# qopt-prefetch-distance, Qopt-prefetch-distance

Specifies the prefetch distance to be used for compiler-generated prefetches inside loops. This feature is only available for ifort.

## Syntax

Linux: | -qopt-prefetch-distance=n1[, n2]

macOS: | None

Windows: | /Qopt-prefetch-distance:n1[, n2]

## Arguments

n1, n2 | Is the prefetch distance in terms of the number of (possibly-vectorized) iterations. Possible values are non-negative numbers >=0. n2 is optional. n1 = 0 turns off all compiler issued prefetches from memory to L2. n2 = 0 turns off all compiler issued prefetches from L2 to L1. If n2 is specified and n1 > 0, n1 should be >= n2.

## Default

OFF | The compiler uses default heuristics to determine the prefetch distance.

## Description

This option specifies the prefetch distance to be used for compiler-generated prefetches inside loops. The unit (n1 and optionally n2) is the number of iterations. If the loop is vectorized by the compiler, the unit is the number of vectorized iterations.

The value of n1 will be used as the distance for prefetches from memory to L2 (for example, the vprefetch1 instruction). If n2 is specified, it will be used as the distance for prefetches from L2 to L1 (for example, the vprefetch0 instruction).

This option is ignored if option -qopt-prefetch=0 (Linux*) or /Qopt-prefetch:0 (Windows*) is specified.

## IDE Equivalent

None

## Alternate Options

None

## Example

Consider the following Linux* examples:

    -qopt-prefetch-distance=64,32

The above causes the compiler to use a distance of 64 iterations for memory to L2 prefetches, and a distance of 32 iterations for L2 to L1 prefetches.

    -qopt-prefetch-distance=24

The above causes the compiler to use a distance of 24 iterations for memory to L2 prefetches. The distance for L2 to L1 prefetches will be determined by the compiler.

    -qopt-prefetch-distance=0,4

The above turns off all memory to L2 prefetches inserted by the compiler inside loops. The compiler will use a distance of 4 iterations for L2 to L1 prefetches.

    -qopt-prefetch-distance=16,0

The above causes the compiler to use a distance of 16 iterations for memory to L2 prefetches. No L2 to L1 loop prefetches are issued by the compiler.

Parent topic: Advanced Optimization Options

## See Also

qopt-prefetch, Qopt-prefetch compiler option
PREFETCH directive