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cmuratori's profile
Casey Muratori
Casey Muratori
Casey Muratori
@cmuratori

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Casey Muratori

@cmuratori

I'm worried that the baby thinks people can't change.

Seattle
caseymuratori.com
Joined March 2009

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    2. Stefan Werner‏ @stefan_3d 7 Apr 2020
      Replying to @dgmdavid @cmuratori and

      1. No. 2. No. 3. No. Those things are true most of the time, but not all of the time. Profiling and benchmarks are the tools I use to find out whether it's me or the compiler being stupid.

      0 replies 0 retweets 1 like
    3. This Tweet is unavailable.
    4. Stefan Werner‏ @stefan_3d 7 Apr 2020
      Replying to @dgmdavid @cmuratori and

      If your starting assumption is that you are smarter than the compiler, then your code should run the fastest as-is, and any compiler optimization flags could only make it slower.

      2 replies 0 retweets 0 likes
    5. Casey Muratori‏ @cmuratori 7 Apr 2020
      Replying to @stefan_3d @dgmdavid and

      That's not at all how compilers work. If you do not turn optimizations on, it will store and load every value to and from the stack on every line. There is no way to write performance oriented code without turning optimizations on unless you use inline ASM.

      2 replies 0 retweets 18 likes
    6. Stefan Werner‏ @stefan_3d 7 Apr 2020
      Replying to @cmuratori @dgmdavid and

      Then how would you compile code that you think can't be optimized no more? -O0 with a few select flags at best. -O1 (at least on gcc) already implies things like -freorder-blocks, which is letting the compiler make decisions for you.

      1 reply 0 retweets 0 likes
    7. Casey Muratori‏ @cmuratori 7 Apr 2020
      Replying to @stefan_3d @dgmdavid and

      Does this help?https://godbolt.org/z/6DAVpR 

      1 reply 0 retweets 2 likes
    8. Stefan Werner‏ @stefan_3d 7 Apr 2020
      Replying to @cmuratori @dgmdavid and

      I know godbolt, yes. My question is though, how would you compile hand-optimized C if you wanted the compiler to not touch any bits of it other than faithfully translating it line by line to assembly?

      1 reply 0 retweets 0 likes
    9. Casey Muratori‏ @cmuratori 7 Apr 2020
      Replying to @stefan_3d @dgmdavid and

      Did you read the actual specific godbolt I sent?

      1 reply 0 retweets 0 likes
    10. Stefan Werner‏ @stefan_3d 7 Apr 2020
      Replying to @cmuratori @dgmdavid and

      Oops, my apologies.

      2 replies 0 retweets 0 likes
      Casey Muratori‏ @cmuratori 7 Apr 2020
      Replying to @stefan_3d @dgmdavid and

      I was trying to explain that what you're saying doesn't make any sense. There is no such thing as "translate line by line into ASM". There is no direct mapping from C to ASM.

      12:36 AM - 7 Apr 2020
      • 7 Likes
      • ha Rytis Akhil Indurti RunbyAtwirl trfd David Roome 🪐 Ivan Braidi
      2 replies 0 retweets 7 likes
        1. New conversation
        2. Casey Muratori‏ @cmuratori 7 Apr 2020
          Replying to @cmuratori @stefan_3d and

          -O2 _is_ the way you tell the compiler "try to produce the correct ASM for this C". Sometimes it can't figure it out (and it's frustrating). But if you write the C carefully enough, sometimes it can. -O0 _never_ figures out the correct ASM for the C code, pretty much ever.

          1 reply 0 retweets 5 likes
        3. Casey Muratori‏ @cmuratori 7 Apr 2020
          Replying to @cmuratori @stefan_3d and

          So in the godbolt I showed a -O0 and a -O2 of an inner product, and hopefully you can see that -O0 is _not at all_ similar to the C code - it's not even usable. -O2 produces the _actual_ closest translation to the input C code!

          0 replies 0 retweets 6 likes
        4. End of conversation
        1. New conversation
        2. Stefan Werner‏ @stefan_3d 7 Apr 2020
          Replying to @cmuratori @dgmdavid and

          There is no direct mapping from C to ASM, correct. I feel though like some C programmers are convinced that there is, being "close to the metal" and all.

          1 reply 0 retweets 0 likes
        3. Rubén Osorio‏ @osor_io 7 Apr 2020
          Replying to @stefan_3d @cmuratori and

          These are in practice two things though. Telling the computer what to do with the code you write (or use from a lib) and what the compiler does as a following pass to translate it so your machine can run it (and how good of a job it does at that translation).

          1 reply 0 retweets 0 likes
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