RFC ls006 FPR <-> GPR Move/Conversion

• Funded by NLnet under the Privacy and Enhanced Trust Programme, EU Horizon2020 Grant 825310, and NGI0 Entrust No 101069594
• https://libre-soc.org/openpower/sv/int_fp_mv/
• https://libre-soc.org/openpower/sv/rfc/ls006.fpintmv/
• https://bugs.libre-soc.org/show_bug.cgi?id=1015
• https://git.openpower.foundation/isa/PowerISA/issues/todo

Severity: Major

Status: New

Date: 09 Feb 2024 v2

Target: v3.2B

Source: v3.1B

Books and Section affected: UPDATE

• Book I 4.6.5 Floating-Point Move Instructions
• Book I 4.6.7.2 Floating-Point Convert To/From Integer Instructions
• 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

Single-precision Instructions added:

• mffprs -- Move From FPR Single
• mtfprs -- Move To FPR Single
• ctfprs -- Convert To FPR Single

Identical (except Double-precision) Instructions added:

• mffpr -- Move From FPR
• mtfpr -- Move To FPR
• cffpr -- Convert From FPR
• ctfpr -- Convert To FPR

Submitter: Luke Leighton (Libre-SOC)

Requester: Libre-SOC

Impact on processor:

• Addition of three new Single-Precision GPR-FPR-based instructions
• Addition of four new Double-Precision GPR-FPR-based instructions

Impact on software:

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

Keywords:

    GPR, FPR, Move, Conversion, ECMAScript, Saturating

Motivation

CPUs without VSX/VMX lack a way to efficiently transfer data between FPRs and GPRs, they need to go through memory, this proposal adds more efficient data transfer (both bitwise copy and Integer <-> FP conversion) instructions that transfer directly between FPRs and GPRs without needing to go through memory.

IEEE 754 does not specify what results are obtained when converting a NaN or out-of-range floating-point value to integer: consequently, different programming languages and ISAs have made different choices, making binary portability very difficult. Below is an overview of the different variants, listing the languages and hardware that implements each variant.

Notes and Observations:

• These instructions are present in many other ISAs.
• ECMAScript rounding as one instruction saves 32 scalar instructions including seven branch instructions.
• Both sets are orthogonal (no difference except being Single/Double). This allows IBM to follow the pre-existing precedent of allocating separate Major Opcodes (PO) for Double-precision and Single-precision respectively.

Changes

Add the following entries to:

• Book I 4.6.5 Floating-Point Move Instructions
• Book I 4.6.7.2 Floating-Point Convert To/From Integer Instructions
• Book I 1.6.1 and 1.6.2

\newpage{}

Floating-point to Integer Conversion Overview

IEEE 754 does not specify what results are obtained when converting a NaN or out-of-range floating-point value to integer, so different programming languages and ISAs have made different choices. The different conversion modes supported by the cffpr instruction are as follows:

• P-Type:

  Used by most other PowerISA instructions, as well as commonly used floating-point to integer conversions on x86.

• S-Type:

  Used for several notable programming languages:
  • Java's conversion from float/double to long/int¹
  • Rust's as operator²
  • LLVM's llvm.fptosi.sat³ and llvm.fptoui.sat⁴ intrinsics
  • SPIR-V's OpenCL dialect's OpConvertFToU⁵ and OpConvertFToS⁶ instructions when decorated with the SaturatedConversion⁷ decorator.
  • Also WebAssembly's trunc_sat_u⁸ and trunc_sat_s⁹ instructions,

• E-Type:

  Used for ECMAScript's ToInt32 abstract operation¹⁰. Also implemented in ARMv8.3A as the FJCVTZS instruction¹¹.

Floating-point to Integer Conversion Semantics Summary

Let round be the result of bfp_ROUND_TO_INTEGER(rmode, input). Let w be the number of bits in the result's type. The result of Floating-point to Integer conversion is as follows:

+------+------+---------------------------------------------------------------+
|Type| Result | Category of rounding                                          |
|    | Sign   +----------+-----------+----------+-----------+---------+-------+
|    |        | NaN      | +Inf      | -Inf     | > Max     | < Min   | Else  |
|    |        |          |           |          | Possible  | Possible|       |
|    |        |          |           |          | Result    | Result  |       |
+----+--------+----------+-----------+----------+-----------+---------+-------+
|  P |Unsigned| 0        | 2^w - 1   | 0        | 2^w - 1   | 0       | round |
|    +--------+----------+-----------+----------+-----------+---------+-------+
|    | Signed | -2^(w-1) | 2^(w-1)-1 | -2^(w-1) | 2^(w-1)-1 | -2^(w-1)| round |
+----+--------+----------+-----------+----------+-----------+---------+-------+
|  S |Unsigned| 0        | 2^w - 1   | 0        | 2^w - 1   | 0       | round |
|    +--------+----------+-----------+----------+-----------+---------+-------+
|    | Signed | 0        | 2^(w-1)-1 | -2^(w-1) | 2^(w-1)-1 | -2^(w-1)| round |
+----+--------+----------+-----------+----------+-----------+---------+-------+
|  E | Either | 0                               | round & (2^w - 1)           |
+----+--------+---------------------------------+-----------------------------+

