openpower/sv/int_fp_mv/cvt_fp_to_int_overview.mdwn

   1 ## Floating-point to Integer Conversion Overview
   2
   3 <div id="fpr-to-gpr-conversion-mode"></div>
   4
   5 IEEE 754 does not specify what results are obtained when converting a NaN
   6 or out-of-range floating-point value to integer, so different programming
   7 languages and ISAs have made different choices.  The different conversion
   8 modes supported by the `cffpr` instruction are as follows:
   9
  10 * P-Type: <div id="fpr-to-gpr-conversion-p-type"></div>
  11     Used by most other PowerISA instructions,
  12     as well as commonly used floating-point to integer conversions on x86.
  13
  14 * S-Type: <div id="fpr-to-gpr-conversion-s-type"></div>
  15     Used for several notable programming languages:
  16
  17     * Java's conversion from `float`/`double` to `long`/`int`[^java_fp_to_int]
  18     * Rust's `as` operator[^rust_as_operator]
  19     * LLVM's `llvm.fptosi.sat`[^llvm_fptosi_sat] and
  20         `llvm.fptoui.sat`[^llvm_fptoui_sat] intrinsics
  21     * SPIR-V's OpenCL dialect's `OpConvertFToU`[^SPIRV_OpConvertFToU] and
  22         `OpConvertFToS`[^SPIRV_OpConvertFToS] instructions when decorated with
  23         the `SaturatedConversion`[^SPIRV_SaturatedConversion] decorator.
  24     * Also WebAssembly's `trunc_sat_u`[^trunc_sat_u] and
  25         `trunc_sat_s`[^trunc_sat_s] instructions,
  26
  27
  28 * E-Type: <div id="fpr-to-gpr-conversion-e-type"></div>
  29     Used for ECMAScript's `ToInt32` abstract operation[^ECMAScript_ToInt32].
  30     Also implemented in ARMv8.3A as the `FJCVTZS` instruction[^ARM_FJCVTZS].
  31
  32 [^trunc_sat_u]: WASM's `trunc_sat_u`:
  33     <https://webassembly.github.io/spec/core/exec/numerics.html#op-trunc-sat-u>
  34 [^trunc_sat_s]: WASM's `trunc_sat_s`:
  35     <https://webassembly.github.io/spec/core/exec/numerics.html#op-trunc-sat-s>
  36 [^java_fp_to_int]: Java `float`/`double` to `long`/`int` conversion:
  37     <https://docs.oracle.com/javase/specs/jls/se16/html/jls-5.html#jls-5.1.3>
  38 [^rust_as_operator]: Rust's `as` operator:
  39     <https://doc.rust-lang.org/1.70.0/reference/expressions/operator-expr.html#numeric-cast>
  40 [^llvm_fptosi_sat]: LLVM's `llvm.fptosi.sat` intrinsic:
  41     <https://llvm.org/docs/LangRef.html#llvm-fptosi-sat-intrinsic>
  42 [^llvm_fptoui_sat]: LLVM's `llvm.fptoui.sat` intrinsic:
  43     <https://llvm.org/docs/LangRef.html#llvm-fptoui-sat-intrinsic>
  44 [^SPIRV_OpConvertFToU]: SPIR-V's `OpConvertFToU` instruction:
  45     <https://www.khronos.org/registry/spir-v/specs/unified1/SPIRV.html#OpConvertFToU>
  46 [^SPIRV_OpConvertFToS]: SPIR-V's `OpConvertFToS` instruction:
  47     <https://www.khronos.org/registry/spir-v/specs/unified1/SPIRV.html#OpConvertFToS>
  48 [^SPIRV_SaturatedConversion]: SPIR-V's `SaturatedConversion` decorator:<br/>
  49     <https://www.khronos.org/registry/spir-v/specs/unified1/SPIRV.html#_a_id_decoration_a_decoration>
  50 [^ECMAScript_ToInt32]: ECMAScript's `ToInt32` abstract operation:
  51     <https://262.ecma-international.org/14.0/#sec-toint32>
  52 [^ARM_FJCVTZS]: ARM's `FJCVTZS` instruction:
  53     <https://developer.arm.com/documentation/dui0801/g/hko1477562192868>
  54
  55 ### Floating-point to Integer Conversion Semantics Summary
  56
  57 Let `round` be the result of `bfp_ROUND_TO_INTEGER(rmode, input)`.
  58 Let `w` be the number of bits in the result's type.
  59 The result of Floating-point to Integer conversion is as follows:
  60
  61 ```
  62 +------+------+---------------------------------------------------------------+
  63 |Type| Result | Category of rounding                                          |
  64 |    | Sign   +----------+-----------+----------+-----------+---------+-------+
  65 |    |        | NaN      | +Inf      | -Inf     | > Max     | < Min   | Else  |
  66 |    |        |          |           |          | Possible  | Possible|       |
  67 |    |        |          |           |          | Result    | Result  |       |
  68 +----+--------+----------+-----------+----------+-----------+---------+-------+
  69 |  P |Unsigned| 0        | 2^w - 1   | 0        | 2^w - 1   | 0       | round |
  70 |    +--------+----------+-----------+----------+-----------+---------+-------+
  71 |    | Signed | -2^(w-1) | 2^(w-1)-1 | -2^(w-1) | 2^(w-1)-1 | -2^(w-1)| round |
  72 +----+--------+----------+-----------+----------+-----------+---------+-------+
  73 |  S |Unsigned| 0        | 2^w - 1   | 0        | 2^w - 1   | 0       | round |
  74 |    +--------+----------+-----------+----------+-----------+---------+-------+
  75 |    | Signed | 0        | 2^(w-1)-1 | -2^(w-1) | 2^(w-1)-1 | -2^(w-1)| round |
  76 +----+--------+----------+-----------+----------+-----------+---------+-------+
  77 |  E | Either | 0                               | round & (2^w - 1)           |
  78 +----+--------+---------------------------------+-----------------------------+
  79 ```