RAT issues as part of reason 1

Article: ARM Goes 64-bit
By: Paul A. Clayton (paaronclayton.delete@this.gmail.com), August 17, 2012 2:18 pm
Room: Moderated Discussions
David Kanter (dkanter.delete@this.realworldtech.com) on August 17, 2012 12:46 pm wrote:
[snip]
> Can anyone add other drawbacks?

In the subthread starting here, issues involving the RAT were mentioned (a part of your reason 1). A larger architected register set also increases the overhead of making checkpoints for speculation recovery (if any are provided besides the retirement checkpoint) and restoring from checkpoints.

RAT and register file issues can be reduced by having an asymmetric register file. E.g., an ISA could provide an L2 register file which is only readable for one source operand in each instruction. Banking hints and guarantees might be useful (even in the register file of an OoO design). Preferential use of specific registers could also allow some optimizations (e.g., recent x86 optimizations with respect to the stack pointer). (An L2 register file might be handled something like a store queue and cache rather than with a complete RAT; if updates are infrequent, the extra energy to write values into the main L2 RF at commit might be less than the cost of memory operations that would replace L2 accesses.)

Providing the ability to zero registers would allow early register release (other small values might likewise be "inlined" into the 'physical' register name), reducing the overhead when registers are not actually being used (or are being used only for small values). (There are a lot of papers on dealing with register file size, power, and latency issues, especially for wide OoO designs.) I seem to recall that AVX provides special zeroing instructions; such could be used to free physical registers early and keep them free (using a special is_zero name in the RAT or a is_zero bit).

In theory, an ISA could moderate the multithreading penalty by supporting smaller contexts. Registers could be allocated to threads in groups of (e.g.) 8 registers with unallocated registers being treated as zero registers (or perhaps as shared registers [possibly with only the owner thread being allowed to write into such shared registers or with only privileged code being able to perform such writes]). (One could even imagine the use of such register blocks as something like register windows.)
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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
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          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
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            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
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        New Article: ARM Goes 64-bitRichard Cownie08/14/12 06:53 AM
          OT: Conrad's "Youth"Richard Cownie08/14/12 07:20 AM
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        New Article: ARM Goes 64-bitmpx08/14/12 08:59 AM
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        New Article: ARM Goes 64-bitanonymou508/14/12 11:03 AM
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            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
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                        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
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                                    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
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              Semantic Dictionary EncodingRay11/19/12 10:37 PM
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                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
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                        A JIT exampleEduardoS11/23/12 10:13 AM
                          A JIT exampleWilco11/23/12 01:41 PM
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            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
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    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
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          64-bit pointers eat some performancebakaneko08/17/12 08:37 AM
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              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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