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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. William D. Jones‏ @cr1901 24 Nov 2015
      Replying to @CopperheadOS

      @CopperheadSec @RichFelker Why would -fwrapv have a performance cost period?

      1 reply 0 retweets 0 likes
    2. CopperheadOS‏ @CopperheadOS 24 Nov 2015
      Replying to @cr1901

      @cr1901 @RichFelker It removes the ability for the compiler to assume that signed overflow doesn't happen, causing missed optimizations.

      1 reply 0 retweets 0 likes
    3. William D. Jones‏ @cr1901 24 Nov 2015
      Replying to @CopperheadOS

      @CopperheadSec @RichFelker Oh hmmm, I thought the predictability of signed overflow would've led to some new optimizations...

      2 replies 0 retweets 0 likes
    4. CopperheadOS‏ @CopperheadOS 24 Nov 2015
      Replying to @cr1901

      @cr1901 @RichFelker Having it undefined gives the compiler the freedom to assume whatever it wants about it on a case-by-case basis.

      1 reply 0 retweets 0 likes
    5. CopperheadOS‏ @CopperheadOS 24 Nov 2015
      Replying to @CopperheadOS

      @cr1901 @RichFelker Undefined doesn't just mean unspecified. It means it's guaranteed to never happen.

      1 reply 0 retweets 0 likes
    6. CopperheadOS‏ @CopperheadOS 24 Nov 2015
      Replying to @CopperheadOS

      @cr1901 @RichFelker The compiler can interpret x + INT_MAX as a guarantee that x <= 0. And that's starting to be what actually happens.

      1 reply 2 retweets 1 like
    7. CopperheadOS‏ @CopperheadOS 24 Nov 2015
      Replying to @CopperheadOS

      @cr1901 @RichFelker Compilers are really bad at integer range analysis right now... but as they get better, lots of code is going to break.

      1 reply 1 retweet 0 likes
    8. William D. Jones‏ @cr1901 24 Nov 2015
      Replying to @CopperheadOS

      @CopperheadSec @RichFelker The C-Pocalypse. I mean, the C-hate is already pretty bad if Twitter is any indication.

      2 replies 0 retweets 0 likes
    9. CopperheadOS‏ @CopperheadOS 24 Nov 2015
      Replying to @cr1901

      @cr1901 @RichFelker Yeah, they already make the assumptions that are going to break incorrect code. They just aren't yet optimizing enough.

      1 reply 0 retweets 0 likes
    10. CopperheadOS‏ @CopperheadOS 24 Nov 2015
      Replying to @CopperheadOS

      @cr1901 @RichFelker The adoption of link-time optimization will uncover a lot of these issues too. Not just compiler improvements.

      2 replies 0 retweets 0 likes
      Rich Felker‏ @RichFelker 24 Nov 2015
      Replying to @CopperheadOS

      @CopperheadSec @cr1901 Indeed, most UB is undetectable by the compiler without explicit runtime checks since extern acts as compiler barrier

      2:35 PM - 24 Nov 2015
      2 replies 0 retweets 0 likes
        1. Rich Felker‏ @RichFelker 24 Nov 2015
          Replying to @RichFelker

          @CopperheadSec @cr1901 LTO is completely game-changing here. I'm proactively trying to find anything in @musllibc assuming extern-as-barrier

          0 replies 0 retweets 0 likes
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        2. William D. Jones‏ @cr1901 24 Nov 2015
          Replying to @RichFelker

          @RichFelker @CopperheadSec barrier?

          1 reply 0 retweets 0 likes
        3. CopperheadOS‏ @CopperheadOS 24 Nov 2015
          Replying to @cr1901

          @cr1901 @RichFelker Functions from other files (other than inline in headers) are opaque to the compiler without link-time optimization.

          1 reply 0 retweets 0 likes
        4. CopperheadOS‏ @CopperheadOS 24 Nov 2015
          Replying to @CopperheadOS

          @cr1901 @RichFelker But with LTO, the compiler can optimize the program as a whole and that makes subtle cases of UB much more dangerous.

          1 reply 0 retweets 0 likes
        5. CopperheadOS‏ @CopperheadOS 24 Nov 2015
          Replying to @CopperheadOS

          @cr1901 @RichFelker Without LTO, issues are usually confined to a single source file if there's no memory corruption.

          1 reply 0 retweets 0 likes
        6. William D. Jones‏ @cr1901 24 Nov 2015
          Replying to @CopperheadOS

          @CopperheadSec @RichFelker Oh, you're assuming the compiler can prove that signed overflow can't occur if it knows all the places >>

          1 reply 0 retweets 0 likes
        7. William D. Jones‏ @cr1901 24 Nov 2015
          Replying to @cr1901

          @CopperheadSec @RichFelker where side effects to an extern signed int occurs globally?

          1 reply 0 retweets 0 likes
        8. CopperheadOS‏ @CopperheadOS 24 Nov 2015
          Replying to @cr1901

          @cr1901 @RichFelker For example, a call to foo(a, b). Lets say it does a + b internally. Now foo(a, b) is a guarantee to the compiler.

          1 reply 0 retweets 0 likes
        9. CopperheadOS‏ @CopperheadOS 24 Nov 2015
          Replying to @CopperheadOS

          @cr1901 @RichFelker Without LTO, these kinds of assumptions can't be made across source files. It greatly limits the potential for breakage.

          2 replies 0 retweets 0 likes
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