[[!tag standards]]
+Note on considered alternative naming schemes: we decided to switch to using the reduced mnemonic naming scheme (over some people's objections) since it would be 5 instructions instead of dozens, though we did consider trying to match PowerISA's existing naming scheme for the instructions rather than only for the instruction aliases. <https://bugs.libre-soc.org/show_bug.cgi?id=1015#c7>
+
# 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=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
+* <https://bugs.libre-soc.org/show_bug.cgi?id=1015> int-fp RFC
* [[int_fp_mv/appendix]]
+* [[sv/rfc/ls002]] - `fmvis` and `fishmv` External RFC Formal Submission
+* [[sv/rfc/ls006]] - int-fp-mv 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 compact
-format float immediate is up for consideration using BF16 as a base.
+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,
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 load/store
+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 3D GPU this kills any modern competitive edge.
+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 to the minimum seems necessary.
+of instructions required is a priority.
Therefore, we are proposing adding:
-* FPR load-immediate using `BF16` as the constant
+* 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
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
+* Int -> Float
+ * **standard IEEE754** - used by most languages and CPUs
+* Float -> Int
+ * **standard OpenPOWER** - saturation with NaN
+ converted to minimum valid integer
+ * **Java/Saturating** - 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 is 32
-scalar instructions, including seven branch instructions.
-
-## FP -> Integer conversions
-
-Different programming languages turn out to have completely different
-semantics for FP to Integer conversion. This section gives 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 senantics.
-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).
-
-### 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>
-
-### Other languages
-
-TODO: review and investigate other language semantics
+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) compated to existing Scalar Power ISA.
+(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
The interactions with SVP64
are explained in the [[int_fp_mv/appendix]]
-## 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, then writes the 32-bit float to
-`RT`. Effectively, `fmvtgs` is a macro-fusion of `frsp fmvtg`
-and therefore has the exact same exception and flags behaviour of `frsp`
-
-Unlike `frsp` however, with RT being 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, then writes the 64-bit float to
-`FRT`. Effectively, `fmvfgs` is a macro-fusion of `fmvfg frsp` and
-therefore has the exact same exception and flags behaviour of `frsp`
-
-Rc=1 tests FRT and sets CR1
-
-TODO: clear statement on evaluation as to whether exceptions or flags raised as part of the **FP** conversion (not the int bitcopy part, the conversion part. the semantics should really be the same as frsp)
-
-v3.0C section 4.6.7.1 states:
-
-FPRF is set to the class and sign of the result, except for Invalid Operation Exceptions when VE=1.
-
- Special Registers Altered:
- FPRF FR FI
- FX OX UX XX VXSNAN
- CR1 (if Rc=1)
-
-## Float load immediate <a name="fmvis"></a>
-
-This is like a variant of `fmvfg`
-
-`fmvis FRT, FI`
-
-Reinterprets `FI << 16` as a 32-bit float, which is then converted to a
-64-bit float and written to `FRT`. This is equivalent to reinterpreting
-`FI` as a `BF16` and converting to 64-bit float.
-
+# 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. This frees up one extra bit in the X-Form format for packing
-a full `BF16`.
+instruction.
Example:
to instead write `+0.0` if `RA` is register `0`, at least
allowing clearing FPRs.
-`fmvis` fits well with DX-Form:
-
-| 0-5 | 6-10 | 11-15 | 16-25 | 26-30 | 31 | Form |
-|--------|------|-------|-------|-------|-----|-----|
-| Major | FRT | d1 | d0 | XO | d2 | DX-Form |
-
- bf16 = d0 || d1 || d2
- fp32 = bf16 || [0]*16
- FRT = Single_to_Double(fp32)
-
-## FPR to GPR conversions
-
-<div id="fpr-to-gpr-conversion-mode"></div>
-
-X-Form:
-
-| 0-5 | 6-10 | 11-15 | 16-25 | 26-30 | 31 |
-|--------|------|--------|-------|-------|----|
-| Major | RT | //Mode | FRA | XO | Rc |
-| Major | FRT | //Mode | RA | XO | Rc |
-
-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
-
-* `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`]
-* `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`]
-* `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`]
-* `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`]
-* `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
-
-## GPR to FPR conversions
-
-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`.
-
-# 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` |
-| `int::MIN_VALUE` | `int` | the minimum value `int` can store (`0` if unsigned, `-2^(int::BITS-1)` if signed) |
-| `int::MAX_VALUE` | `int` | the maximum value `int` can store (`2^int::BITS - 1` if unsigned, `2^(int::BITS-1) - 1` if signed) |
-| `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):
+`fmvis` fits with DX-Form:
-```
-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)
-```
+| 0-5 | 6-10 | 11-15 | 16-25 | 26-30 | 31 | Form |
+|--------|------|-------|-------|-------|-----|---------|
+| Major | FRS | d1 | d0 | XO | d2 | DX-Form |
-<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):
+Pseudocode:
-```
-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)
-```
+ bf16 = d0 || d1 || d2 # create BF16 immediate
+ fp32 = bf16 || [0]*16 # convert BF16 to FP32
+ FRS = DOUBLE(fp32) # convert FP32 to FP64
-<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):
+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:
```
-def fp_to_int_java_script<fp, int>(v: fp) -> int:
- if v is NaN or infinite:
- return 0
- v = rint(v, rounding_mode)
- v = v mod int::VALUE_COUNT # 2^32 for i32, 2^64 for i64, result is non-negative
- bits = (uint)v
- return (int)bits
+# 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
```
+[[!inline pages="openpower/sv/int_fp_mv/moves_and_conversions" raw=yes ]]
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