opt-gather-scatter-unroll, Qopt-gather-scatter-unroll

Lets you specify an alternative loop unroll sequence for gather and scatter loops on Intel® MIC Architecture. This option only applies to Intel® MIC Architecture.

Architecture Restrictions

Only available on Intel® 64 architecture targeting Intel® MIC Architecture

Syntax

Linux OS:

-opt-gather-scatter-unroll=n

-no-opt-gather-scatter-unroll

OS X:

None

Windows OS:

/Qopt-gather-scatter-unroll:n

/Qopt-gather-scatter-unroll-

Arguments

`n`	Is the unroll factor for the gather and scatter loops. It must be an integer between 0 and 8. If you specify value 0 for `n`, it is the same as specifying the negative form of the option.

Default

-no-opt-gather-scatter-unroll or /Qopt-gather-scatter-unroll

The compiler uses default heuristics when unrolling gather and scatter loops.

Description

This option lets you specify an alternative loop unroll sequence for gather and scatter loops on Intel® MIC Architecture.

This option may improve performance of gather/scatter operations.

The value of n that provides the best performance is data-dependent.

In cases where the gather/scatter operation accesses data in a small number of cache-lines (such as 1 or 2), the default sequence (using a small value for n) works best. In cases where each individual data item falls in a different cache-line, it may be better to use a large value for n.

IDE Equivalent

None

Alternate Options

None

Example

Normally, on Intel® MIC Architecture, there are no "one-shot" gather/scatter instructions, so the compiler generates a loop to perform complete gather/scatter. By default, the loop looks as follows:

L1:
   gather
   jkz L2
   gather
   jknz L1
L2:

For some applications, this loop would be faster if it was unrolled; and different applications may benefit from different unroll factors. Also, when the loop is unrolled, adding gather/scatter hint instructions before the loop provides additional benefits.

If you specify the [Q]opt-gather-scatter-unroll option, the compiler will generate a similar loop unrolled by the number specified in n. The following example shows what happens when the -opt-gather-scatter-unroll=3 (Linux* OS) or /Qopt-gather-scatter-unroll:3 option (Windows* OS) is specified. Notice that the alternate sequence also generates two gather/scatter hint instructions preceding the loop:

   gather hint
   gather hint
   nop
L1:
   gather
   jkz L2
   gather |
   gather |  -> the number of gathers specified by n
   gather |
   jknz L1
L2: