IFMA and Division

By: rwessel (, July 26, 2022 8:16 pm
Room: Moderated Discussions
Bill G ( on July 26, 2022 4:25 pm wrote:
> Adrian ( on July 26, 2022 2:53 am wrote:
> > Aarch64 and any other ISA must also add IFMA instructions to be
> > able to have a big number performance comparable with x86-64,
> > because there is no way to have a comparable throughput when using scalar multiplications.
> Why is a new IFMA instruction needed, instead of just doing 52-bit integer
> multiplication with a 64-bit floating-point FMA vector instruction?

You really want the whole 104-bit product. The regular vector FP instructions don't provide that, and is specifically what the two IFMA instructions do.

> Michael S ( on July 26, 2022 5:55 am wrote:
> > ... But it has something else - fully pipelined FP double-precision
> > division unit. Could it be used to speed up big-integer division
> > for above-mentioned lengths? My answer is "I don't know before I
> > try.". And "try" here does not mean something quick, but rather
> > a week or four of hard work of writing and comparing several alternatives.
> Digit based division, where a digit here would be 52 bits, is quite complicated. I can see keeping a pipelined
> division unit full when doing multiple big integer divides in parallel but I can’t see how to keep a pipelined
> division unit full for a single big integer division. This would be a good thesis project for someone.
> Adrian ( on July 26, 2022 7:59 am wrote:
> > Unlike for multiplication, for division or square root it is less likely
> > that vector instructions could be helpful in big number computations,
> > as in the algorithms that use such instructions their operands are typically
> > dependent on earlier results. So I do not expect any future
> > ISA improvements comparable to the introduction of IFMA, but only
> > slightly faster implementations of the existing instructions.
> In order to use the fast multiplies provided by vector multiply instructions, what
> would you think of implementing big integer division with Newton-Raphson iterations
> followed by a fix up step at the end to get the correctly rounded answer? A Newton-Raphson
> iteration computes 1/a with two multiplications per iteration like this:
> X(n+1) = X(n) ( 2 - a X(n) )

That's what everyone does. NR approximately doubles the number of accurate bits in the inverse with each iteration, and you don't have to do the early iterations full width. Another multiplication to do the actual division, and (often) a bit of a fixup step.) Basically that makes division (time) equivalent to 4-8 multiplications of the same size. A good initial estimate (which is not infrequent done with the FP divider), is definitely useful. Of course, this requires a fast multiplication to be useful.
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Yitian 710 anonymous22021/10/20 08:57 PM
  Yitian 710 Adrian2021/10/21 12:20 AM
  Yitian 710 Wilco2021/10/21 03:47 AM
    Yitian 710 Rayla2021/10/21 05:52 AM
      Yitian 710 Wilco2021/10/21 11:59 AM
        Yitian 710 anon22021/10/21 05:16 PM
        Yitian 710 Wilco2022/07/16 12:21 PM
          Yitian 710 Anon2022/07/16 08:22 PM
            Yitian 710 Rayla2022/07/17 09:10 AM
              Yitian 710 Anon2022/07/17 12:04 PM
                Yitian 710 Rayla2022/07/17 12:08 PM
                  Yitian 710 Wilco2022/07/17 01:16 PM
                    Yitian 710 Anon2022/07/17 01:32 PM
                      Yitian 710 Wilco2022/07/17 02:22 PM
                        Yitian 710 Anon2022/07/17 02:47 PM
                          Yitian 710 Wilco2022/07/17 03:50 PM
                            Yitian 710 Anon2022/07/17 08:46 PM
                              Yitian 710 Wilco2022/07/18 03:01 AM
                                Yitian 710 Anon2022/07/19 11:21 AM
                                  Yitian 710 Wilco2022/07/19 06:15 PM
                                    Yitian 710 Anon2022/07/21 01:25 AM
