vector

Syntax

Arguments

Description

The vector pragma indicates that the loop should be vectorized, if it is legal to do so, ignoring normal heuristic decisions about profitability. The vector pragma takes several argument keywords to specify the kind of loop vectorization required. These keywords are aligned, unaligned, always, temporal, and nontemporal. The compiler does not apply the vector pragma to nested loops, each nested loop needs a preceding pragma statement. Place the pragma before the loop control statement.

Using the aligned/unaligned keywords

When the aligned/unaligned argument keyword is used with this pragma, it indicates that the loop should be vectorized using aligned/unaligned data movement instructions for all array references. Specify only one argument keyword: aligned or unaligned.

Using the always keyword

When the always argument keyword is used, the pragma controls the vectorization of the subsequent loop in the program. If assert is added, the compiler will generate an error-level assertion test to display a message saying that the compiler efficiency heuristics indicate that the loop cannot be vectorized.

Using the nontemporal/temporal keywords

The nontemporal and temporal argument keywords are used to control how the "stores" of register contents to storage are performed (streaming versus non-streaming) on systems based on IA-32 and Intel® 64 architectures.

On systems based on Intel® MIC Architecture, #pragma vector nontemporal directs the compiler to generate clevict (cache-line-evict) instructions after the stores based on the non-temporal pragma when the compiler knows that the store addresses are aligned; optionally takes a comma separated list of variables

By default, the compiler automatically determines whether a streaming store should be used for each variable.

Streaming stores may cause significant performance improvements over non-streaming stores for large numbers on certain processors. However, the misuse of streaming stores can significantly degrade performance.

Using the [no]vecremainder keyword

If the vector always pragma and keyword are specified, the following occurs:

• If the vecremainder clause is specified, the compiler vectorizes both the main and remainder loops.

• If the novecremainder clause is specified, the compiler vectorizes the main loop, but it does not vectorize the remainder loop.

Using the [no]mask_readwrite keyword

If the vector pragma and mask_readwrite or nomask_readwrite keyword are specified, the following occurs:

• If the mask_readwrite clause is specified, the compiler generates masked loads and stores within all conditions in the loop.

• If the nomask_readwrite clause is specified, the compiler generates unmasked loads and stores for increased performance.

Examples

In the following example, the aligned argument keyword is used to request that the loop be vectorized with aligned instructions.

Note that the arrays are declared in such a way that the compiler could not normally prove this would be safe to vectorize.

void vec_aligned(float *a, int m, int c) {
  int i;
  // Instruct compiler to ignore assumed vector dependencies.
  #pragma vector aligned
  for (i = 0; i < m; i++)
    a[i] = a[i] * c;
  // Alignment unknown but compiler can still align.
  for (i = 0; i < 100; i++)
    a[i] = a[i] + 1.0f; 
}

void vec_always(int *a, int *b, int m) {
  #pragma vector always
  for(int i = 0; i <= m; i++)
    a[32*i] = b[99*i]; 
}

float a[1000]; 
void foo(int N){
  int i;
  #pragma vector nontemporal
  for (i = 0; i < N; i++) {
    a[i] = 1;
  } 
}

A float-type loop together with the generated assembly is shown in the following example. For large N, significant performance improvements result on systems with Intel® Pentium® 4 processors over non-streaming implementations.

  .B1.2: 
movntps XMMWORD PTR _a[eax], xmm0 
movntps XMMWORD PTR _a[eax+16], xmm0 
add eax, 32 
cmp eax, 4096 
jl .B1.2

double A[1000]; 
double B[1000]; 
void foo(int n){
  int i; 
#pragma vector nontemporal (A, B)
  for (i=0; i<n; i++){
    A[i] = 0;
    B[i] = i;
  } 
}