OpenCL™ Developer Guide for Intel® Core™ and Intel® Xeon® Processors

  • 10/30/2018
  • Public Content
Contents

Use Lower Math Precision

OpenCL™ offers two basic ways to trade precision for speed:
  • native_*
     and
    half_*
     math built-ins, which have lower precision, but are faster than their un-prefixed variants
  • The compiler optimization options that enable optimizations for floating-point arithmetic for the whole OpenCL program, for example, the
    -cl-fast-relaxed-math
     flag.
In general, while the
-cl-fast-relaxed-math
 flag is a quick way to get performance gains for kernels with many math operations, it does not permit fine numeric accuracy control. Consider experimenting with the
native_*
 equivalents separately for each specific case, keeping track of the resulting accuracy.
Native_
 versions of math built-ins are supported in hardware and run substantially faster, while offering lower accuracy. Use native trigonometry and transcendental functions, such as
sin
,
cos
,
exp
, and
log
, when performance is more important than precision.
For a full list of OpenCL build options and option descriptions, refer to the the OpenCL specification. For the instructions on how to use these options with the Intel® SDK for OpenCL™ Applications, refer to the following pages in the Developer Guide for Intel® SDK for OpenCL™ Applications: Build with OpenCL Offline Compiler Command Line Interface (for Intel® SDK for OpenCL™ Applications standalone version), Configuring OpenCL™ Build Options (for Intel® Code Builder for OpenCL™ API plugin for Microsoft Visual Studio*), Configuring Build Options (for Intel® Code Builder for OpenCL™ API plugin for Eclipse*).

Product and Performance Information

1

Intel's compilers may or may not optimize to the same degree for non-Intel microprocessors for optimizations that are not unique to Intel microprocessors. These optimizations include SSE2, SSE3, and SSSE3 instruction sets and other optimizations. Intel does not guarantee the availability, functionality, or effectiveness of any optimization on microprocessors not manufactured by Intel. Microprocessor-dependent optimizations in this product are intended for use with Intel microprocessors. Certain optimizations not specific to Intel microarchitecture are reserved for Intel microprocessors. Please refer to the applicable product User and Reference Guides for more information regarding the specific instruction sets covered by this notice.

Notice revision #20110804