Number of GPRs

Article: ARM Goes 64-bit
By: bakaneko (nyan.delete@this.hyan.wan), August 17, 2012 3:09 am
Room: Moderated Discussions



Kenneth Jonsson (kj.delete@this.localhost.org) on August 16, 2012 2:35 pm wrote:
> I have never developed a compiler, but looking at code generated by GCC for ARM,
> x86_64, MIPS and PPC begs the question: is there really any real benefit in
> having more than 16 GPRs? I cannot really see that the 16GPRs that ARM and
> x86_64 has is a limitation in close to 100% of the functions, a large number of
> the functions uses less than 8 registers. This is also true on PPC and MIPS
> where 32 GPRs are available, most register seem to just sit there
> unused.
>
> More register does increase the cost of switching context, so unused
> register cannot be a good thing.

A context switch is a simple read/write from a linear
region.

What makes them expensive is all the lost state for
prefetching, predicting and the page tables.

The decision to use 32 64bit registers is pragmatic, but I
don't think it is good reasoning:

Warning: the following is all assumption

ARM wants to use OoO, thus they have to treat banked
registers the same as normal registers and let them use
the same set of shadow registers, otherwise the micro-
architecture won't be efficient.

In 64bit mode there are no banked registers. But they still
use shadow registers if at any point in time any 32bit
legacy code ran.

If there was no access to the banked registers as normal
registers in 64bit mode, they would be lost outside 32bit
mode.

64bit mode needs more/bigger registers anyway (they
implement a 64bit register as 2x32bit registers if I
interpret the ISA correctly), so losing shadow registers
would hurt performance.

ARM needs to make sure the banked registers don't waste
important resources and the easiest way is to give them to
the programmer as normal registers.

And because they want to make it easy to program 32bit mode
from 64bit mode, they give these former shadowed registers
R16-R31.

And this is were legacy support bites them. 32 registers
aren't necessary
. 16-24 could be good enough, with the
other 8 reserved for special encodings. But because it works
so well with the microarchitecture and allows legacy support
they do it.

ARM isn't x86, and 64bit could have given them a clean
break with the legacy interrupt mechanism, which isn't
usable for embedded code anyway which wants 64bit mode and
fast interrupts at the same time.

But the virtualisation 4x2 matrix is as much of a nightmare,
so I'm assuming it's just bad decisions everywhere.

