IEEE 754 doesn't 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. Below is an overview
-of the different variants, listing the languages and hardware that
-implements each variant.
-
-For convenience, those different conversion semantics will be given names
-based on which common ISA or programming language uses them, since there
-may not be an established name for them:
-
-**Standard OpenPower conversion**
-
-This conversion 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/Saturating 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/Saturating conversion
-semantics](#fp-to-int-java-saturating-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)
- (only for long/int results)
-* 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>
-
-**Rc=1 and OE=1**
-
-All of these instructions 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.
-
-\newpage{}
-
-### FP to Integer Conversion Simplified 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 1009 (page 1035) of
-Power ISA v3.1B):
-
-```
- 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-saturating-conversion-semantics"></div>
-[Java/Saturating conversion semantics](https://docs.oracle.com/javase/specs/jls/se16/html/jls-5.html#jls-5.1.3)
-(only for long/int results)
-(with adjustment to add non-truncate rounding modes):
-
-```
- def fp_to_int_java_saturating<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):
+languages and ISAs have made different choices. The different conversion
+modes supported by the `cffpr` instruction are as follows:
+
+* P-Type:
+ This type of conversion is that used by most other PowerISA instructions,
+ as well as commonly used floating-point to integer conversions on x86.
+
+* S-Type:
+ This type of conversion is used for WebAssembly's
+ [`trunc_sat_u`](https://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)
+ instructions, as well as several notable programming languages:
+
+ * Java's conversion from
+ [`float`/`double` to `long`/`int`](https://docs.oracle.com/javase/specs/jls/se16/html/jls-5.html#jls-5.1.3)
+ * Rust's [`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).
+
+* E-Type:
+ This type of conversion is used for
+ [ECMAScript's `ToInt32` abstract operation](https://262.ecma-international.org/14.0/#sec-toint32).
+
+ This type of conversion is also implemented in ARMv8.3A as the `FJCVTZS`
+ instruction.
+ <https://developer.arm.com/documentation/dui0801/g/hko1477562192868>
+
+### Floating-point to Integer Conversion Semantics Summary
+
+Let `rounded` 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:
```
- 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
++--------+------------+-----------------------------------------------------------------------+
+| Kind | Result's | Category of rounded |
+| | Signedness +-----------+-----------+-----------+-----------+-----------+-----------+
+| | | NaN | +Infinity | -Infinity | > Maximum | < Minimum | Otherwise |
+| | | | | | Possible | Possible | |
+| | | | | | Result | Result | |
++--------+------------+-----------+-----------+-----------+-----------+-----------+-----------+
+| P-Type | Unsigned | 0 | 2^w - 1 | 0 | 2^w - 1 | 0 | rounded |
+| +------------+-----------+-----------+-----------+-----------+-----------+-----------+
+| | Signed | -2^(w-1) | 2^(w-1)-1 | -2^(w-1) | 2^(w-1)-1 | -2^(w-1) | rounded |
++--------+------------+-----------+-----------+-----------+-----------+-----------+-----------+
+| S-Type | Unsigned | 0 | 2^w - 1 | 0 | 2^w - 1 | 0 | rounded |
+| +------------+-----------+-----------+-----------+-----------+-----------+-----------+
+| | Signed | 0 | 2^(w-1)-1 | -2^(w-1) | 2^(w-1)-1 | -2^(w-1) | rounded |
++--------+------------+-----------+-----------+-----------+-----------+-----------+-----------+
+| E-Type | Either | 0 | rounded & (2^w - 1) |
++--------+------------+-----------------------------------+-----------------------------------+
```
projects / libreriscv.git / commitdiff