--- /dev/null
+[[!tag standards]]
+
+# FPR-to-GPR and GPR-to-FPR
+
+TODO special constants instruction (e, tau/N, ln 2, sqrt 2, etc.) -- exclude any constants available through fmvis
+
+**Draft Status** under development, for submission as an RFC
+
+Links:
+
+* <https://bugs.libre-soc.org/show_bug.cgi?id=650>
+* <https://bugs.libre-soc.org/show_bug.cgi?id=230#c71>
+* <https://bugs.libre-soc.org/show_bug.cgi?id=230#c74>
+* <https://bugs.libre-soc.org/show_bug.cgi?id=230#c76>
+* <https://bugs.libre-soc.org/show_bug.cgi?id=887> fmvis
+* [[int_fp_mv/appendix]]
+* [[sv/rfc/ls002]] - `fmvis` and `fishmv` External RFC Formal Submission
+
+Trademarks:
+
+* Rust is a Trademark of the Rust Foundation
+* Java and Javascript are Trademarks of Oracle
+* LLVM is a Trademark of the LLVM Foundation
+* SPIR-V is a Trademark of the Khronos Group
+* OpenCL is a Trademark of Apple, Inc.
+
+Referring to these Trademarks within this document
+is by necessity, in order to put the semantics of each language
+into context, and is considered "fair use" under Trademark
+Law.
+
+Introduction:
+
+High-performance CPU/GPU software needs to often convert between integers
+and floating-point, therefore fast conversion/data-movement instructions
+are needed. Also given that initialisation of floats tends to take up
+considerable space (even to just load 0.0) the inclusion of two compact
+format float immediate instructions is up for consideration using 16-bit
+immediates. BF16 is one of the formats: a second instruction allows a full
+accuracy FP32 to be constructed.
+
+Libre-SOC will be compliant with the
+**Scalar Floating-Point Subset** (SFFS) i.e. is not implementing VMX/VSX,
+and with its focus on modern 3D GPU hybrid workloads represents an
+important new potential use-case for OpenPOWER.
+
+Prior to the formation of the Compliancy Levels first introduced
+in v3.0C and v3.1
+the progressive historic development of the Scalar parts of the Power ISA assumed
+that VSX would always be there to complement it. However With VMX/VSX
+**not available** in the newly-introduced SFFS Compliancy Level, the
+existing non-VSX conversion/data-movement instructions require
+a Vector of load/store
+instructions (slow and expensive) to transfer data between the FPRs and
+the GPRs. For a modern 3D GPU this kills any possibility of a
+competitive edge.
+Also, because SimpleV needs efficient scalar instructions in
+order to generate efficient vector instructions, adding new instructions
+for data-transfer/conversion between FPRs and GPRs multiplies the savings.
+
+In addition, the vast majority of GPR <-> FPR data-transfers are as part
+of a FP <-> Integer conversion sequence, therefore reducing the number
+of instructions required is a priority.
+
+Therefore, we are proposing adding:
+
+* FPR load-immediate instructions, one equivalent to `BF16`, the
+ other increasing accuracy to `FP32`
+* FPR <-> GPR data-transfer instructions that just copy bits without conversion
+* FPR <-> GPR combined data-transfer/conversion instructions that do
+ Integer <-> FP conversions
+
+If adding new Integer <-> FP conversion instructions,
+the opportunity may be taken to modernise the instructions and make them
+well-suited for common/important conversion sequences:
+
+* **standard IEEE754** - used by most languages and CPUs
+* **standard OpenPOWER** - saturation with NaN
+ converted to minimum valid integer
+* **Java** - saturation with NaN converted to 0
+* **JavaScript** - modulo wrapping with Inf/NaN converted to 0
+
+The assembly listings in the [[int_fp_mv/appendix]] show how costly
+some of these language-specific conversions are: Javascript, the
+worst case, is 32 scalar instructions including seven branch instructions.
+
+# Proposed New Scalar Instructions
+
+All of the following instructions use the standard OpenPower conversion to/from 64-bit float format when reading/writing a 32-bit float from/to a FPR. All integers however are sourced/stored in the *GPR*.
+
+Integer operands and results being in the GPR is the key differentiator between the proposed instructions
+(the entire rationale) compared to existing Scalar Power ISA.
+In all existing Power ISA Scalar conversion instructions, all
+operands are FPRs, even if the format of the source or destination
+data is actually a scalar integer.