\newpage{}

Immediate Tables

Tables that are used by mffpr[s][.]/mtfpr[s]/cffpr[o][.]/ctfpr[s][.]:

IT -- Integer Type

CVM -- Float to Integer Conversion Mode

\newpage{}

Move To/From Floating-Point Register Instructions

These instructions perform a copy from one register file to another, as if by using a GPR/FPR store, followed by a FPR/GPR load.

Move From Floating-Point Register

    mffpr RT, FRB
    mffpr. RT, FRB

    RT <- (FRB)

The contents of FPR[FRB] are placed into GPR[RT].

Special Registers altered:

    CR0     (if Rc=1)

Architecture Note:

mffpr is equivalent to the combination of stfd followed by ld.

Architecture Note:

mffpr is a separate instruction from mfvsrd because mfvsrd requires VSX which may not be available on simpler implementations. Additionally, SVP64 may treat VSX instructions differently than SFFS instructions in a future version of the architecture.

Move From Floating-Point Register Single

    mffprs RT, FRB
    mffprs. RT, FRB

    RT <- [0] * 32 || SINGLE((FRB))

The contents of FPR[FRB] are converted to BFP32 by using SINGLE, then zero-extended to 64-bits, and the result stored in GPR[RT].

Special Registers altered:

    CR0     (if Rc=1)

Architecture Note:

mffprs is equivalent to the combination of stfs followed by lwz.

\newpage{}

Move To Floating-Point Register

    mtfpr FRT, RB

    FRT <- (RB)

The contents of GPR[RB] are placed into FPR[FRT].

Special Registers altered:

    None

Architecture Note:

mtfpr is equivalent to the combination of std followed by lfd.

Architecture Note:

mtfpr is a separate instruction from mtvsrd because mtvsrd requires VSX which may not be available on simpler implementations. Additionally, SVP64 may treat VSX instructions differently than SFFS instructions in a future version of the architecture.

Move To Floating-Point Register Single

    mtfprs FRT, RB

    FRT <- DOUBLE((RB)[32:63])

The contents of bits 32:63 of GPR[RB] are converted to BFP64 by using DOUBLE, then the result is stored in GPR[RT].

Special Registers altered:

    None

Architecture Note:

mtfprs is equivalent to the combination of stw followed by lfs.

\newpage{}

Conversion To/From Floating-Point Register Instructions

Convert To Floating-Point Register

    ctfpr FRT, RB, IT
    ctfpr. FRT, RB, IT

    if IT[0] = 0 then  # 32-bit int -> 64-bit float
        # rounding never necessary, so don't touch FPSCR
        # based off xvcvsxwdp
        if IT = 0 then  # Signed 32-bit
            src <- bfp_CONVERT_FROM_SI32((RB)[32:63])
        else  # IT = 1 -- Unsigned 32-bit
            src <- bfp_CONVERT_FROM_UI32((RB)[32:63])
        FRT <- bfp64_CONVERT_FROM_BFP(src)
    else
        # rounding may be necessary. based off xscvuxdsp
        reset_xflags()
        switch(IT)
            case(0):  # Signed 32-bit
                src <- bfp_CONVERT_FROM_SI32((RB)[32:63])
            case(1):  # Unsigned 32-bit
                src <- bfp_CONVERT_FROM_UI32((RB)[32:63])
            case(2):  # Signed 64-bit
                src <- bfp_CONVERT_FROM_SI64((RB))
            default:  # Unsigned 64-bit
                src <- bfp_CONVERT_FROM_UI64((RB))
        rnd <- bfp_ROUND_TO_BFP64(0b0, FPSCR.RN, src)
        result <- bfp64_CONVERT_FROM_BFP(rnd)
        cls <- fprf_CLASS_BFP64(result)

        if xx_flag = 1 then SetFX(FPSCR.XX)

        FRT <- result
        FPSCR.FPRF <- cls
        FPSCR.FR <- inc_flag
        FPSCR.FI <- xx_flag

Convert from a unsigned/signed 32/64-bit integer in RB to a 64-bit float in FRT.

If converting from a unsigned/signed 32-bit integer to a 64-bit float, rounding is never necessary, so FPSCR is unmodified and exceptions are never raised. Otherwise, FPSCR is modified and exceptions are raised as usual.

Rc=1 tests FRT and sets CR1, exactly like all other Scalar Floating-Point operations.

