One 512-bit vector unit versus 2 256-bit vector units, re Zen 4 AVX-512

By: Jeffrey Bosboom (firstinitiallastname.delete@this.firstnamelastname.com), November 4, 2022 5:18 pm
Room: Moderated Discussions
From a recent Agner Fog forum post:

The support for the new AVX512 instructions is quite good, and it includes many of the extra subsets of AVX512. Here, I have to correct a common misunderstanding. The Zen 4 does not execute a 512-bit vector instruction by using a 256-bit execution unit twice, but by using two 256-bit units simultaneously. It does not split a 512-bit instruction into two 256-bit micro-operations, like the Zen 1 that splits 256-bit instructions into two 128-bit micro-operations. The Zen 4 has four 256-bit execution units. Two of these units can do floating point addition, and the other two can do floating point multiplication. All four can do integer vector addition etc. This gives a maximum throughput for 512-bit vectors of one floating point vector multiplication and one floating point vector addition, or two integer vector additions, per clock cycle. This throughput is doubled for vectors of 256 bits or less. It is still advantageous to use 512-bit instructions if the throughput is limited by instruction decoding or micro-operation queues or code cache or something else. It is rare that execution unit throughput is the bottleneck.


I understand how cracking a 2n-bit instruction into two n-bit instructions and executing them sequentially saves area compared to a full 2n-bit-wide unit. But what is the difference between one 2n-bit unit and two n-bit units that can execute a 2n-bit instruction at full rate when paired? Or from the other direction, why wouldn't a full 2n-bit unit also be designed to execute two n-bit instructions simultaneously?
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One 512-bit vector unit versus 2 256-bit vector units, re Zen 4 AVX-512Jeffrey Bosboom2022/11/04 05:18 PM
  Clarification?Mark Roulo2022/11/04 07:34 PM
    Expanded question about design pointsJeffrey Bosboom2022/11/04 09:37 PM
      Expanded question about design pointsAnon2022/11/04 09:53 PM
        Expanded question about design pointsJeffrey Bosboom2022/11/04 10:05 PM
          Expanded question about design pointsAnon2022/11/04 10:30 PM
            Expanded question about design pointsChester2022/11/05 03:24 PM
              Expanded question about design pointsAnon2022/11/05 03:43 PM
              Expanded question about design pointsLinus Torvalds2022/11/06 01:18 PM
                Expanded question about design pointsAdrian2022/11/07 03:38 AM
                  Expanded question about design pointsanon2022/11/07 11:34 AM
                    Expanded question about design pointsAdrian2022/11/08 03:34 AM
                      Expanded question about design pointsChester2022/11/08 07:29 AM
                      Expanded question about design pointsanon2022/11/08 08:01 AM
                        Expanded question about design pointsAdrian2022/11/08 08:53 AM
                          Expanded question about design pointsLinus Torvalds2022/11/08 10:35 AM
                            Expanded question about design pointsBrett2022/11/08 11:33 AM
                              Expanded question about design pointsBrett2022/11/08 11:48 AM
                              Expanded question about design points---2022/11/09 10:41 AM
                            Expanded question about design pointsAdrian2022/11/08 11:45 AM
                              Expanded question about design pointsLinus Torvalds2022/11/08 12:29 PM
                                Expanded question about design pointsanon2022/11/08 12:58 PM
                              Zen 4cJames2022/11/09 02:54 AM
                                Zen 4cAndrew Clough2022/11/09 04:59 AM
                                  Zen 4canonymou52022/11/09 11:29 AM
                                    Zen 4cChester2022/11/09 08:12 PM
                            Expanded question about design pointsBjörn Ragnar Björnsson2022/11/08 08:24 PM
                              FP Adders are not so cheap compared to FP multipliersHeikki Kultala2022/11/09 08:07 AM
                                FP Adders are not so cheap compared to FP multipliersBjörn Ragnar Björnsson2022/11/09 11:10 PM
                          Expanded question about design pointsAnon2022/11/08 05:31 PM
      Expanded question about design pointsAdrian2022/11/05 02:00 AM
        Expanded question about design pointsAnon2022/11/05 02:27 AM
          Expanded question about design pointsAdrian2022/11/05 02:50 AM
            Expanded question about design pointsAnon2022/11/05 03:10 AM
              Expanded question about design pointsAdrian2022/11/05 06:34 AM
        Expanded question about design pointshobold2022/11/06 03:48 AM
          Expanded question about design pointsAdrian2022/11/07 03:19 AM
            Expanded question about design pointsAdrian2022/11/07 08:07 AM
  One 512-bit vector unit versus 2 256-bit vector units, re Zen 4 AVX-512Anon2022/11/04 07:49 PM
  One 512-bit vector unit versus 2 256-bit vector units, re Zen 4 AVX-512noko2022/11/04 08:49 PM
  One 512-bit vector unit versus 2 256-bit vector units, re Zen 4 AVX-512Brendan2022/11/05 01:07 AM
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