Compiler Methodology for Intel® MIC Architecture

Selective Use of gatherhint/scatterhint Instructions

Overview

 This note documents a known hardware issue with early alpha hardware of the Intel® Xeon® Phi™ coprocessor (A0 stepping from 2011) and an undocumented option to work around it.

The -mGLOB_default_function_attrs=”use_gather_scatter_hint=on” compiler option can be used to generate gatherhint/scatterhint instructions.  This is useful if your code is doing non-unit stride accesses and/or uses indirect addressing via pointers or index arrays. 

Topics

Here is the compiler behavior related to gatherhint/scatterhint generation:

Behavior in the compiler:

1. By default compiler does not generate gatherhint and nop

2. When -mGLOB_default_function_attrs=”use_gather_scatter_hint=on”  is specified, it generates gatherhint with a nop.

gatherhint
gatherhint
nop
gather

One can get the benefit of gather hints in current compilers using this option.

Take Aways

The gatherhint/scatterhint instructions are useful for codes with non-unit stride memory accesses, and codes using indirect addressing through pointers or index arrays. 

NEXT STEPS

It is essential that you read this guide from start to finish using the built-in hyperlinks to guide you along a path to a successful port and tuning of your application(s) on Intel® Xeon Phi™Coprocessors.  The paths provided in this guide reflect the steps necessary to get best possible application performance.

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