80 bit FP

Article: PhysX87: Software Deficiency
By: David Kanter (dkanter.delete@this.realworldtech.com), July 19, 2010 11:33 am
Room: Moderated Discussions
Ian Ollmann (iano@apple.com) on 7/19/10 wrote:
---------------------------
>Kevin G (kevin@cubitdesigns.com) on 7/8/10 wrote:
>---------------------------
>>David Kanter (dkanter@realworldtech.com) on 7/8/10 wrote:
>>---------------------------
>>>
>>>But I can totally understand why an ABI would preserve 80b FP for those who *really
>>>need* it. But 99% of people are fine with 64b, so I think pushing people in that
>>>direction and making it difficult to use x87 is a reasonable choice.
>>
>>For those that were wanting 80 bit precision, I thought that there were rumblings
>>that the successor to AVX would support 128 bit precision. Such a change would finally
>>allow x87 support to be phased out.
>
>Yeah, but do you seriously believe that a quad precision part would be faster than
>x87? Obviously, its possible if you throw enough hardware at it -- and maybe Intel
>would do it to claim the third IEEE-754-2008 binary format or something; They do
>take the standard more seriously than some -- but there is no market for quad to
>support that sort of investment in anything other than boutique parts and certainly
>the perf/Watt story would not be good. It would be better to spend the silicon
>on other features like a larger ROB or vector unit or unsigned vector compares or
>making LSU false aliasing less common or something. These things would benefit
>more code. I think their level of investment in x87 is about right. Its enough
>to make sure legacy code still functions without eating up >a lot of chip area.

I would expect hardware for decimal floating point before 128b precision arithmetic. The former just seems more useful and marketable, especially since IBM has gone down that route already.

>Math libraries at times will use the x87 in order to deliver highly accurate results
>for double precision math functions. The power function is a nice example, where
>you need more bits in the intermediate representation than in the final representation
>to get an accurate result. Without x87, you have to get the extra precision from
>elsewhere, either by storing values as a high + low sum and doing what amounts to
>multiprecision arithmetic on it (read: expensive), or where you are sufficiently
>talented and fortunate to be able to use, leverage some other higher precision hardware
>feature such as a fused multiply-add. Obviously, Intel >doesn't currently do FMA so the choices are not good.

Is vectorized multi-precision arithmetic really going to be slower than x87 for the power function? I guess it depends on the operand size (SP vs. DP) and whether there is any data parallelism or not. I would think handling 4 floats at a time through vectorized SSE libraries might be faster than x87...

More generally, I wonder where the trade-off is between vectorized multi-precision arithmetic and a scalar implementation.


