openpower/sv/rfc/ls006.fpintmv.mdwn

   1 # RFC ls006 FPR <-> GPR Move/Conversion
   2
   3 **URLs**:
   4
   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 * `fmvtgs` -- Single-Precision Floating Move To GPR
  34 * `fmvfgs` -- Single-Precision Floating Move From GPR
  35 * `fcvtfgs` -- Single-Precision Floating Convert From Integer In GPR
  36
  37 Identical (except Double-precision) Instructions added:
  38
  39 * `fmvtg` -- Double-Precision Floating Move To GPR
  40 * `fmvfg` -- Double-Precision Floating Move From GPR
  41 * `fcvttg` -- Double-Precision Floating Convert To Integer In GPR
  42 * `fcvtfg` -- Double-Precision Floating Convert From Integer In GPR
  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, JavaScript
  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 doesn't specify what results are obtained when converting a NaN
  73 or out-of-range floating-point value to integer, so different programming
  74 languages and ISAs have made different choices.  Below is an overview
  75 of the different variants, listing the languages and hardware that
  76 implements each variant.
  77
  78 **Notes and Observations**:
  79
  80 * These instructions are present in many other ISAs.
  81 * JavaScript rounding as one instruction saves 32 scalar instructions
  82   including seven branch instructions.
  83 * Both sets are orthogonal (no difference except being Single/Double).
  84   This allows IBM to follow the pre-existing precedent of allocating
  85   separate Major Opcodes (PO) for Double-precision and Single-precision
  86   respectively.
  87
  88 **Changes**
  89
  90 Add the following entries to:
  91
  92 * Book I 4.6.5 Floating-Point Move Instructions
  93 * Book I 4.6.7.2 Floating-Point Convert To/From Integer Instructions
  94 * Book I 1.6.1 and 1.6.2
  95
  96 ----------------
  97
  98 \newpage{}
  99
 100 [[!inline pages="openpower/sv/int_fp_mv/moves_and_conversions" raw=yes ]]
 101
 102 ----------
 103
 104 \newpage{}
 105
 106 ----------
 107
 108 # Instruction Formats
 109
 110 Add the following entries to Book I 1.6.1.19 XO-FORM:
 111
 112 ```
 113     |0    |6   |11  |13   |16   |21  |22  |31  |
 114     | PO  | RT | IT | CVM | FRB | OE | XO | Rc |
 115 ```
 116
 117 Add the following entries to Book I 1.6.1.15 X-FORM:
 118
 119 ```
 120     |0   |6    |11  |13  |16   |21  |31  |
 121     | PO | FRT | IT | // | RB  | XO | Rc |
 122     | PO | FRT | //      | RB  | XO | Rc |
 123     | PO | RT  | //      | FRB | XO | Rc |
 124 ```
 125
 126 # Instruction Fields
 127
 128 Add XO to FRB's Formats list in Book I 1.6.2 Word Instruction Fields.
 129
 130 Add XO to FRT's Formats list in Book I 1.6.2 Word Instruction Fields.
 131
 132 Add new fields:
 133
 134 ```
 135     IT (11:12)
 136         Field used to specify integer type for FPR <-> GPR conversions.
 137
 138         Formats: X, XO
 139
 140     CVM (13:15)
 141         Field used to specify conversion mode for
 142         integer -> floating-point conversion.
 143
 144         Formats: XO
 145 ```
 146
 147 ----------
 148
 149 \newpage{}
 150
 151 ----------
 152
 153 # Appendices
 154
 155     Appendix E Power ISA sorted by opcode
 156     Appendix F Power ISA sorted by version
 157     Appendix G Power ISA sorted by Compliancy Subset
 158     Appendix H Power ISA sorted by mnemonic
 159
 160 |Form| Book | Page | Version | mnemonic | Description |
 161 |----|------|------|---------|----------|-------------|
 162 |VA  | I    | #    | 3.2B    |todo   | |
 163
 164 ----------------
 165
 166 [[!tag opf_rfc]]