Special Registers altered:

    CR1               (if Rc=1)
    FPRF FR FI FX XX  (if IT[0]=1)

Assembly Aliases

\newpage{}

Convert To Floating-Point Register Single

    ctfprs FRT, RB, IT
    ctfprs. FRT, RB, IT

    # rounding may be necessary. based off xscvuxdsp
    reset_xflags()
    switch(IT)
        case(0):  # Signed 32-bit
            src <- bfp_CONVERT_FROM_SI32((RB)[32:63])
        case(1):  # Unsigned 32-bit
            src <- bfp_CONVERT_FROM_UI32((RB)[32:63])
        case(2):  # Signed 64-bit
            src <- bfp_CONVERT_FROM_SI64((RB))
        default:  # Unsigned 64-bit
            src <- bfp_CONVERT_FROM_UI64((RB))
    rnd <- bfp_ROUND_TO_BFP32(FPSCR.RN, src)
    result32 <- bfp32_CONVERT_FROM_BFP(rnd)
    cls <- fprf_CLASS_BFP32(result32)
    result <- DOUBLE(result32)

    if xx_flag = 1 then SetFX(FPSCR.XX)

    FRT <- result
    FPSCR.FPRF <- cls
    FPSCR.FR <- inc_flag
    FPSCR.FI <- xx_flag

Convert from a unsigned/signed 32/64-bit integer in RB to a 32-bit float in FRT, following the usual 32-bit float in 64-bit float format. FPSCR is modified and exceptions are raised as usual.

Rc=1 tests FRT and sets CR1, exactly like all other Scalar Floating-Point operations.

Special Registers altered:

    CR1     (if Rc=1)
    FPRF FR FI FX XX

Assembly Aliases

\newpage{}

Convert From Floating-Point Register

    cffpr RT, FRB, CVM, IT
    cffpr. RT, FRB, CVM, IT
    cffpro RT, FRB, CVM, IT
    cffpro. RT, FRB, CVM, IT

    # based on xscvdpuxws
    reset_xflags()
    src <- bfp_CONVERT_FROM_BFP64((FRB))

    switch(IT)
        case(0):  # Signed 32-bit
            range_min <- bfp_CONVERT_FROM_SI32(0x8000_0000)
            range_max <- bfp_CONVERT_FROM_SI32(0x7FFF_FFFF)
            js_mask <- 0x0000_0000_FFFF_FFFF
        case(1):  # Unsigned 32-bit
            range_min <- bfp_CONVERT_FROM_UI32(0)
            range_max <- bfp_CONVERT_FROM_UI32(0xFFFF_FFFF)
            js_mask <- 0x0000_0000_FFFF_FFFF
        case(2):  # Signed 64-bit
            range_min <- bfp_CONVERT_FROM_SI64(-0x8000_0000_0000_0000)
            range_max <- bfp_CONVERT_FROM_SI64(0x7FFF_FFFF_FFFF_FFFF)
            js_mask <- 0xFFFF_FFFF_FFFF_FFFF
        default:  # Unsigned 64-bit
            range_min <- bfp_CONVERT_FROM_UI64(0)
            range_max <- bfp_CONVERT_FROM_UI64(0xFFFF_FFFF_FFFF_FFFF)
            js_mask <- 0xFFFF_FFFF_FFFF_FFFF

    if (CVM[2] = 1) | (FPSCR.RN = 0b01) then
        rnd <- bfp_ROUND_TO_INTEGER_TRUNC(src)
    else if FPSCR.RN = 0b00 then
        rnd <- bfp_ROUND_TO_INTEGER_NEAR_EVEN(src)
    else if FPSCR.RN = 0b10 then
        rnd <- bfp_ROUND_TO_INTEGER_CEIL(src)
    else if FPSCR.RN = 0b11 then
        rnd <- bfp_ROUND_TO_INTEGER_FLOOR(src)

