too. Only gcc 8 fails to collapse it. I wonder if this is new logic to prevent wrongly applying "pointer provenance" stuff to pointers cast to integers.

        1 reply 0 retweets 3 likes
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      7. End of conversation
      1. New conversation
      2. John Regehr‏ @johnregehr Sep 14
        Replying to @RichFelker

        it's not that bad is it?https://gcc.godbolt.org/z/j_0a5i 

        1 reply 0 retweets 0 likes
      3. Rich Felker‏ @RichFelker Sep 14
        Replying to @johnregehr

        It is when you expect https://gcc.godbolt.org/z/M2JzTF  and it's in an extreme hot path.

        2 replies 0 retweets 1 like
      4. Michael Foukarakis‏ @mfukar Sep 15
        Replying to @RichFelker @johnregehr

        I may regret asking, but what kind of code has this in a hot path?

        1 reply 0 retweets 1 like
      5. Rich Felker‏ @RichFelker Sep 15
        Replying to @mfukar @johnregehr

        For the related comparison issue, getc, scanf, etc. The subtraction is probably less hot.

        0 replies 0 retweets 0 likes
      6. End of conversation
      1. Harold Aptroot‏ @HaroldAptroot Sep 14
        Replying to @RichFelker

        The overall thing is bad enough, but with that mov edx,0 I feel like GCC is just trolling us.

        0 replies 0 retweets 0 likes
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