-qsimd
Category
Pragma equivalent
#pragma nosimd
Purpose
Controls whether the compiler can automatically take advantage of vector instructions for processors that support them.
These instructions can offer higher performance when used with algorithmic-intensive tasks such as multimedia applications.
Defaults
Whether -qsimd is specified or not, -qsimd=auto is implied at the -O3 or higher optimization level; -qsimd=noauto is implied at the -O2 or lower optimization level.
Usage
The -qsimd=auto option enables automatic generation of vector instructions for processors that support them. When -qsimd=auto is in effect, the compiler converts certain operations that are performed in a loop on successive elements of an array into vector instructions. These instructions calculate several results at one time, which is faster than calculating each result sequentially. These options are useful for applications with significant image processing demands.
The -qsimd=noauto option disables the conversion of loop array operations into vector instructions. To achieve finer control, use -qstrict=ieeefp, -qstrict=operationprecision, and -qstrict=vectorprecision. For details, see -qstrict.
- Specifying -qsimd without any suboption is equivalent to -qsimd=auto.
- Specifying -qsimd=auto does not guarantee that autosimdization will occur.
- Using vector instructions to calculate several results at one time might delay or even miss detection of floating-point exceptions on some architectures. If detecting exceptions is important, do not use -qsimd=auto.
Rules
If you enable IPA and specify -qsimd=auto at the IPA compile step, but specify -qsimd=noauto at the IPA link step, the compiler automatically sets -qsimd=auto at the IPA link step. Similarly, if you enable IPA and specify -qsimd=noauto at the IPA compile step, but specify -qsimd=auto at the IPA link step, the compiler automatically sets -qsimd=auto at the compile step.
Predefined macros
None.
Examples
- xlc -O3 -qsimd
- xlc -O2 -qhot=level=0 -qsimd=auto
- xlc -O2 -qsimd=auto
...
#pragma nosimd
for (i=1; i<1000; i++) {
/* program code */
}