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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 3 Jun 2017
      Replying to @cr88192 @oshepherd and

      No, that's a common misconception. It's mostly sloppiness, along with a small but nonzero amount of willful wrongness.

      1 reply 0 retweets 1 like
    2. Rich Felker‏ @RichFelker 3 Jun 2017
      Replying to @RichFelker @cr88192 and

      If you actually want to express wrapping signed arithmetic (rare that it's wanted) there are easy, zero-cost ways to express it without UB.

      1 reply 0 retweets 1 like
    3. Brendan G Bohannon‏ @cr88192 3 Jun 2017
      Replying to @RichFelker @oshepherd and

      you mean using unsigned and casts?... maybe. I was thinking for example signed ((x<<31)>>31) and similar, (x/(1<<31)) may use a divide, ...

      2 replies 0 retweets 0 likes
    4. Brendan G Bohannon‏ @cr88192 3 Jun 2017
      Replying to @cr88192 @RichFelker and

      partial nvm: '/' wouldn't work here, would need ex: ((x>=0)?0:-1) or similar, but similar issue. saner IMO to just make all this be defined.

      1 reply 0 retweets 0 likes
    5. Rich Felker‏ @RichFelker 3 Jun 2017
      Replying to @cr88192 @oshepherd and

      Use unsigned types whenever you're doing bitwise arithmetic. Then it's just -(x>>31).

      1 reply 0 retweets 1 like
    6. Rich Felker‏ @RichFelker 3 Jun 2017
      Replying to @RichFelker @cr88192 and

      That simplifies to a single SAR instruction on any decent compiler architecture.

      1 reply 0 retweets 1 like
    7. Brendan G Bohannon‏ @cr88192 3 Jun 2017
      Replying to @RichFelker @oshepherd and

      but signed >> works on most targets and compilers I have used (excepting for MSP430 and AVR and similar), and a lot of code relies on it.

      1 reply 0 retweets 0 likes
    8. Rich Felker‏ @RichFelker 3 Jun 2017
      Replying to @cr88192 @oshepherd and

      >> on signed is implementation-defined, not undefined, and fairly safe to rely upon. << on negative is undefined. Overflow is always undef.

      1 reply 0 retweets 0 likes
    9. Rich Felker‏ @RichFelker 3 Jun 2017
      Replying to @RichFelker @cr88192 and

      When x is signed, x<<n is equivalent to x*(1<<n) except the former is defined on a smaller domain; just use the latter if you care.

      1 reply 0 retweets 0 likes
    10. Brendan G Bohannon‏ @cr88192 3 Jun 2017
      Replying to @RichFelker @oshepherd and

      usually << on signed is functionally equivalent to the unsigned case. some targets also have expensive '*' (if not optimized away).

      1 reply 0 retweets 0 likes
      Rich Felker‏ @RichFelker 3 Jun 2017
      Replying to @cr88192 @oshepherd and

      If a compiler can't generate "SHL n" for "*(1<<n)" it's not suitable for real-world use. You write C to the language, not to a bad compiler.

      10:35 AM - 3 Jun 2017
      • 1 Like
      • Clifford Wolf
      1 reply 0 retweets 1 like
        1. New conversation
        2. Brendan G Bohannon‏ @cr88192 3 Jun 2017
          Replying to @RichFelker @oshepherd and

          I have seen CC's which would fail to optimize this, for targets with neither a "SHL n" op, nor a "MUL" op, so both end up as runtime calls.

          1 reply 0 retweets 0 likes
        3. Rich Felker‏ @RichFelker 3 Jun 2017
          Replying to @cr88192 @oshepherd and

          If this optimization or lack thereof happens at the target backend level rather than on high-level IR, the compiler is hopelessly broken.

          1 reply 0 retweets 1 like
        4. Brendan G Bohannon‏ @cr88192 3 Jun 2017
          Replying to @RichFelker @oshepherd and

          I am thinking it is going pretty ok if the CC's work and accept modern'ish C syntax (luckily K&R only CC's are pretty much dead now...).

          2 replies 0 retweets 0 likes
        5. Rich Felker‏ @RichFelker 3 Jun 2017
          Replying to @cr88192 @oshepherd and

          You're a decade or two late on these discussions...

          1 reply 0 retweets 1 like
        6. Brendan G Bohannon‏ @cr88192 3 Jun 2017
          Replying to @RichFelker @oshepherd and

          old stuff lingers for a while, it seems only pretty recent that using GCC/etc became semi-universal (vs LCC variants and one-off compilers).

          1 reply 0 retweets 0 likes
        7. Brendan G Bohannon‏ @cr88192 3 Jun 2017
          Replying to @cr88192 @RichFelker and

          actually, FWIW, in a class I am taking there are still some machines around using orange-on-black CRTs and floppy drives. epic new tech...

          0 replies 0 retweets 0 likes
        8. End of conversation

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