*otherwise they all could point to an imaginary 0 register.
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TopicPosted ByDate
New Article: ARM Goes 64-bitDavid Kanter08/14/12 12:04 AM
  New Article: ARM Goes 64-bitnone08/14/12 12:44 AM
    New Article: ARM Goes 64-bitDavid Kanter08/14/12 01:04 AM
    MIPS MT-ASEPaul A. Clayton08/14/12 09:01 AM
      MONITOR/MWAITEduardoS08/14/12 10:08 AM
        MWAIT not specifically MTPaul A. Clayton08/14/12 10:36 AM
          MWAIT not specifically MTEduardoS08/15/12 03:16 PM
        MONITOR/MWAITanonymou508/14/12 11:07 AM
          MONITOR/MWAITEduardoS08/15/12 03:20 PM
      MIPS MT-ASErwessel08/14/12 10:14 AM
  New Article: ARM Goes 64-bitSHK08/14/12 02:01 AM
  New Article: ARM Goes 64-bitanon08/14/12 02:37 AM
    New Article: ARM Goes 64-bitRichard Cownie08/14/12 03:57 AM
      New Article: ARM Goes 64-bitanon08/14/12 04:29 AM
      New Article: ARM Goes 64-bitnone08/14/12 04:44 AM
        New Article: ARM Goes 64-bitanon08/14/12 05:28 AM
          New Article: ARM Goes 64-bitanon08/14/12 05:32 AM
            New Article: ARM Goes 64-bitEduardoS08/14/12 06:06 AM
          New Article: ARM Goes 64-bitnone08/14/12 05:40 AM
            AArch64 select better than cmovPaul A. Clayton08/14/12 06:08 AM
            New Article: ARM Goes 64-bitanon08/14/12 06:12 AM
              New Article: ARM Goes 64-bitnone08/14/12 06:25 AM
                Predicated ld/store are usefulPaul A. Clayton08/14/12 06:48 AM
                  Predicated ld/store are usefulnone08/14/12 06:56 AM
                    Predicated ld/store are usefulanon08/14/12 07:07 AM
                    Predicated stores might not be that badPaul A. Clayton08/14/12 07:27 AM
                      Predicated stores might not be that badDavid Kanter08/15/12 01:14 AM
                        Predicated stores might not be that badMichael S08/15/12 11:41 AM
                        Predicated stores might not be that badR Byron08/17/12 04:09 AM
                New Article: ARM Goes 64-bitanon08/14/12 06:54 AM
                  New Article: ARM Goes 64-bitnone08/14/12 07:04 AM
                    New Article: ARM Goes 64-bitanon08/14/12 07:43 AM
          New Article: ARM Goes 64-bitEduardoS08/14/12 06:07 AM
            New Article: ARM Goes 64-bitanon08/14/12 06:20 AM
              New Article: ARM Goes 64-bitnone08/14/12 06:29 AM
                New Article: ARM Goes 64-bitanon08/14/12 07:00 AM
            New Article: ARM Goes 64-bitMichael S08/14/12 03:43 PM
        New Article: ARM Goes 64-bitRichard Cownie08/14/12 06:53 AM
          OT: Conrad's "Youth"Richard Cownie08/14/12 07:20 AM
      New Article: ARM Goes 64-bitEduardoS08/14/12 06:04 AM
        New Article: ARM Goes 64-bitmpx08/14/12 08:59 AM
          New Article: ARM Goes 64-bitAntti-Ville Tuunainen08/14/12 09:16 AM
        New Article: ARM Goes 64-bitanonymou508/14/12 11:03 AM
          New Article: ARM Goes 64-bitname9911/17/12 03:31 PM
            Microarchitecting a counter registerPaul A. Clayton11/17/12 07:37 PM
    New Article: ARM Goes 64-bitbakaneko08/14/12 04:21 AM
      New Article: ARM Goes 64-bitname9911/17/12 03:40 PM
        New Article: ARM Goes 64-bitEduardoS11/17/12 04:52 PM
        New Article: ARM Goes 64-bitDoug S11/17/12 05:48 PM
        New Article: ARM Goes 64-bitbakaneko11/18/12 05:40 PM
          New Article: ARM Goes 64-bitWilco11/19/12 07:59 AM
            New Article: ARM Goes 64-bitEduardoS11/19/12 08:23 AM
              New Article: ARM Goes 64-bitWilco11/19/12 09:31 AM
                Downloading µarch-specific binaries?Paul A. Clayton11/19/12 11:21 AM
                New Article: ARM Goes 64-bitEduardoS11/19/12 11:41 AM
                  New Article: ARM Goes 64-bitWilco11/21/12 07:44 AM
                    JIT vs. static compilation (Was: New Article: ARM Goes 64-bit)VMguy11/22/12 03:21 AM
                      JIT vs. static compilation (Was: New Article: ARM Goes 64-bit)David Kanter11/22/12 12:12 PM
                        JIT vs. static compilation (Was: New Article: ARM Goes 64-bit)Gabriele Svelto11/23/12 03:50 AM
                    New Article: ARM Goes 64-bitEduardoS11/23/12 10:09 AM
                      New Article: ARM Goes 64-bitEBFE11/26/12 01:24 AM
