Developer Guide

Intel oneAPI FPGA Handbook

ID 785441
Date 2/07/2024
Public

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Document Table of Contents

Operations with Explicit Precision Controls

The following operations leave the precision of the input ap_float-type variables untouched and allow you to control the output precision:

Rounding Mode Control For ap_float to ap_float Conversions

Syntax

convert_to<output_exponent_width, output_mantissa_width, rounding_mode>

Description

Use this method to override the rounding mode set for an ap_float variable when converting the variable to a different precision. By default, ap_float to ap_float conversions use the rounding mode that you specified when you declared the variable.

Multiplication

Syntax

ihc::ap_float<output_exponent_width, output_mantissa_width>::mul <[accuracy_setting], [subnormal_setting]> (ap_float_a, ap_float_b)

Description

This math function supplements the basic multiplication operation performed by the multiplication (*) operator. Multiplies ap_float_a and ap_float_b without changing the input types and outputs an ap_float at the specified precision. The optional parameters are defined as follows:

  • subnormal_setting: Optional parameter to specify whether input and output numbers are flushed to zero when carrying out basic binary operations explicitly. Set this parameter with one of the following values:
    subnormal_setting Parameter Values
    subnormal_setting Values Description
    ihc::fp_config::FP_Subnormal::ON Input and output numbers in the subnormal range are preserved. The target FPGA device must have subnormal support. Subnormal support might require more FPGA area.
    ihc::fp_config::FP_Subnormal::OFF Input or output numbers in the subnormal range are flushed to zero.
    ihc::fp_config::FP_Subnormal::AUTO With this setting, the Intel® oneAPI DPC++/C++ Compiler enables subnormal support only under the folllowing conditions:
    • The target FPGA device directly supports it.
    • It does not incur any extra FPGA area overhead.

    If you do not set the subnormal_setting parameter, the ihc::fp_config::FP_Subnormal::AUTO subnormal setting is used by default.

  • accuracy_setting: Optional parameter that influences trade-offs between the accuracy of the result due to different rounding decisions in the intermediary calculations and the FPGA area utilized by the generated hardware. Floating-point operations with less accurate results typically use fewer logic elements. For example, a divider with a high accuracy might use 20% more FPGA area than a divider with low accuracy. The low accuracy divider has a higher error bound [1 unit of least precision (ULP)] than a high accuracy divider (0.5 ULP).

    Set this parameter with one of the following values:

    accuracy_setting Parameter Values
    accuracy_setting Values Description
    ihc::fp_config::FP_Accuracy::HIGH

    Uses the high precision version of the floating-point math operations. This is the default setting.

    ihc::fp_config::FP_Accuracy::LOW Allows the compiler to use a higher error bound to save on-chip area.

    If you do not set the accuracy_setting parameter, ihc::fp_config::FP_Accuracy::HIGH accuracy setting is used by default.

Addition/Subtraction/Division

Syntax

ihc::ap_float<output_exponent_width, output_mantissa_width>::add <[optional parameters]> (ap_float_a, ap_float_b) 
ihc::ap_float<output_exponent_width, output_mantissa_width>::sub <[optional parameters]> (ap_float_a, ap_float_b) 
ihc::ap_float<output_exponent_width, output_mantissa_width>::div <[optional parameters]> (ap_float_a, ap_float_b)

Description

These math functions supplement the basic math operations performed by the addition/subtraction/division (+ / //) operators. Adds/subtracts/divides ap_float_a and ap_float_b by first casting ap_float_a and ap_float_b to the specified ap_float precision. The operation and output are at the specified precision.

You can also specify the optional parameters accuracy_setting and subnormal_setting described earlier.

NOTE:

If your SYCL code performs computations on constant/literal native floating-point values, the compiler can sometimes combine them at compile time and save area. This is a compiler optimization technique called constant folding or constant propagation. This optimization does not work for ap_float even when the operands are constant. You should compute your constant arithmetic in native types or precompute manually.