should say: (new LINK)

By: Travis Downs (, January 22, 2020 3:28 pm
Room: Moderated Discussions
anon ( on January 17, 2020 8:12 pm wrote:
> Travis Downs ( on January 17, 2020 10:55 am wrote:
> > should say: (new LINK)
> Have you tested if using new registers (xmm16-31) is any different from old xmms?

The upper 16 registers they are different in that sense that dirtying them doesn't cause you to suffer from the implicit widening effect.

That is, if you dirty the upper bits of zmm0 to zmm15, all future SIMD and FP instructions will be widened to 512-bits (yes, this means that 128-bit SIMD FP instructions will cause you to use L2, they are just as heavy as 512-bit instructions despite calculating only 128 bits of result).

However, if you dirty the uppers of zmm16 to 31, this effect doesn't happen: there is no implicit widening. This is probably because legacy instructions only access 0-15 and the whole vzeroupper and associated tracking and merging scenarios applies only to 0-15.

I believe the same is true for ymm16-31 too: if you dirty those there is no implicit widening to 256-bits for subsequent instructions. I haven't tested it though.

Note that this applies to the dirtying instructions, not the subsequent "widened" instructions. If you dirty the uppers of 0-15, then use xmm16+ or ymm16+, implicit widening stills occurs, since it is a CPU-wide state.

Does it answer your question?

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    AVX-512 downclocking postTravis Downs2020/01/17 01:10 PM
  AVX-512 downclocking postEtienne2020/01/17 03:16 AM
    Thanks, typos fixed and credited (NT)Travis Downs2020/01/17 01:15 PM
  Title suggestions welcome (NT)Travis Downs2020/01/17 08:54 AM
  AVX-512 downclocking postanonymou52020/01/17 10:53 AM
    AVX-512 downclocking postTravis Downs2020/01/17 11:14 AM
      AVX-512 downclocking postYoav2020/01/17 11:50 AM
        AVX-512 downclocking postTravis Downs2020/01/17 01:14 PM
      AVX-512 downclocking postanonymou52020/01/17 04:26 PM
        AVX-512 downclocking postTravis Downs2020/01/22 08:19 PM
          AVX-512 downclocking postanonymou52020/01/23 12:56 AM
            AVX-512 downclocking postFoyle2020/01/23 05:51 AM
              AVX-512 downclocking postanonymou52020/01/23 06:57 AM
                AVX-512 downclocking postTravis Downs2020/01/24 12:49 PM
            finer-grained licensesTravis Downs2020/01/24 01:03 PM
              finer-grained licensesanonymou52020/01/24 04:28 PM
                finer-grained licensesTravis Downs2020/01/25 09:46 AM
  post published (new line)Travis Downs2020/01/17 11:55 AM
    should say: (new LINK) (NT)Travis Downs2020/01/17 11:55 AM
      should say: (new LINK)Tim McCaffrey2020/01/17 01:44 PM
        Thanks, fixed and credited (NT)Travis Downs2020/01/17 02:54 PM
      should say: (new LINK)anon2020/01/17 09:12 PM
        should say: (new LINK)Travis Downs2020/01/22 03:28 PM
          Thanks!anon2020/01/22 08:06 PM
            Thanks!Travis Downs2020/01/22 08:16 PM
              Thanks!anon2020/01/22 10:20 PM
                Thanks!Travis Downs2020/01/23 01:51 AM
                  Thanks!Linus Torvalds2020/01/23 05:33 PM
                    Thanks!Travis Downs2020/01/24 12:44 PM
  Throttling dispatchGabriele Svelto2020/01/22 01:40 PM
    Itanium also used itDavid Kanter2020/01/22 02:04 PM
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