                        New Article: ARM Goes 64-bitGabriele Svelto11/26/12 03:33 AM
                          New Article: ARM Goes 64-bitEBFE11/27/12 11:17 PM
                            New Article: ARM Goes 64-bitGabriele Svelto11/28/12 02:32 AM
                        New Article: ARM Goes 64-bitEduardoS11/26/12 12:16 PM
                          New Article: ARM Goes 64-bitEBFE11/28/12 12:33 AM
                            New Article: ARM Goes 64-bitEduardoS11/28/12 05:53 AM
                              New Article: ARM Goes 64-bitMichael S11/28/12 06:15 AM
                                New Article: ARM Goes 64-bitEduardoS11/28/12 07:33 AM
                                  New Article: ARM Goes 64-bitMichael S11/28/12 09:16 AM
                                    New Article: ARM Goes 64-bitEduardoS11/28/12 09:53 AM
                                    New Article: ARM Goes 64-bitEugene Nalimov11/28/12 05:58 PM
                                      Amazing!EduardoS11/28/12 07:25 PM
                                        Amazing! (non-italic response)EduardoS11/28/12 07:25 PM
                                        Amazing!EBFE11/28/12 08:20 PM
                                          Undefined behaviour doubles downEduardoS11/28/12 09:10 PM
                              New Article: ARM Goes 64-bitEBFE11/28/12 07:54 PM
                                New Article: ARM Goes 64-bitEduardoS11/28/12 09:21 PM
                Have you heard of Transmeta?David Kanter11/19/12 03:47 PM
            New Article: ARM Goes 64-bitbakaneko11/19/12 09:08 AM
            New Article: ARM Goes 64-bitDavid Kanter11/19/12 03:40 PM
              Semantic Dictionary EncodingRay11/19/12 10:37 PM
              New Article: ARM Goes 64-bitRohit11/20/12 04:48 PM
                New Article: ARM Goes 64-bitDavid Kanter11/20/12 11:07 PM
                  New Article: ARM Goes 64-bitWilco11/21/12 06:41 AM
                    New Article: ARM Goes 64-bitDavid Kanter11/21/12 10:12 AM
                    A JIT exampleMark Roulo11/21/12 10:30 AM
                      A JIT exampleWilco11/21/12 07:04 PM
                        A JIT examplerwessel11/21/12 09:05 PM
                        A JIT exampleGabriele Svelto11/23/12 03:53 AM
                        A JIT exampleEduardoS11/23/12 10:13 AM
                          A JIT exampleWilco11/23/12 01:41 PM
                            A JIT exampleEduardoS11/23/12 02:06 PM
                            A JIT exampleGabriele Svelto11/23/12 04:09 PM
                              A JIT exampleSymmetry11/26/12 05:58 AM
            New Article: ARM Goes 64-bitRay11/19/12 10:27 PM
    New Article: ARM Goes 64-bitDavid Kanter08/14/12 09:11 AM
  v7-M is Thumb-onlyPaul A. Clayton08/14/12 06:58 AM
  Minor suggested correctionPaul A. Clayton08/14/12 08:33 AM
    Minor suggested correctionanon08/14/12 08:57 AM
  New Article: ARM Goes 64-bitExophase08/14/12 08:33 AM
    New Article: ARM Goes 64-bitDavid Kanter08/14/12 09:16 AM
      New Article: ARM Goes 64-bitjigal08/15/12 01:49 PM
  Correction re ARM and BBC MicroPaul08/14/12 08:59 PM
    Correction re ARM and BBC MicroPer Hesselgren08/15/12 03:27 AM
  Memory BW so lowPer Hesselgren08/15/12 03:14 AM
    Memory BW so lownone08/15/12 11:16 AM
  New Article: ARM Goes 64-bitdado08/15/12 10:25 AM
  Number of GPRsKenneth Jonsson08/16/12 02:35 PM
    Number of GPRsExophase08/16/12 02:52 PM
      Number of GPRsKenneth Jonsson08/17/12 02:41 AM
        Ooops, missing link...Kenneth Jonsson08/17/12 02:44 AM
        64-bit pointers eat some performancePaul A. Clayton08/17/12 06:19 AM
          64-bit pointers eat some performancebakaneko08/17/12 08:37 AM
            Brute force seems to workPaul A. Clayton08/17/12 10:08 AM
              Brute force seems to workbakaneko08/17/12 11:15 AM
          64-bit pointers eat some performanceRichard Cownie08/17/12 08:46 AM
            Pointer compression is atypicalPaul A. Clayton08/17/12 10:43 AM
              Pointer compression is atypicalRichard Cownie08/17/12 12:57 PM
                Pointer compression is atypicalHoward Chu08/22/12 10:17 PM
                  Pointer compression is atypicalRichard Cownie08/23/12 04:48 AM
                    Pointer compression is atypicalHoward Chu08/23/12 06:51 AM
              Pointer compression is atypicalWilco08/17/12 02:41 PM