                                      Yitian 710 none2022/07/21 01:49 AM
                                        Yitian 710 Anon2022/07/21 03:03 AM
                                          Yitian 710 none2022/07/21 04:34 AM
                                      Yitian 710 James2022/07/21 02:29 AM
                                        Yitian 710 Anon2022/07/21 03:05 AM
                                      Yitian 710 Wilco2022/07/21 04:31 AM
                                        Yitian 710 Anon2022/07/21 05:17 AM
                                          Yitian 710 Wilco2022/07/21 05:33 AM
                                            Yitian 710 Anon2022/07/21 05:50 AM
                                              Yitian 710 Wilco2022/07/21 06:07 AM
                                                Yitian 710 Anon2022/07/21 06:20 AM
                                                  Yitian 710 Wilco2022/07/21 10:02 AM
                                                    Yitian 710 Anon2022/07/21 10:22 AM
                    Yitian 710 Adrian2022/07/17 11:09 PM
                      Yitian 710 Wilco2022/07/18 01:15 AM
                        Yitian 710 Adrian2022/07/18 02:35 AM
          Yitian 710 Adrian2022/07/16 11:19 PM
            Computations on Big IntegersBill G2022/07/25 10:06 PM
              Computations on Big Integersnone2022/07/25 11:35 PM
                x86 MUL 64x64 Eric Fink2022/07/26 01:06 AM
                  x86 MUL 64x64 Adrian2022/07/26 02:27 AM
                  x86 MUL 64x64 none2022/07/26 02:38 AM
                    x86 MUL 64x64 Jörn Engel2022/07/26 10:17 AM
                      x86 MUL 64x64 Linus Torvalds2022/07/27 10:13 AM
                        x86 MUL 64x64 2022/07/28 09:40 AM
                        x86 MUL 64x64 Jörn Engel2022/07/28 10:18 AM
                          More than 3 registers per instruction-.-2022/07/28 07:01 PM
                            More than 3 registers per instructionAnon2022/07/28 10:39 PM
                            More than 3 registers per instructionJörn Engel2022/07/28 10:42 PM
                              More than 3 registers per instruction-.-2022/07/29 04:31 AM
                Computations on Big IntegersBill G2022/07/26 01:40 AM
                  Computations on Big Integersnone2022/07/26 02:17 AM
                    Computations on Big IntegersBill G2022/07/26 03:52 AM
                    Computations on Big Integers---2022/07/26 09:57 AM
                  Computations on Big IntegersAdrian2022/07/26 02:53 AM
                    Computations on Big IntegersBill G2022/07/26 03:39 AM
                      Computations on Big IntegersAdrian2022/07/26 04:21 AM
                    Computations on Big Integers in Apple AMX UnitsBill G2022/07/26 04:28 AM
                      Computations on Big Integers in Apple AMX UnitsAdrian2022/07/26 05:13 AM
                        TypoAdrian2022/07/26 05:20 AM
                          IEEE binary64 is 53 bits rather than 52. (NT)Michael S2022/07/26 05:34 AM
                            IEEE binary64 is 53 bits rather than 52.Adrian2022/07/26 07:32 AM
                              IEEE binary64 is 53 bits rather than 52.Michael S2022/07/26 10:02 AM
                                IEEE binary64 is 53 bits rather than 52.Adrian2022/07/27 06:58 AM
                                  IEEE binary64 is 53 bits rather than 52.none2022/07/27 07:14 AM
                                    IEEE binary64 is 53 bits rather than 52.Adrian2022/07/27 07:55 AM
                                      Thanks a lot for the link to the article! (NT)none2022/07/27 08:09 AM
                          TypozArchJon2022/07/26 09:51 AM
                            TypoMichael S2022/07/26 10:25 AM
                              TypozArchJon2022/07/26 11:52 AM
                                TypoMichael S2022/07/26 01:02 PM
                    Computations on Big IntegersMichael S2022/07/26 05:55 AM
                      Computations on Big IntegersAdrian2022/07/26 07:59 AM
                        IFMA and DivisionBill G2022/07/26 04:25 PM
                          IFMA and Divisionrwessel2022/07/26 08:16 PM
                          IFMA and DivisionAdrian2022/07/27 07:25 AM
                      Computations on Big Integersnone2022/07/27 01:22 AM
                    Big integer multiplication with vector IFMABill G2022/07/29 01:06 AM
                      Big integer multiplication with vector IFMAAdrian2022/07/29 01:35 AM
                        Big integer multiplication with vector IFMA-.-2022/07/29 04:32 AM
                          Big integer multiplication with vector IFMAAdrian2022/07/29 09:47 PM
                            Big integer multiplication with vector IFMAAnon2022/07/30 08:12 AM
                              Big integer multiplication with vector IFMAAdrian2022/07/30 09:27 AM
                                AVX-512 unfriendly to heter-performance coresPaul A. Clayton2022/07/31 03:20 PM
                                  AVX-512 unfriendly to heter-performance coresAnon2022/07/31 03:33 PM
                                    AVX-512 unfriendly to heter-performance coresanonymou52022/07/31 05:03 PM
                                  AVX-512 unfriendly to heter-performance coresBrett2022/07/31 07:26 PM
                                  AVX-512 unfriendly to heter-performance coresAdrian2022/08/01 01:45 AM
                                    Why can't E-cores have narrow/slow AVX-512? (NT)anonymous22022/08/01 03:37 PM
                                      Why can't E-cores have narrow/slow AVX-512?Ivan2022/08/02 12:09 AM
                                        Why can't E-cores have narrow/slow AVX-512?anonymou52022/08/02 10:13 AM
                                        Why can't E-cores have narrow/slow AVX-512?Dummond D. Slow2022/08/02 03:02 PM
                                    AVX-512 unfriendly to heter-performance coresPaul A. Clayton2022/08/02 01:19 PM
                                      AVX-512 unfriendly to heter-performance coresAnon2022/08/02 09:09 PM
                                      AVX-512 unfriendly to heter-performance coresAdrian2022/08/03 12:50 AM
                                        AVX-512 unfriendly to heter-performance coresAnon2022/08/03 09:15 AM
                                          AVX-512 unfriendly to heter-performance cores-.-2022/08/03 08:17 PM
                                            AVX-512 unfriendly to heter-performance coresAnon2022/08/03 09:02 PM
                        IFMA: empty promises from Intel as usualKent R2022/07/29 07:15 PM
                          No hype lasts foreverAnon2022/07/30 08:06 AM
                        Big integer multiplication with vector IFMAme2022/07/30 09:15 AM
                Computations on Big Integers---2022/07/26 09:48 AM
                  Computations on Big Integersnone2022/07/27 01:10 AM
                    Computations on Big Integers---2022/07/28 11:43 AM
                      Computations on Big Integers---2022/07/28 06:44 PM
              Computations on Big Integersdmcq2022/07/26 02:27 PM
                Computations on Big IntegersAdrian2022/07/27 08:15 AM
                  Computations on Big IntegersBrett2022/07/27 11:07 AM
      Yitian 710 Wes Felter2021/10/21 12:51 PM
        Yitian 710 Adrian2021/10/21 01:25 PM
    Yitian 710 Anon2021/10/21 06:08 AM
      Strange definition of the word single. (NT)anon22021/10/21 05:00 PM
        AMD Epyc uses chiplets. This is why "strange"?Mark Roulo2021/10/21 05:08 PM
          AMD Epyc uses chiplets. This is why "strange"?anon22021/10/21 05:34 PM
            Yeah. Blame, too, though!Mark Roulo2021/10/21 05:58 PM
              Yeah. Blame, too, though!anon22021/10/21 08:07 PM
                Yeah. Blame, too, though!Björn Ragnar Björnsson2022/07/17 06:23 AM
              Yeah. Blame, too, though!Rayla2022/07/17 09:13 AM
                Yeah. Blame, too, though!Anon2022/07/17 12:01 PM
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