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GCC 10 Lands Support For Emulating MMX With SSE Instructions

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  • GCC 10 Lands Support For Emulating MMX With SSE Instructions

    Phoronix: GCC 10 Lands Support For Emulating MMX With SSE Instructions

    The GCC 10 code compiler merged support to begin emulating MMX intrinsics using SSE...

    http://www.phoronix.com/scan.php?pag...g-MMX-With-SSE

  • #2
    Why?

    would it not make more sense to modernize code that is still using MMX?

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    • #3
      this should first be seen on ocean cove cores, i dunno when they are coming tough

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      • #4
        This isn't going to help applications that are already built for MMX and not open source. So dropping it would be retarded.

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        • #5
          Originally posted by wizard69 View Post
          Why?

          would it not make more sense to modernize code that is still using MMX?
          Are you paying for the refactoring?

          Because recompiling with different flags is pretty cheap.

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          • #6
            Originally posted by Weasel View Post
            This isn't going to help applications that are already built for MMX and not open source. So dropping it would be retarded.
            I would hazard a guess:
            having the compiler do this conversion is very likely going to yield faster performance than relying on the "software MMX" (aka provided by microcode) that will be the "MMX" support in newer CPUs that don't really support MMX in hardware anymore.

            Stuff that can't be recompiled will get fucked, but at least it will still run so that's not that bad I guess.

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            • #7
              There are some CPUs like Pentium-M 32bit (~2004) where MMX is much faster than SSE... for the rest it should be the same performance, with the added issue that SSE opcodes are bigger in size than MMX which then affects the caches and by extension might affect performance.

              The 8 byte vectorizer gain in the article, I don't understand. What does this even mean? The EMMS instruction that isn't there? Possible context switches that don't need to save MMX state?

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              • #8
                Can't Linux emulate MMX if processor doesn't support it? At least in the early days Linux could emulate x87 instructions if user didn't have an FPU. If that can be implemented for MMX too, then in theory it really could be dropped. I hope that no performance critical application is using these instructions anymore. Really old software should run fast enough with emulation. Fast enough meaning faster than with the hardware it was designed for. Or at least that's what I would expect but correct me if you know better.

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                • #9
                  Originally posted by wizard69 View Post
                  Why?

                  would it not make more sense to modernize code that is still using MMX?
                  This was my immediate thought too. MMX is super old. Who is asking for this? (A genuine question out of curiosity.)

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                  • #10
                    Originally posted by _Alex_ View Post
                    The 8 byte vectorizer gain in the article, I don't understand. What does this even mean? The EMMS instruction that isn't there? Possible context switches that don't need to save MMX state?
                    MMX isn't the same speed as SSE. It is much slower instruction for instruction (2x slower in best if cases) So doing 8 byte vectors with SSE instead of MMX will give a significant performance boost. And even better if the code is mixing 8 byte vectors with 16 byte vectors which code originally written for NEON might.

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