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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. Rich Felker‏ @RichFelker Sep 14

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

      3 replies 0 retweets 4 likes
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    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
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    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
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      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.

      9:47 PM - 14 Sep 2018
      • 1 Retweet
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      • ██████████████████████████████████████████████████ Siddhesh Poyarekar
      3 replies 1 retweet 1 like
        1. 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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        1. New conversation
        2. Peter Barfuss 𒀱‏ @bofh453 Sep 14
          Replying to @RichFelker

          Under what condition would (void*)a - (void*)a not equal (intptr_t)a - (intptr_t)a == 0? NULL is just (void*)0 on any platform in use today (or, ever, I think) so I don't understand how it could pose a problem.

          1 reply 0 retweets 0 likes
        3. Rich Felker‏ @RichFelker Sep 14
          Replying to @bofh453

          Difference of void* pointers is not defined at all. Difference of pointers to complete types is defined only when they both point to elements of the same array. The null pointer does not point to the element of any array, so differences involving it are undefined.

          2 replies 0 retweets 4 likes
        4. Rich Felker‏ @RichFelker Sep 14
          Replying to @RichFelker @bofh453

          The point of this whole line of investigation is to be able to write a-b efficiently in a form that's defined given that one of the following is true: either a and b both point to elements of the same array, or they're both null pointers.

          1 reply 0 retweets 0 likes
        5. Rich Felker‏ @RichFelker Sep 14
          Replying to @RichFelker @bofh453

          Really, failure of C to define a-b when both a and b are null pointers is a defect IMO. The right resolution is probably for someone to write a defect report and get it accepted. @jfbastien? @gustedt?

          2 replies 1 retweet 2 likes
        6. Jens Gustedt‏ @gustedt Sep 14
          Replying to @RichFelker

          It would be a new feature, not a defect. Including this would have to be well argued. Do you know what C++ does, here?

          1 reply 0 retweets 1 like
        7. Rich Felker‏ @RichFelker Sep 14
          Replying to @gustedt

          If all existing implementations work this way (null-null==0) it seems like an oversight that it wasn't specified to begin with. BTW a related issue is that relational operators should be defined on null,null (false for <,>; true for <=,>=).

          1 reply 0 retweets 0 likes
        8. Jens Gustedt‏ @gustedt Sep 14
          Replying to @RichFelker

          I wouldn't be sure that all implementations do this. There are still implementations that have several representations for null pointers. Your idea would put the strain of doing a case analysis on all code that does pointer subtraction.

          1 reply 0 retweets 0 likes
        9. 1 more reply
        1. New conversation
        2. friend void‏ @volatile_void Sep 17
          Replying to @RichFelker

          (ptrdiff_t)((uintptr_t)a - (uintptr_t)b) generates the assembly code that you expect.

          1 reply 0 retweets 2 likes
        3. friend void‏ @volatile_void Sep 17
          Replying to @volatile_void @RichFelker

          This is only convenient for pointers to char types, and it makes assumptions about the representation of pointers, but I think that musl is making these assumptions elsewhere already.

          1 reply 0 retweets 1 like
        4. Rich Felker‏ @RichFelker Sep 17
          Replying to @volatile_void

          Yes, but I like to compartmentalize assumptions like this. This isn't documented well now, but there shouldn't be a reason that stdio can't be reused unmodified on a system with different address space model & pointer representation.

          0 replies 0 retweets 1 like
        5. End of conversation

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