FPGA Optimization Guide for Intel® oneAPI Toolkits

Developer Guide

Contents

Control Hardware Implementation of the Supported Data Types and Math Operations

The
Intel® oneAPI
DPC++/C++
 Compiler
 allows you to control (at global and local scope) whether the implementation of the supported data type and math functions uses DSPs or soft logic using ALMs. You can find this implementation in the Details pane of the corresponding math instruction nodes in the System Viewer of the FPGA optimization report (
report.html
).

Global-scope Control

Include the
-Xsdsp-mode=
[default|prefer-dsp|prefer-softlogic]
 flag in your
dpcpp
 command to control the hardware implementation of the supported data types and math operations of all kernels in your source code.
Example
dpcpp -fintelfpga –Xshardware -Xsdsp-mode=<option> <source_file>.cpp
where the
<option>
 is one of the following values:
Option
Description
default
Default option if you do not pass this command-line flag manually. The compiler determines the implementation based on the data type and math operation.
prefer-dsp
Prefer math operations to be implemented in DSPs. If a math operation is implemented by DSPs by default, you see no difference in resource utilization or area. Otherwise, you will notice a decrease in the use of soft-logic resources and an increase in the use of DSPs.
prefer-softlogic
Prefer math operations to be implemented in soft-logic. If a math operation is implemented without DSPs by default, you see no difference in resource utilization or area. Otherwise, you will notice a decrease in the use of DSPs and an increase in the use of soft-logic resources.

Local-scope Control

You can control DSP usage of the supported math operations on a local scope by using the library function
sycl::ext::intel::math_dsp_control(<
Preference::<enum>
,
Propagate::<enum>
>)
 defined in the
fpga_dsp_control.hpp
 header file, which is included in the
fpga_extensions.hpp
 header file. This library function provides control at the block level within a single kernel. A reference-capturing lambda expression is passed as the argument to this library function. Inside the lambda expression, the implementation preference of math operations that support DSP control is determined by two template arguments.
The
<
Preference::<enum>
>
 argument takes up an
enum
 data type with one of the following values:
Value
Description
cl::sycl::ext::intel::Preference::DSP
Prefer math operations to be implemented in DSPs. Its behavior on a math operation is equivalent to the global control
-Xsdsp-mode=prefer-dsp
.
The
cl::sycl::ext::intel::Preference::DSP
 option is automatically applied if you do not specify the template argument
Preference
 manually.
cl::sycl::ext::intel::Preference::Softlogic
Prefer math operations to be implemented in soft logic. Its behavior on a math operation is equivalent to the global control
-Xsdsp-mode=prefer-softlogic
.
cl::sycl::ext::intel::Preference::Compiler_default
Compiler determines the implementation based on the data type and math operation. Its behavior on a math operation is equivalent to the global control
-Xsdsp-mode=default
.
Local control overrides global control on a controlled math operation. For example: 
// dpcpp -Xsdsp-mode=prefer-softlogic ...
using namespace sycl;
cgh.single_task<class LocalControl>([=]() {
  out_acc[0] = in_acc[0] + inp_acc[1]; // Addition implemented in soft-logic.
  ext::intel::math_dsp_control<ext::intel::Preference::DSP>([&] {
    out_acc[1] = in_acc[0] + in_acc[1]; // Addition implemented in DSP.
  });
});
The
<
Propagate::<enum>
>
 argument is a boolean value that determines the propagation of the
Preference::<enum>
 value to function calls in the lambda expression as follows:
Value
Description
cl::sycl::ext::intel::Propagate::On
DSP control recursively applies to controllable math operations in all function calls in the lambda expression function. This value is the default and it is applied if you do not specify the argument
<
Propagate::<enum>
>
.
cl::sycl::ext::intel::Propagate::Off
DSP control applies only to the controllable math operations directly inside the lambda expression. Math operations in function calls inside the lambda expression are not affected by this DSP control.
Example
sycl::ext::intel::math_dsp_control<sycl::ext::intel::Preference::Softlogic,
                                   sycl::ext::intel::Propagate::On>([&]{
  a += 1.23f;
});
A nested
math_dsp_control<>()
 call is controlled only by its own
Preference
 argument. The
Preference
 of the parent
math_dsp_control<>()
 function does not affect the nested
math_dsp_control<>()
 function, even if the parent has the argument
Propagate::On
. For example: 
using namespace sycl;
cgh.single_task<class LocalControl>([=]() {
  ext::intel::math_dsp_control<ext::intel::Preference::DSP, 
                               ext::intel::Propagate::On>([&] {
    out_acc[0] = in_acc[0] + in_acc[1]; // Addition implemented in DSP.
    ext::intel::math_dsp_control<ext::intel::Preference::Softlogic>([&] {
      // Addition implemented in soft-logic. Preference::DSP on the parent 
      // math_dsp_control<>() call does not affect this math_dsp_control<>().
      out_acc[1] = in_acc[0] + in_acc[1]; 
    });
  });
});

Supported Data Type and Math Operations

Data types
Math Operations
float
Addition, subtraction, multiplication by a constant
ap_float<8, 23>
Addition, subtraction, multiplication by a constant
int
Multiplication by a constant
ac_int
Multiplication by a constant
ac_fixed
Multiplication by a constant
For additional information, refer to the FPGA tutorial sample
DSP Control
 listed in the Intel® oneAPI Samples Browser on Linux* or Windows*, or access the code sample in GitHub.

Product and Performance Information

1

Performance varies by use, configuration and other factors. Learn more at www.Intel.com/PerformanceIndex.