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Introduction
Check-list for OpenCL™ Optimizations
Tips and Tricks for Kernel Development
Application-Level Optimizations
Debugging OpenCL™ Kernels on Linux* OS
Performance Debugging with Intel® SDK for OpenCL™ Applications
Coding for the Intel® Architecture Processors
Why Optimizing Kernels Is Important?
Avoid Spurious Operations in Kernels
Avoid Handling Edge Conditions in Kernels
Use the Preprocessor for Constants
Prefer (32-bit) Signed Integer Data Types
Prefer Row-Wise Data Accesses
Use Built-In Functions
Avoid Extracting Vector Components
Task-Parallel Programming Model Hints
Common Mistakes in OpenCL™ Applications
Introduction for OpenCL™ Coding on Intel® Architecture Processors
Vectorization Basics for Intel® Architecture Processors
Vectorization: SIMD Processing Within a Work Group
Benefitting from Implicit Vectorization
Vectorizer Knobs
Environment Variables
Attribute Extensions
See Also
Targeting a Different CPU Architecture
Using Vector Data Types
Writing Kernels to Directly Target the Intel® Architecture Processors
Work-Group Size Considerations
Threading: Achieving Work-Group Level Parallelism
Efficient Data Layout
Using the Blocking Technique
Intel® Turbo Boost Technology Support
Global Memory Size
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Vectorizer Knobs
Several environment variables and one attribute extension are related to vectorizer.
Environment Variables
This variable can be set to False and True respectively. Notice that just like any other environment variables this one affects the behavior of the vectorizer of the entire system (or shell instances) until variable gets unset explicitly (or shell(s) terminates).
This variable affects code generation for CPU OpenCL device only. It effectively sets the vectorization “width” (when CL_CONFIG_USE_VECTORIZER = True):
- CL_CONFIG_CPU_VECTORIZER_MODE = 0 (default). The compiler makes heuristic decisions whether to vectorize each kernel, and if so which vector width to use.
- CL_CONFIG_CPU_VECTORIZER_MODE = 1. No vectorization by compiler. Explicit vector data types in kernels are left intact. This mode is the same as CL_CONFIG_USE_VECTORIZER = False.
- CL_CONFIG_CPU_VECTORIZER_MODE = 4. Disables heuristic and vectorizes to the width of 4.
- CL_CONFIG_CPU_VECTORIZER_MODE = 8. Disables heuristic and vectorizes to the width of 8.
- CL_CONFIG_CPU_VECTORIZER_MODE = 16. Disables heuristic and vectorizes to the width of 16.
Attribute Extensions
The goal of this extension is to allow programmers to specify vector length the kernel should be vectorized to. This information may be used by the compiler to generate more optimal code.
For a device that supports the extension, the function clGetDeviceInfo with the parameter CL_DEVICE_EXTENSION returns a space separated list of extensions names that contains cl_intel_vec_len_hint.
Use the OpenCL C optional attribute qualifier __attribute__((intel_vec_len_hint(<int>))), where <int> is a vector length that the kernel should be vectorized to.
You can set one the following value to the variable:
- 0 - the compiler makes heuristic decisions whether to vectorize each kernel, and if so which vector length to use.
- 1 - no vectorization by compiler. Explicit vector data types in kernels are left intact.
- 4 - disables heuristic and vectorizes to the length of 4.
- 8 - disables heuristic and vectorizes to the length of 8.
- 16 - disables heuristic and vectorizes to the length of 16.
NOTE:
Note
NOTE:
If the work group size is not evenly divisible by the specified vector length hint, loop remainder might not be executed in vector code iterations.
NOTE:
Note
NOTE:
If you specify simultaneously the intel_vec_len_hint and vec_type_hint attributes, the compiler ignores vec_type_hint attribute.
The examples below illustrate valid and invalid uses of the extension:
- Vectorizing to the length of 8:
__attribute__((intel_vec_len_hint(8))) __kernel void kr1(…) { … } - Specifying both intel_vec_len_hint and vec_type_hint. In the example, the compiler ignores vec_type_hint and vectorize to the length of 4.
__attribute__((intel_vec_len_hint(4))) __attribute__((vec_type_hint (float8))) __kernel void kr2(…) { … }