+
+*(The existing Scalar instructions being FP-FP only is based on an assumption
+that VSX will be implemented, and VSX is not part of the SFFS Compliancy
+Level. An earlier version of the Power ISA used to have similar
+FPR<->GPR instructions to these:
+they were deprecated due to this incorrect assumption that VSX would
+always be present).*
+
+Note that source and destination widths can be overridden by SimpleV
+SVP64, and that SVP64 also has Saturation Modes *in addition*
+to those independently described here. SVP64 Overrides and Saturation
+work on *both* Fixed *and* Floating Point operands and results.
+ The interactions with SVP64
+are explained in the [[int_fp_mv/appendix]]
+
+# Float load immediate <a name="fmvis"></a>
+
+These are like a variant of `fmvfg` and `oris`, combined.
+Power ISA currently requires a large
+number of instructions to get Floating Point constants into registers.
+`fmvis` on its own is equivalent to BF16 to FP32/64 conversion,
+but if followed up by `fishmv` an additional 16 bits of accuracy in the
+mantissa may be achieved.
+
+These instructions **always** save
+resources compared to FP-load for exactly the same reason
+that `li` saves resources: an L1-Data-Cache and memory read
+is avoided.
+
+*IBM may consider it worthwhile to extend these two instructions to
+v3.1 Prefixed (`pfmvis` and `pfishmv`: 8RR, imm0 extended).
+If so it is recommended that
+`pfmvis` load a full FP32 immediate and `pfishmv` supplies the three high
+missing exponent bits (numbered 8 to 10) and the lower additional
+29 mantissa bits (23 to 51) needed to construct a full FP64 immediate.
+Strictly speaking the sequence `fmvis fishmv pfishmv` achieves the
+same effect in the same number of bytes as `pfmvis pfishmv`,
+making `pfmvis` redundant.*
+
+Just as Floating-point Load does not set FP Flags neither does fmvis or fishmv.
+As fishmv is specifically intended to work in conjunction with fmvis
+to provide additional accuracy, all bits other than those which
+would have been set by a prior fmvis instruction are deliberately ignored.
+(If these instructions involved reading from registers rather than immediates
+it would be a different story).
+
+## Load BF16 Immediate
+
+`fmvis FRS, D`
+
+Reinterprets `D << 16` as a 32-bit float, which is then converted to a
+64-bit float and written to `FRS`. This is equivalent to reinterpreting
+`D` as a `BF16` and converting to 64-bit float.
+There is no need for an Rc=1 variant because this is an immediate loading
+instruction.
+
+Example:
+
+```
+# clearing a FPR
+fmvis f4, 0 # writes +0.0 to f4
+# loading handy constants
+fmvis f4, 0x8000 # writes -0.0 to f4
+fmvis f4, 0x3F80 # writes +1.0 to f4
+fmvis f4, 0xBF80 # 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
+
+# clearing 128 FPRs with 2 SVP64 instructions
+# by issuing 32 vec4 (subvector length 4) ops
+setvli VL=MVL=32
+sv.fmvis/vec4 f0, 0 # writes +0.0 to f0-f127
+```
+Important: If the float load immediate instruction(s) are left out,
+change all [GPR to FPR conversion instructions](#GPR-to-FPR-conversions)
+to instead write `+0.0` if `RA` is register `0`, at least
+allowing clearing FPRs.
+
+`fmvis` fits with DX-Form:
+
+| 0-5 | 6-10 | 11-15 | 16-25 | 26-30 | 31 | Form |
+|--------|------|-------|-------|-------|-----|---------|
+| Major | FRS | d1 | d0 | XO | d2 | DX-Form |
+
+Pseudocode:
+
+ bf16 = d0 || d1 || d2 # create BF16 immediate
+ fp32 = bf16 || [0]*16 # convert BF16 to FP32
+ FRS = DOUBLE(fp32) # convert FP32 to FP64
+
+Special registers altered:
+
+ None
+
+## Float Immediate Second-Half MV <a name="fishmv"></a>
+
+`fishmv FRS, D`
+
+DX-Form:
+
+| 0-5 | 6-10 | 11-15 | 16-25 | 26-30 | 31 | Form |
+|--------|------|-------|-------|-------|-----|---------|
+| Major | FRS | d1 | d0 | XO | d2 | DX-Form |
+
+Strategically similar to how `oris` is used to construct
+32-bit Integers, an additional 16-bits of immediate is
+inserted into `FRS` to extend its accuracy to
+a full FP32 (stored as usual in FP64 Format within the FPR).
+If a prior `fmvis` instruction had been used to
+set the upper 16-bits of an FP32 value, `fishmv` contains the
+lower 16-bits.
