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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. Wojciech Muła‏ @pshufb Mar 21

      GCC: is there a way to tell a compiler what range of number I expect? For instance there's a uint8_t in range [2, 17]. I expect the compiler will optimize code for that range and consider out-of-range values as undefined. #cpp

      5 replies 0 retweets 0 likes
      Show this thread
      Rich Felker‏ @RichFelker Mar 21
      Replying to @pshufb

      if (x<2 || x>17) 0/0;

      9:50 PM - 21 Mar 2018
      • 3 Likes
      • Wojciech Muła Mark Kriegsman reabstraction thunk golem
      4 replies 0 retweets 3 likes
        1. New conversation
        2. reabstraction thunk golem‏ @jckarter Mar 21
          Replying to @RichFelker @pshufb

          Why not shorten it to 0/(x >= 2 && x <= 17);

          1 reply 0 retweets 0 likes
        3. Rich Felker‏ @RichFelker Mar 21
          Replying to @jckarter @pshufb

          Because I like the idiom for "undefined" that you can also drop in case labels, etc.

          0 replies 0 retweets 1 like
        4. End of conversation
        1. New conversation
        2. John Regehr‏ @johnregehr Mar 21
          Replying to @RichFelker @pshufb

          this is @volatile_void's preferred idiom!

          2 replies 0 retweets 1 like
        3. Rich Felker‏ @RichFelker Mar 21
          Replying to @johnregehr @pshufb @volatile_void

          Specifically the 0/0? Or just the [insert UB here] aspect?

          1 reply 0 retweets 0 likes
        4. John Regehr‏ @johnregehr Mar 21
          Replying to @RichFelker @pshufb @volatile_void

          well he may not have used that one exactly, I don't remember

          0 replies 0 retweets 0 likes
        5. End of conversation
        1. New conversation
        2. CopperheadOS‏ @CopperheadOS Mar 21
          Replying to @RichFelker @pshufb

          If you do it like that, the compiler will almost always optimize it out before it can be used for optimization. GCC has special rules to preserve branches to __builtin_unreachable longer than usual. Clang has __builtin_assume to tell the compiler not to remove it right away.

          1 reply 0 retweets 1 like
        3. CopperheadOS‏ @CopperheadOS Mar 21
          Replying to @CopperheadOS @RichFelker @pshufb

          You can slow down code by using __builtin_assume (or the GCC idiom) because it ends up preserving the code through the initial optimizations to avoid invalidating the guarantees it gets from it. Preserving it can hinder other optimizations even though it gets removed at the end.

          1 reply 0 retweets 1 like
        4. John Regehr‏ @johnregehr Mar 21
          Replying to @CopperheadOS @RichFelker @pshufb

          SUPER tricky to nail the phase ordering here...

          1 reply 0 retweets 1 like
        5. CopperheadOS‏ @CopperheadOS Mar 21
          Replying to @johnregehr @RichFelker @pshufb

          https://llvm.org/docs/LangRef.html#llvm-assume-intrinsic … https://clang.llvm.org/docs/LanguageExtensions.html#builtin-assume … The same kind of issue happens with the branch metadata from __builtin_expect. Clang lowers it from the intrinsic to branch metadata very early on (the early function passes, before interprocedural optimization) & loses it easily.

          1 reply 0 retweets 2 likes
        6. CopperheadOS‏ @CopperheadOS Mar 21
          Replying to @CopperheadOS @johnregehr and

          Can output a bunch of fancy alias analysis metadata, etc. but the compiler likely isn't going to cripple optimizations to keep it around, so a lot of it will end up invalidated by other transformations before getting to where it could make a big difference.

          1 reply 0 retweets 2 likes
        7. CopperheadOS‏ @CopperheadOS Mar 21
          Replying to @CopperheadOS @johnregehr and

          It's a bit amusing that the warning against using it in only in the LLVM IR reference and not there for Clang __builtin_assume. It can also ridiculously slow down compile time if you put it in something used everywhere. Remember that happening with Rc<T> in Rust.

          0 replies 0 retweets 1 like
        8. End of conversation
        1. New conversation
        2. Siddhesh Poyarekar‏ @siddhesh_p Mar 22
          Replying to @RichFelker @pshufb

          __builtin_unreachable is probably a nicer way than 0/0

          1 reply 0 retweets 1 like
        3. Laurent Bercot‏ @laurentbercot Mar 22
          Replying to @siddhesh_p @RichFelker @pshufb

          But it's not in the C standard and depends on your compiler, whereas 0/0 works everywhere.

          1 reply 0 retweets 0 likes
        4. Siddhesh Poyarekar‏ @siddhesh_p Mar 22
          Replying to @laurentbercot @RichFelker @pshufb

          Well he asked gcc...

          0 replies 0 retweets 0 likes
        5. End of conversation

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