openpower/sv/rfc/ls006.fpintmv.mdwn

   1 # RFC ls006 FPR <-> GPR Move/Conversion
   2
   3 * Funded by NLnet under the Privacy and Enhanced Trust Programme, EU
   4   Horizon2020 Grant 825310, and NGI0 Entrust No 101069594
   5 * <https://libre-soc.org/openpower/sv/int_fp_mv/>
   6 * <https://libre-soc.org/openpower/sv/rfc/ls006.fpintmv/>
   7 * <https://bugs.libre-soc.org/show_bug.cgi?id=1015>
   8 * <https://git.openpower.foundation/isa/PowerISA/issues/todo>
   9
  10 **Severity**: Major
  11
  12 **Status**: New
  13
  14 **Date**: 20 Oct 2022
  15
  16 **Target**: v3.2B
  17
  18 **Source**: v3.1B
  19
  20 **Books and Section affected**: **UPDATE**
  21
  22 * Book I 4.6.5 Floating-Point Move Instructions
  23 * Book I 4.6.7.2 Floating-Point Convert To/From Integer Instructions
  24 * Appendix E Power ISA sorted by opcode
  25 * Appendix F Power ISA sorted by version
  26 * Appendix G Power ISA sorted by Compliancy Subset
  27 * Appendix H Power ISA sorted by mnemonic
  28
  29 **Summary**
  30
  31 Single-precision Instructions added:
  32
  33 * `mffprs` -- Move From FPR Single
  34 * `mtfprs` -- Move To FPR Single
  35 * `ctfprs` -- Convert To FPR Single
  36
  37 Identical (except Double-precision) Instructions added:
  38
  39 * `mffpr` -- Move From FPR
  40 * `mtfpr` -- Move To FPR
  41 * `cffpr` -- Convert From FPR
  42 * `ctfpr` -- Convert To FPR
  43
  44 **Submitter**: Luke Leighton (Libre-SOC)
  45
  46 **Requester**: Libre-SOC
  47
  48 **Impact on processor**:
  49
  50 * Addition of three new Single-Precision GPR-FPR-based instructions
  51 * Addition of four new Double-Precision GPR-FPR-based instructions
  52
  53 **Impact on software**:
  54
  55 * Requires support for new instructions in assembler, debuggers,
  56   and related tools.
  57
  58 **Keywords**:
  59
  60 ```
  61     GPR, FPR, Move, Conversion, ECMAScript, Saturating
  62 ```
  63
  64 **Motivation**
  65
  66 CPUs without VSX/VMX lack a way to efficiently transfer data between
  67 FPRs and GPRs, they need to go through memory, this proposal adds more
  68 efficient data transfer (both bitwise copy and Integer <-> FP conversion)
  69 instructions that transfer directly between FPRs and GPRs without needing
  70 to go through memory.
  71
  72 IEEE 754 does not specify what results are obtained when converting a NaN
  73 or out-of-range floating-point value to integer: consequently, different
  74 programming
  75 languages and ISAs have made different choices, making binary portability
  76 very difficult.  Below is an overview
  77 of the different variants, listing the languages and hardware that
  78 implements each variant.
  79
  80 **Notes and Observations**:
  81
  82 * These instructions are present in many other ISAs.
  83 * ECMAScript rounding as one instruction saves 32 scalar instructions
  84   including seven branch instructions.
  85 * Both sets are orthogonal (no difference except being Single/Double).
  86   This allows IBM to follow the pre-existing precedent of allocating
  87   separate Major Opcodes (PO) for Double-precision and Single-precision
  88   respectively.
  89
  90 **Changes**
  91
  92 Add the following entries to:
  93
  94 * Book I 4.6.5 Floating-Point Move Instructions
  95 * Book I 4.6.7.2 Floating-Point Convert To/From Integer Instructions
  96 * Book I 1.6.1 and 1.6.2
  97
  98 ----------------
  99
 100 \newpage{}
 101
 102 [[!inline pages="openpower/sv/int_fp_mv/cvt_fp_to_int_overview" raw=yes ]]
 103
 104 ----------
 105
 106 \newpage{}
 107
 108 [[!inline pages="openpower/sv/int_fp_mv/moves_and_conversions" raw=yes ]]
 109
 110 ----------
 111
 112 \newpage{}
 113
 114 # Instruction Formats
 115
 116 Add the following entries to Book I 1.6.1.19 XO-FORM:
 117
 118 ```
 119     |0    |6   |11  |13   |16   |21  |22  |31  |
 120     | PO  | RT | IT | CVM | FRB | OE | XO | Rc |
 121 ```
 122
 123 Add the following entries to Book I 1.6.1.15 X-FORM:
 124
 125 ```
 126     |0   |6    |11  |13  |16   |21  |31  |
 127     | PO | FRT | IT | // | RB  | XO | Rc |
 128     | PO | FRT | //      | RB  | XO | Rc |
 129     | PO | RT  | //      | FRB | XO | Rc |
 130 ```
 131
 132 # Instruction Fields
 133
 134 Add XO to FRB's Formats list in Book I 1.6.2 Word Instruction Fields.
 135
 136 Add XO to FRT's Formats list in Book I 1.6.2 Word Instruction Fields.
 137
 138 Add new fields:
 139
 140 ```
 141     IT (11:12)
 142         Field used to specify integer type for FPR <-> GPR conversions.
 143
 144         Formats: X, XO
 145
 146     CVM (13:15)
 147         Field used to specify conversion mode for
 148         integer -> floating-point conversion.
 149
 150         Formats: XO
 151 ```
 152
 153 ----------
 154
 155 \newpage{}
 156
 157 # Appendices
 158
 159     Appendix E Power ISA sorted by opcode
 160     Appendix F Power ISA sorted by version
 161     Appendix G Power ISA sorted by Compliancy Subset
 162     Appendix H Power ISA sorted by mnemonic
 163
 164 |Form| Book | Page | Version | mnemonic | Description |
 165 |----|------|------|---------|----------|-------------|
 166 |VA  | I    | #    | 3.2B    |todo   | |
 167
 168 ----------------
 169
 170 [[!tag opf_rfc]]