                Pointer compression is atypicalRichard Cownie08/17/12 04:13 PM
                  Pointer compression is atypicalRicardo B08/19/12 10:44 AM
                  Pointer compression is atypicalHoward Chu08/22/12 10:08 PM
                    Unified libraries?Paul A. Clayton08/23/12 07:49 AM
                    Pointer compression is atypicalRichard Cownie08/23/12 08:44 AM
                      Pointer compression is atypicalHoward Chu08/23/12 05:17 PM
                        Pointer compression is atypicalanon08/23/12 08:15 PM
                          Pointer compression is atypicalHoward Chu08/23/12 09:33 PM
            64-bit pointers eat some performanceFoo_08/18/12 12:09 PM
              64-bit pointers eat some performanceRichard Cownie08/18/12 05:25 PM
                64-bit pointers eat some performanceRichard Cownie08/18/12 05:32 PM
            Page-related benefit of small pointersPaul A. Clayton08/23/12 08:36 AM
        Number of GPRsWilco08/17/12 06:31 AM
          Number of GPRsKenneth Jonsson08/17/12 11:54 AM
            Number of GPRsExophase08/17/12 12:44 PM
              Number of GPRsKenneth Jonsson08/17/12 01:22 PM
                Number of GPRsWilco08/17/12 02:53 PM
        What about dynamic utilization?Exophase08/17/12 09:30 AM
          Compiler vs. assembly aliasing knowledge?Paul A. Clayton08/17/12 10:20 AM
            Compiler vs. assembly aliasing knowledge?Exophase08/17/12 11:09 AM
            Compiler vs. assembly aliasing knowledge?anon08/18/12 02:23 AM
              Compiler vs. assembly aliasing knowledge?Ricardo B08/19/12 11:02 AM
                Compiler vs. assembly aliasing knowledge?anon08/19/12 06:07 PM
                  Compiler vs. assembly aliasing knowledge?Ricardo B08/19/12 07:26 PM
                    Compiler vs. assembly aliasing knowledge?anon08/19/12 10:03 PM
                      Compiler vs. assembly aliasing knowledge?anon08/20/12 01:59 AM
        Number of GPRsDavid Kanter08/17/12 12:46 PM
          RAT issues as part of reason 1Paul A. Clayton08/17/12 02:18 PM
        Number of GPRsname9911/17/12 06:37 PM
          Large ARFs increase renaming costPaul A. Clayton11/17/12 09:23 PM
    Number of GPRsDavid Kanter08/16/12 03:31 PM
    Number of GPRsRichard Cownie08/16/12 05:17 PM
    32 GPRs ~2-3%Paul A. Clayton08/16/12 06:27 PM
      Oops, Message-ID: aaed6e38-c7bd-467e-ba41-f40cf1020e5e@googlegroups.com (NT)Paul A. Clayton08/16/12 06:29 PM
      32 GPRs ~2-3%Exophase08/16/12 10:06 PM
        R31 as SP/zero is kind of neat (NT)Paul A. Clayton08/17/12 06:23 AM
        32 GPRs ~2-3%rwessel08/17/12 08:24 AM
          32 GPRs ~2-3%Exophase08/17/12 09:16 AM
            32 GPRs ~2-3%Max08/17/12 04:19 PM
      32 GPRs ~2-3%name9911/17/12 07:43 PM
    Number of GPRsmpx08/17/12 01:11 AM
      Latency and powerPaul A. Clayton08/17/12 06:54 AM
    Number of GPRsbakaneko08/17/12 03:09 AM
  New Article: ARM Goes 64-bitSteve08/17/12 02:12 PM
    New Article: ARM Goes 64-bitDavid Kanter08/19/12 12:42 PM
      New Article: ARM Goes 64-bitDoug S08/19/12 02:02 PM
      New Article: ARM Goes 64-bitAnon08/19/12 07:16 PM
      New Article: ARM Goes 64-bitSteve08/30/12 07:51 AM
  Scalar vs Vector registersRobert David Graham08/19/12 05:19 PM
    Scalar vs Vector registersDavid Kanter08/19/12 05:29 PM
  New Article: ARM Goes 64-bitBaserock ARM servers08/21/12 04:13 PM
    Baserock ARM serversSysanon08/21/12 04:14 PM
    A-15 virtualization and LPAE?Paul A. Clayton08/21/12 06:13 PM
      A-15 virtualization and LPAE?Anon08/21/12 07:13 PM
        Half-depth advantages?Paul A. Clayton08/21/12 08:42 PM
          Half-depth advantages?Anon08/22/12 03:33 PM
            Thanks for the information (NT)Paul A. Clayton08/22/12 04:04 PM
      A-15 virtualization and LPAE?C. Ladisch08/23/12 11:12 AM
        A-15 virtualization and LPAE?Paul08/23/12 03:17 PM
        Excessive pessimismPaul A. Clayton08/23/12 04:08 PM
          Excessive pessimismDavid Kanter08/23/12 05:05 PM
    New Article: ARM Goes 64-bitMichael S08/22/12 07:12 AM
      BTW, Baserock==product, Codethink==company (NT)Paul A. Clayton08/22/12 08:56 AM
  New Article: ARM Goes 64-bitReinoud Zandijk08/21/12 11:27 PM
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