    switch(CVM)
        case(0, 1):  # P-Type
            if IsNaN(rnd) then
                result <- si64_CONVERT_FROM_BFP(range_min)
            else if bfp_COMPARE_GT(rnd, range_max) then
                result <- ui64_CONVERT_FROM_BFP(range_max)
            else if bfp_COMPARE_LT(rnd, range_min) then
                result <- si64_CONVERT_FROM_BFP(range_min)
            else if IT[1] = 1 then  # Unsigned 32/64-bit
                result <- ui64_CONVERT_FROM_BFP(rnd)
            else  # Signed 32/64-bit
                result <- si64_CONVERT_FROM_BFP(rnd)
        case(2, 3):  # S-Type
            if IsNaN(rnd) then
                result <- [0] * 64
            else if bfp_COMPARE_GT(rnd, range_max) then
                result <- ui64_CONVERT_FROM_BFP(range_max)
            else if bfp_COMPARE_LT(rnd, range_min) then
                result <- si64_CONVERT_FROM_BFP(range_min)
            else if IT[1] = 1 then  # Unsigned 32/64-bit
                result <- ui64_CONVERT_FROM_BFP(rnd)
            else  # Signed 32/64-bit
                result <- si64_CONVERT_FROM_BFP(rnd)
        default:  # E-Type
            # CVM = 6, 7 are illegal instructions
            # using a 128-bit intermediate works here because the largest type
            # this instruction can convert from has 53 significand bits, and
            # the largest type this instruction can convert to has 64 bits,
            # and the sum of those is strictly less than the 128 bits of the
            # intermediate result.
            limit <- bfp_CONVERT_FROM_UI128([1] * 128)
            if IsInf(rnd) | IsNaN(rnd) then
                result <- [0] * 64
            else if bfp_COMPARE_GT(bfp_ABSOLUTE(rnd), limit) then
                result <- [0] * 64
            else
                result128 <- si128_CONVERT_FROM_BFP(rnd)
                result <- result128[64:127] & js_mask

    switch(IT)
        case(0):  # Signed 32-bit
            result <- EXTS64(result[32:63])
            result_bfp <- bfp_CONVERT_FROM_SI32(result[32:63])
        case(1):  # Unsigned 32-bit
            result <- EXTZ64(result[32:63])
            result_bfp <- bfp_CONVERT_FROM_UI32(result[32:63])
        case(2):  # Signed 64-bit
            result_bfp <- bfp_CONVERT_FROM_SI64(result)
        default:  # Unsigned 64-bit
            result_bfp <- bfp_CONVERT_FROM_UI64(result)

    overflow <- 0  # signals SO only when OE = 1
    if IsNaN(src) | ¬bfp_COMPARE_EQ(rnd, result_bfp) then
        overflow <- 1  # signals SO only when OE = 1
        vxcvi_flag <- 1
        xx_flag <- 0
        inc_flag <- 0
    else
        xx_flag <- ¬bfp_COMPARE_EQ(src, result_bfp)
        inc_flag <- bfp_COMPARE_GT(bfp_ABSOLUTE(result_bfp), bfp_ABSOLUTE(src))

    if vxsnan_flag = 1 then SetFX(FPSCR.VXSNAN)
    if vxcvi_flag = 1 then SetFX(FPSCR.VXCVI)
    if xx_flag = 1 then SetFX(FPSCR.XX)

    vx_flag <- vxsnan_flag | vxcvi_flag
    vex_flag <- FPSCR.VE & vx_flag
    if vex_flag = 0 then
        RT <- result
        FPSCR.FPRF <- undefined
        FPSCR.FR <- inc_flag
        FPSCR.FI <- xx_flag
    else
        FPSCR.FR <- 0
        FPSCR.FI <- 0

Convert from 64-bit float in FRB to a unsigned/signed 32/64-bit integer in RT, with the conversion overflow/rounding semantics following the chosen CVM value. FPSCR is modified and exceptions are raised as usual.

This instruction has an Rc=1 mode which sets CR0 in the normal way for any instructions producing a GPR result. Additionally, when OE=1, if the numerical value of the FP number is not 100% accurately preserved (due to truncation or saturation and including when the FP number was NaN) then this is considered to be an Integer Overflow condition, and CR0.SO, XER.SO and XER.OV are all set as normal for any GPR instructions that overflow. When RT is not written (vex_flag = 1), all CR0 bits except SO are undefined.

Special Registers altered:

    CR0              (if Rc=1)
    XER SO, OV, OV32 (if OE=1)
    FPRF=0bUUUUU FR FI FX XX VXSNAN VXCV

Assembly Aliases

\newpage{}

Instruction Formats

Add the following entries to Book I 1.6.1.19 XO-FORM:

    |0    |6   |11  |13   |16   |21  |22  |31  |
    | PO  | RT | IT | CVM | FRB | OE | XO | Rc |

Add the following entries to Book I 1.6.1.15 X-FORM:

    |0   |6    |11  |13  |16   |21  |31  |
    | PO | FRT | IT | // | RB  | XO | Rc |
    | PO | FRT | //      | RB  | XO | Rc |
    | PO | RT  | //      | FRB | XO | Rc |

Instruction Fields

Add XO to FRB's Formats list in Book I 1.6.2 Word Instruction Fields.

Add XO to FRT's Formats list in Book I 1.6.2 Word Instruction Fields.

Add new fields:

    IT (11:12)
        Field used to specify integer type for FPR <-> GPR conversions.

        Formats: X, XO

    CVM (13:15)
        Field used to specify conversion mode for
        integer -> floating-point conversion.

        Formats: XO

\newpage{}

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