+
+The key difference between using `li` and `oris` to construct 32-bit
+GPR Immediates and `fishmv` is that the `fmvis` will have converted
+the `BF16` immediate to FP64 (Double) format.
+This is taken into consideration
+as can be seen in the pseudocode below.
+
+Pseudocode:
+
+ fp32 <- SINGLE((FRS)) # convert to FP32
+ fp32[16:31] <- d0 || d1 || d2 # replace LSB half
+ FRS <- DOUBLE(fp32) # convert back to FP64
+
+Special registers altered:
+
+ None
+
+**This instruction performs a Read-Modify-Write.** *FRS is read, the additional
+16 bit immediate inserted, and the result also written to FRS*
+
+Example:
+
+```
+# these two combined instructions write 0x3f808000
+# into f4 as an FP32 to be converted to an FP64.
+# 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 an FP32
+fishmv f4, 0x8000 # writes +1.00390625 to f4
+```
+
+# Moves
+
+These instructions perform a straight unaltered bit-level copy from one Register
+File to another.
+
+# FPR to GPR moves
+
+* `fmvtg RT, FRA`
+* `fmvtg. RT, FRA`
+
+move a 64-bit float from a FPR to a GPR, just copying bits directly.
+As a direct bitcopy, no exceptions occur and no status flags are set.
+
+Rc=1 tests RT and sets CR0, exactly like all other Scalar Fixed-Point
+operations.
+
+* `fmvtgs RT, FRA`
+* `fmvtgs. RT, FRA`
+
+move a 32-bit float from a FPR to a GPR, just copying bits. Converts the
+64-bit float in `FRA` to a 32-bit float, using the same method as `stfs`,
+then writes the 32-bit float to `RT`, setting the high 32-bits to zeros.
+Effectively, `fmvtgs` is a macro-fusion of `stfs` and `lwz` and therefore
+does not behave like `frsp` and does not set any fp exception flags.
+
+Since RT is a GPR, Rc=1 follows standard *integer* behaviour, i.e.
+tests RT and sets CR0.
+
+# GPR to FPR moves
+
+`fmvfg FRT, RA`
+
+move a 64-bit float from a GPR to a FPR, just copying bits. No exceptions
+are raised, no flags are altered of any kind.
+
+Rc=1 tests FRT and sets CR1
+
+`fmvfgs FRT, RA`
+
+move a 32-bit float from a GPR to a FPR, just copying bits. Converts the
+32-bit float in `RA` to a 64-bit float, using the same method as `lfs`,
+then writes the 64-bit float to `FRT`. Effectively, `fmvfgs` is a
+macro-fusion of `stw` and `lfs` and therefore no fp exception flags are set.
+
+Rc=1 tests FRT and sets CR1, following usual fp Rc=1 semantics.
+
+# Conversions
+
+Unlike the move instructions
+these instructions perform conversions between Integer and
+Floating Point. Truncation can therefore occur, as well
+as exceptions.
+
+Mode values:
+
+| Mode | `rounding_mode` | Semantics |
+|------|-----------------|----------------------------------|
+| 000 | from `FPSCR` | [OpenPower semantics] |
+| 001 | Truncate | [OpenPower semantics] |
+| 010 | from `FPSCR` | [Java semantics] |
+| 011 | Truncate | [Java semantics] |
+| 100 | from `FPSCR` | [JavaScript semantics] |
+| 101 | Truncate | [JavaScript semantics] |
+| rest | -- | illegal instruction trap for now |
+
+[OpenPower semantics]: #fp-to-int-openpower-conversion-semantics
+[Java semantics]: #fp-to-int-java-conversion-semantics
+[JavaScript semantics]: #fp-to-int-javascript-conversion-semantics
+
+## GPR to FPR conversions
+
+**Format**
+
+| 0-5 | 6-10 | 11-15 | 16-25 | 26-30 | 31 | Form |
+|--------|------|--------|-------|-------|----|------|
+| Major | FRT | //Mode | RA | XO | Rc |X-Form|
+
+All of the following GPR to FPR conversions use the rounding mode from `FPSCR`.
+
+* `fcvtfgw FRT, RA`
+ Convert from 32-bit signed integer in the GPR `RA` to 64-bit float in
+ `FRT`.
+* `fcvtfgws FRT, RA`
+ Convert from 32-bit signed integer in the GPR `RA` to 32-bit float in
+ `FRT`.
+* `fcvtfguw FRT, RA`
+ Convert from 32-bit unsigned integer in the GPR `RA` to 64-bit float in
+ `FRT`.
