RFC ls002.fmi v2 Floating-Point Load-Immediate

URLs:

• https://libre-soc.org/openpower/sv/int_fp_mv/#fmvis
• https://libre-soc.org/openpower/sv/rfc/ls002.fmi/
• https://bugs.libre-soc.org/show_bug.cgi?id=1092
• https://git.openpower.foundation/isa/PowerISA/issues/87

Severity: Major

Status: New

Date: 05 Oct 2022 v3 TODO

Target: v3.2B

Source: v3.0B

Books and Section affected:

    Book I Scalar Floating-Point 4.6.2.1
    Appendix E Power ISA sorted by opcode
    Appendix F Power ISA sorted by version
    Appendix G Power ISA sorted by Compliancy Subset
    Appendix H Power ISA sorted by mnemonic

Summary

    Instructions added
    fmvis - Floating-Point Move Immediate, Shifted
    fishmv - Floating-Point Immediate, Second-half Move

Submitter: Luke Leighton (Libre-SOC)

Requester: Libre-SOC

Impact on processor:

    Addition of two new FPR-based instructions

Impact on software:

    Requires support for new instructions in assembler, debuggers,
    and related tools.

Keywords:

    FPR, Floating-point, Load-immediate, BF16, bfloat16, BFP32

Motivation

Similar to lxvkq but extended to a bfloat16 with one 32-bit instruction and a full FP32 in two 32-bit instructions these instructions always save a Data Load and associated L1 and TLB lookup. Even quickly clearing an FPR to zero presently needs Load.

Notes and Observations:

1. There is no need for an Rc=1 variant because this is an immediate loading instruction (an FPR equivalent to li)
2. There is no need for Special Registers (FP Flags) because this is an immediate loading instruction. No FPR Load Operations alter FPSCR, neither does lxvkq, and on that basis neither should these instructions.
3. fishmv as a FRT-only Read-Modify-Write (instead of an unnecessary FRT,FRA pair) saves five potential bits, making the difference between a 5-bit XO (VA/DX-Form) and requiring an entire Primary Opcode.

Changes

Add the following entries to:

• the Appendices of Book I
• Instructions of Book I as a new Section 4.6.2.1
• DX-Form of Book I Section 1.6.1.6 and 1.6.2
• Floating-Point Data a Format of Book I Section 4.3.1

\newpage{}

Floating-Point Move Immediate

fmvis FRT, D

Pseudocode:

    bf16 <- d0 || d1 || d2  # create bfloat16 immediate
    bfp32 <- bf16 || [0]*16 # convert bfloat16 to BFP32
    FRT <- DOUBLE(bfp32)    # convert BFP32 to BFP64

Special registers altered:

None

The value D << 16 is interpreted as a 32-bit float, converted to a 64-bit float and written to FRT. This is equivalent to reinterpreting D as a bfloat16 and converting to 64-bit float.

Examples:

    fmvis f4, 0 # writes +0.0 to f4 (clears an FPR)
    fmvis f4, 0x8000 # writes -0.0 to f4
    fmvis f4, 0x3F80 # writes +1.0 to f4
    fmvis f4, 0xBFC0 # writes -1.5 to f4
    fmvis f4, 0x7FC0 # writes +qNaN to f4
    fmvis f4, 0x7F80 # writes +Infinity to f4
    fmvis f4, 0xFF80 # writes -Infinity to f4
    fmvis f4, 0x3FFF # writes +1.9921875 to f4

Floating-Point Immediate Second-Half Move

fishmv FRT, D

DX-Form:

Pseudocode:

    n <- (FRT)                      # read FRT
    bfp32 <- SINGLE(n)              # convert to BFP32
    bfp32[16:31] <- d0 || d1 || d2  # replace LSB half
    FRT <- DOUBLE(bfp32)            # convert back to BFP64

Special registers altered:

None

An additional 16-bits of immediate is inserted into the low-order half of the single-format value corresponding to the contents of FRT.

This instruction performs a Read-Modify-Write on FRT. In hardware, fishmv may be macro-op-fused with fmvis.

Programmer's note: The use of these two instructions is strategically similar to how li combined with oris may be used to construct 32-bit Integers. If a prior fmvis instruction had been used to set the upper 16-bits from a BFP32 value, fishmv may be used to set the lower 16-bits. Example:

    # these two combined instructions write 0x3f808000
    # into f4 as a BFP32 to be converted to a BFP64.
    # actual contents in f4 after conversion: 0x3ff0_1000_0000_0000
    # first the upper bits, happens to be +1.0
    fmvis f4, 0x3F80 # writes +1.0 to f4
    # now write the lower 16 bits of a BFP32
    fishmv f4, 0x8000 # writes +1.00390625 to f4

\newpage{}

DX-Form

Add the following to Book I, 1.6.1.6, DX-Form

  |0    |6   |11   |16   |26   |31
  | PO  | FRT|   d1|   d0|   XO|d2

Add DX to FRT Field in Book I, 1.6.2

 FRT (6:10)
     Field used to specify an FPR to be used as a
     source.
     Formats: D, X, DX

bfloat16 definition

Add the following to Book I, 4.3.1:

The format may be a 16-bit bfloat16, 32-bit single format for a single-precision value...

The bfloat16 format is used as an immediate.

The structure of the bfloat16, single and double formats is shown below.

  |S |EXP| FRACTION|
  |0 |1 8|9      15|

Figure #. Binary floating-point half-precision format (bfloat16)

Appendices

Appendix E Power ISA sorted by opcode
Appendix F Power ISA sorted by version
Appendix G Power ISA sorted by Compliancy Subset
Appendix H Power ISA sorted by mnemonic