Regards,


David
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TopicPosted ByDate
A bit off baseJohn Mann2010/07/07 07:04 AM
  A bit off baseDavid Kanter2010/07/07 11:28 AM
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      SSE vs x87Michael S2010/07/07 01:07 PM
        SSE vs x87hobold2010/07/08 05:12 AM
      SSE vs x87David Kanter2010/07/07 02:55 PM
        SSE vs x87Andi Kleen2010/07/08 02:43 AM
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            80 bit FPDavid Kanter2010/07/08 11:14 AM
              80 bit FPKevin G2010/07/08 02:12 PM
                80 bit FPIan Ollmann2010/07/19 12:49 AM
                  80 bit FPDavid Kanter2010/07/19 11:33 AM
                    80 bit FPAnil Maliyekkel2010/07/19 04:49 PM
                      80 bit FPrwessel2010/07/19 05:41 PM
                    80 bit FPMatt Waldhauer2010/07/21 11:11 AM
            80 bit FPEmil Briggs2010/07/22 09:06 AM
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      A bit off baseDavid Kanter2010/07/08 11:27 AM
        A bit off baseIan Ameline2010/07/09 10:10 AM
          A bit off baseMichael S2010/07/10 02:13 PM
            A bit off baseIan Ameline2010/07/11 07:51 AM
  A bit off baseDavid Kanter2010/07/07 09:46 PM
    A bit off baseAnon2010/07/08 12:47 AM
      A bit off baseanon2010/07/08 02:15 AM
        A bit off baseGabriele Svelto2010/07/08 04:11 AM
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                      Physics engine historyfvdbergh2010/07/13 07:27 AM
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          150 GFLOP/s measured?David Kanter2010/07/08 07:53 PM
            150 GFLOP/s measured?Aaron Spink2010/07/08 09:05 PM
              150 GFLOP/s measured?anon2010/07/08 09:31 PM
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                  150 GFLOP/s measured?David Kanter2010/07/08 11:27 PM
                    150 GFLOP/s measured?Ian Ollmann2010/07/19 01:14 AM
                      150 GFLOP/s measured?anon2010/07/19 06:39 AM
                        150 GFLOP/s measured?hobold2010/07/19 07:26 AM
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                      150 GFLOP/s measured?Linus Torvalds2010/07/19 07:36 AM
                        150 GFLOP/s measured?Richard Cownie2010/07/19 08:42 AM
                          150 GFLOP/s measured?Aaron Spink2010/07/19 08:56 AM
                            150 GFLOP/s measured?hobold2010/07/19 09:30 AM
                              150 GFLOP/s measured?Groo2010/07/19 02:31 PM
                                150 GFLOP/s measured?hobold2010/07/19 04:17 PM
                                  150 GFLOP/s measured?Groo2010/07/19 06:18 PM
                              150 GFLOP/s measured?Anon2010/07/19 06:18 PM
                            150 GFLOP/s measured?Mark Roulo2010/07/19 11:47 AM
                              150 GFLOP/s measured?slacker2010/07/19 12:55 PM
                                150 GFLOP/s measured?Mark Roulo2010/07/19 01:00 PM
                              150 GFLOP/s measured?anonymous422010/07/25 12:31 PM
                            150 GFLOP/s measured?Richard Cownie2010/07/19 12:41 PM
                              150 GFLOP/s measured?Linus Torvalds2010/07/19 02:57 PM
                                150 GFLOP/s measured?Richard Cownie2010/07/19 04:10 PM
                                150 GFLOP/s measured?Richard Cownie2010/07/19 04:10 PM
                                  150 GFLOP/s measured?hobold2010/07/19 04:25 PM
                                  150 GFLOP/s measured?Linus Torvalds2010/07/19 04:31 PM
                                    150 GFLOP/s measured?Richard Cownie2010/07/20 06:04 AM
                                150 GFLOP/s measured?jrl2010/07/20 01:18 AM
                            150 GFLOP/s measured?anonymous422010/07/25 12:00 PM
                              150 GFLOP/s measured?David Kanter2010/07/25 12:52 PM
                          150 GFLOP/s measured?Anon2010/07/19 06:15 PM
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                              150 GFLOP/s measured?Anon2010/07/19 09:54 PM
                                150 GFLOP/s measured?anon2010/07/19 11:45 PM
                        150 GFLOP/s measured?hobold2010/07/19 09:14 AM
                          150 GFLOP/s measured?Linus Torvalds2010/07/19 11:56 AM
                            150 GFLOP/s measured?a reader2010/07/21 08:16 PM
                              150 GFLOP/s measured?Linus Torvalds2010/07/21 09:05 PM
                                150 GFLOP/s measured?anon2010/07/22 02:09 AM
                                  150 GFLOP/s measured?a reader2010/07/22 07:53 PM
                                    150 GFLOP/s measured?gallier22010/07/23 05:58 AM
                                      150 GFLOP/s measured?a reader2010/07/25 08:35 AM
                                        150 GFLOP/s measured?David Kanter2010/07/25 11:49 AM
                                          150 GFLOP/s measured?a reader2010/07/26 07:03 PM
                                            150 GFLOP/s measured?Michael S2010/07/28 01:38 AM
                                              150 GFLOP/s measured?Gabriele Svelto2010/07/28 01:44 AM
                                    150 GFLOP/s measured?anon2010/07/23 04:55 PM
                                      150 GFLOP/s measured?slacker2010/07/24 12:48 AM
                                        150 GFLOP/s measured?anon2010/07/24 02:36 AM