+* `fcvtfguws FRT, RA`
+ Convert from 32-bit unsigned integer in the GPR `RA` to 32-bit float in
+ `FRT`.
+* `fcvtfgd FRT, RA`
+ Convert from 64-bit signed integer in the GPR `RA` to 64-bit float in
+ `FRT`.
+* `fcvtfgds FRT, RA`
+ Convert from 64-bit signed integer in the GPR `RA` to 32-bit float in
+ `FRT`.
+* `fcvtfgud FRT, RA`
+ Convert from 64-bit unsigned integer in the GPR `RA` to 64-bit float in
+ `FRT`.
+* `fcvtfguds FRT, RA`
+ Convert from 64-bit unsigned integer in the GPR `RA` to 32-bit float in
+ `FRT`.
+
+## FPR to GPR (Integer) conversions
+
+<div id="fpr-to-gpr-conversion-mode"></div>
+
+Different programming languages turn out to have completely different
+semantics for FP to Integer conversion. Below is an overview
+of the different variants, listing the languages and hardware that
+implements each variant.
+
+**Standard IEEE754 conversion**
+
+This conversion is outlined in the IEEE754 specification. It is used
+by nearly all programming languages and CPUs. In the case of OpenPOWER,
+the rounding mode is read from FPSCR
+
+**Standard OpenPower conversion**
+
+This conversion, instead of exact IEEE754 Compliance, performs
+"saturation with NaN converted to minimum valid integer". This
+is also exactly the same as the x86 ISA conversion semantics.
+OpenPOWER however has instructions for both:
+
+* rounding mode read from FPSCR
+* rounding mode always set to truncate
+
+**Java conversion**
+
+For the sake of simplicity, the FP -> Integer conversion semantics generalized from those used by Java's semantics (and Rust's `as` operator) will be referred to as
+[Java conversion semantics](#fp-to-int-java-conversion-semantics).
+
+Those same semantics are used in some way by all of the following languages (not necessarily for the default conversion method):
+
+* Java's
+ [FP -> Integer conversion](https://docs.oracle.com/javase/specs/jls/se16/html/jls-5.html#jls-5.1.3)
+* Rust's FP -> Integer conversion using the
+ [`as` operator](https://doc.rust-lang.org/reference/expressions/operator-expr.html#semantics)
+* LLVM's
+ [`llvm.fptosi.sat`](https://llvm.org/docs/LangRef.html#llvm-fptosi-sat-intrinsic) and
+ [`llvm.fptoui.sat`](https://llvm.org/docs/LangRef.html#llvm-fptoui-sat-intrinsic) intrinsics
+* SPIR-V's OpenCL dialect's
+ [`OpConvertFToU`](https://www.khronos.org/registry/spir-v/specs/unified1/SPIRV.html#OpConvertFToU) and
+ [`OpConvertFToS`](https://www.khronos.org/registry/spir-v/specs/unified1/SPIRV.html#OpConvertFToS)
+ instructions when decorated with
+ [the `SaturatedConversion` decorator](https://www.khronos.org/registry/spir-v/specs/unified1/SPIRV.html#_a_id_decoration_a_decoration).
+* WebAssembly has also introduced
+ [trunc_sat_u](ttps://webassembly.github.io/spec/core/exec/numerics.html#op-trunc-sat-u) and
+ [trunc_sat_s](https://webassembly.github.io/spec/core/exec/numerics.html#op-trunc-sat-s)
+
+**JavaScript conversion**
+
+For the sake of simplicity, the FP -> Integer conversion semantics generalized from those used by JavaScripts's `ToInt32` abstract operation will be referred to as [JavaScript conversion semantics](#fp-to-int-javascript-conversion-semantics).
+
+This instruction is present in ARM assembler as FJCVTZS
+<https://developer.arm.com/documentation/dui0801/g/hko1477562192868>
+
+**Format**
+
+| 0-5 | 6-10 | 11-15 | 16-25 | 26-30 | 31 | Form |
+|--------|------|--------|-------|-------|----|------|
+| Major | RT | //Mode | FRA | XO | Rc |X-Form|
+
+**Rc=1 and OE=1**
+
+All of these insructions have 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.
