# simdoff

Specifies a block of code in the SIMD loop or SIMD-enabled function that should be executed serially, in a logical order of SIMD lanes.

## Syntax

#pragma simdoff

structured-block

## Arguments

None.

## Description

The simdoff block will use a single SIMD lane to execute operations in the order of the loop iterations, or logical lanes of a SIMD-enabled function. This preserves ordering of operations in the block with respect to each other, and correlates with iteration space of the enclosing SIMD construct. The ordered simd block is executed in order, with respect to each SIMD lane or each loop iteration. The operations within the ordered simd or simdoff block can be re-ordered by optimizations, as long as the original execution semantics are preserved.

simdoff blocks allow the isolation and resolution of situations prohibited from SIMD execution. This includes cross-iteration data dependencies, function calls with side effects, such as OpenMP, oneTBB and native thread synchronization primitives.

simdoff sections are useful for resolving cross-iteration data dependencies in otherwise data-parallel computations. For example, the section may handle histogram updates as shown below:

Example
    #pragma simd
    for (int i = 0; i < N; i++)
    {
        float amount = compute_amount(i);
        int   cluster = compute_cluster(i);
    #pragma simdoff
        {
           totals[cluster] += amount; // Requires ordering to process multiple updates for the same cluster
        }
    }

Parent topic: Intel-Specific Pragma Reference