                                    150 GFLOP/s measured?Vincent Diepeveen2010/07/27 05:37 PM
                                      150 GFLOP/s measured??2010/07/27 11:42 PM
                                        150 GFLOP/s measured?slacker2010/07/28 05:55 AM
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                                          Intel's clock rate projectionsAM2010/07/28 10:12 PM
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                          Separate the OoO-ness from speculative-nessMark Christiansen2010/07/20 02:26 PM
                          Separate the OoO-ness from speculative-nessslacker2010/07/20 06:04 PM
                            Separate the OoO-ness from speculative-nessMatt Sayler2010/07/20 06:10 PM
                              Separate the OoO-ness from speculative-nessslacker2010/07/20 09:37 PM
                                Separate the OoO-ness from speculative-ness?2010/07/20 11:51 PM
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                                    Separate the OoO-ness from speculative-ness?2010/07/21 07:05 AM
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                                        Software conventions?2010/07/22 05:43 AM
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                                        Pipelining affects the ISA?2010/07/22 10:58 PM
                                          Pipelining affects the ISA?2010/07/22 11:14 PM
                                          Pipelining affects the ISArwessel2010/07/23 12:03 AM
                                            Pipelining affects the ISA?2010/07/23 05:50 AM
                                            Pipelining affects the ISA?2010/07/23 06:10 AM
                                              Pipelining affects the ISAThiago Kurovski2010/07/23 02:59 PM
                                                Pipelining affects the ISAanon2010/07/24 07:35 AM
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                                            Pipelining affects the ISAIlleglWpns2010/07/26 05:14 AM
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                                      Separate the OoO-ness from speculative-nessanon2010/07/21 05:53 PM
                                        Separate the OoO-ness from speculative-ness?2010/07/22 04:15 AM
                                          Separate the OoO-ness from speculative-nessanon2010/07/22 04:27 AM
                                      Separate the OoO-ness from speculative-nessslacker2010/07/21 07:45 PM
                                        Separate the OoO-ness from speculative-nessanon2010/07/22 01:57 AM
                                        Separate the OoO-ness from speculative-ness?2010/07/22 05:26 AM
                                          Separate the OoO-ness from speculative-nessDan Downs2010/07/22 08:14 AM
                                          Confusing and not very useful definitionDavid Kanter2010/07/22 12:41 PM
                                            Confusing and not very useful definition?2010/07/22 10:58 PM
                                              Confusing and not very useful definitionUngo2010/07/24 12:06 PM
                                                Confusing and not very useful definition?2010/07/25 10:23 PM
                            Separate the OoO-ness from speculative-nesssomeone2010/07/20 08:02 PM
                              Separate the OoO-ness from speculative-nessThiago Kurovski2010/07/21 04:13 PM
            You are just quoting SINGLE precision flops? OMG what planet do you live? Vincent Diepeveen2010/07/19 10:26 AM
              The prior poster was talking about SP (NT)David Kanter2010/07/19 11:34 AM
                All FFT's need double precisionVincent Diepeveen2010/07/19 02:02 PM
                  All FFT's need double precisionDavid Kanter2010/07/19 02:09 PM
                    All FFT's need double precisionVincent Diepeveen2010/07/19 04:06 PM
                  All FFT's need double precision - notMichael S2010/07/20 01:16 AM
                    All FFT's need double precision - notUngo2010/07/21 12:04 AM
                      All FFT's need double precision - notMichael S2010/07/21 02:35 PM
                      All FFT's need double precision - notEduardoS2010/07/21 02:52 PM
                        All FFT's need double precision - notAnon2010/07/21 05:23 PM
                          All FFT's need double precision - notRicardo B2010/07/26 07:46 AM
                        I'm on a boat!anon2010/07/22 11:42 AM
                        All FFT's need double precision - notVincent Diepeveen2010/07/24 11:39 PM
                          All FFT's need double precision - notslacker2010/07/25 03:27 AM
                            All FFT's need double precision - notRicardo B2010/07/26 07:40 AM
                          All FFT's need double precision - notEduardoS2010/07/25 08:37 AM
                            All FFT's need double precision - notMichael S2010/07/25 10:43 AM
                    All FFT's need double precision - notVincent Diepeveen2010/07/24 11:19 PM
      A bit off baseEduardoS2010/07/08 04:08 PM
        A bit off baseGroo2010/07/08 06:11 PM
          A bit off basejohn mann2010/07/08 06:58 PM
            All right...let's cool it...David Kanter2010/07/08 07:54 PM
    A bit off baseVincent Diepeveen2010/07/19 03:36 PM
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