+
+**Instructions**
+
+* `fcvttgw RT, FRA, Mode`
+ Convert from 64-bit float to 32-bit signed integer, writing the result
+ to the GPR `RT`. Converts using [mode `Mode`]. Similar to `fctiw` or `fctiwz`
+* `fcvttguw RT, FRA, Mode`
+ Convert from 64-bit float to 32-bit unsigned integer, writing the result
+ to the GPR `RT`. Converts using [mode `Mode`]. Similar to `fctiwu` or `fctiwuz`
+* `fcvttgd RT, FRA, Mode`
+ Convert from 64-bit float to 64-bit signed integer, writing the result
+ to the GPR `RT`. Converts using [mode `Mode`]. Similar to `fctid` or `fctidz`
+* `fcvttgud RT, FRA, Mode`
+ Convert from 64-bit float to 64-bit unsigned integer, writing the result
+ to the GPR `RT`. Converts using [mode `Mode`]. Similar to `fctidu` or `fctiduz`
+* `fcvtstgw RT, FRA, Mode`
+ Convert from 32-bit float to 32-bit signed integer, writing the result
+ to the GPR `RT`. Converts using [mode `Mode`]
+* `fcvtstguw RT, FRA, Mode`
+ Convert from 32-bit float to 32-bit unsigned integer, writing the result
+ to the GPR `RT`. Converts using [mode `Mode`]
+* `fcvtstgd RT, FRA, Mode`
+ Convert from 32-bit float to 64-bit signed integer, writing the result
+ to the GPR `RT`. Converts using [mode `Mode`]
+* `fcvtstgud RT, FRA, Mode`
+ Convert from 32-bit float to 64-bit unsigned integer, writing the result
+ to the GPR `RT`. Converts using [mode `Mode`]
+
+[mode `Mode`]: #fpr-to-gpr-conversion-mode
+
+## FP to Integer Conversion Pseudo-code
+
+Key for pseudo-code:
+
+| term | result type | definition |
+|---------------------------|-------------|----------------------------------------------------------------------------------------------------|
+| `fp` | -- | `f32` or `f64` (or other types from SimpleV) |
+| `int` | -- | `u32`/`u64`/`i32`/`i64` (or other types from SimpleV) |
+| `uint` | -- | the unsigned integer of the same bit-width as `int` |
+| `int::BITS` | `int` | the bit-width of `int` |
+| `uint::MIN_VALUE` | `uint` | the minimum value `uint` can store: `0` |
+| `uint::MAX_VALUE` | `uint` | the maximum value `uint` can store: `2^int::BITS - 1` |
+| `int::MIN_VALUE` | `int` | the minimum value `int` can store : `-2^(int::BITS-1)` |
+| `int::MAX_VALUE` | `int` | the maximum value `int` can store : `2^(int::BITS-1) - 1` |
+| `int::VALUE_COUNT` | Integer | the number of different values `int` can store (`2^int::BITS`). too big to fit in `int`. |
+| `rint(fp, rounding_mode)` | `fp` | rounds the floating-point value `fp` to an integer according to rounding mode `rounding_mode` |
+
+<div id="fp-to-int-openpower-conversion-semantics"></div>
+OpenPower conversion semantics (section A.2 page 999 (page 1023) of OpenPower ISA v3.1):
+
+```
+def fp_to_int_open_power<fp, int>(v: fp) -> int:
+ if v is NaN:
+ return int::MIN_VALUE
+ if v >= int::MAX_VALUE:
+ return int::MAX_VALUE
+ if v <= int::MIN_VALUE:
+ return int::MIN_VALUE
+ return (int)rint(v, rounding_mode)
+```
+
+<div id="fp-to-int-java-conversion-semantics"></div>
+[Java conversion semantics](https://docs.oracle.com/javase/specs/jls/se16/html/jls-5.html#jls-5.1.3)
+/
+[Rust semantics](https://doc.rust-lang.org/reference/expressions/operator-expr.html#semantics)
+(with adjustment to add non-truncate rounding modes):
+
+```
+def fp_to_int_java<fp, int>(v: fp) -> int:
+ if v is NaN:
+ return 0
+ if v >= int::MAX_VALUE:
+ return int::MAX_VALUE
+ if v <= int::MIN_VALUE:
+ return int::MIN_VALUE
+ return (int)rint(v, rounding_mode)
+```
+
+<div id="fp-to-int-javascript-conversion-semantics"></div>
+Section 7.1 of the ECMAScript / JavaScript
+[conversion semantics](https://262.ecma-international.org/11.0/#sec-toint32) (with adjustment to add non-truncate rounding modes):
+
+```
+def fp_to_int_java_script<fp, int>(v: fp) -> int:
+ if v is NaN or infinite:
+ return 0
+ v = rint(v, rounding_mode) # assume no loss of precision in result
+ v = v mod int::VALUE_COUNT # 2^32 for i32, 2^64 for i64, result is non-negative
+ bits = (uint)v
+ return (int)bits
+```
+
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