vector

Intel® C++ Compiler Classic Developer Guide and Reference

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ID 767249

Date 12/16/2022

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vector

Tells the compiler that the loop should be vectorized according to the argument keywords.

Syntax

#pragma vector {always[assert]|aligned|unaligned|dynamic_align|nodynamic_align|[no] multiple_gather_scatter_by_shuffles|temporal|nontemporal|[no]vecremainder|[no]mask_readwrite|vectorlength(n1[, n2]...)}

#pragma vector nontemporal[(var1[, var2, ...])]

Arguments

always	Instructs the compiler to override any efficiency heuristic during the decision to vectorize or not, and vectorize non-unit strides or very unaligned memory accesses; controls the vectorization of the subsequent loop in the program; optionally takes the keyword assert.
aligned	Instructs the compiler to use aligned data movement instructions for all array references when vectorizing.
unaligned	Instructs the compiler to use unaligned data movement instructions for all array references when vectorizing.
dynamic_align[(`var`)]	Instructs the compiler to perform dynamic alignment optimization for the loop with an optionally specified variable to perform alignment on.
nodynamic_align	Disables dynamic alignment optimization for the loop.
multiple_gather_scatter_by_shuffles	Instructs the optimizer to disable the generation of gather/scatter and to transform gather/scatter into unit-strided loads/stores plus a set of shuffles wherever possible.
nomultiple_gather_scatter_by_shuffles	Instructs the optimizer to enable the generation of gather/scatter instructions and not to transform gather/scatter into unit-strided loads/stores.
nontemporal	Instructs the compiler to use non-temporal (that is, streaming) stores on systems based on all supported architectures, unless otherwise specified; optionally takes a comma-separated list of variables. When this pragma is specified, it is your responsibility to also insert any fences as required to ensure correct memory ordering within a thread or across threads. One typical way to do this is to insert a `_mm_sfence` intrinsic call just after the loops (such as the initialization loop) where the compiler may insert streaming store instructions.
temporal	Instructs the compiler to use temporal (that is, non-streaming) stores on systems based on all supported architectures, unless otherwise specified.
vecremainder	Instructs the compiler to vectorize the remainder loop when the original loop is vectorized.
novecremainder	Instructs the compiler not to vectorize the remainder loop when the original loop is vectorized.
mask_readwrite	Disables memory speculation, causing the generation of masked load and store operations within conditions.
nomask_readwrite	Enables memory speculation, causing the generation of non-masked loads and stores within conditions.
vectorlength (`n1`[, `n2`]...)	Instructs the vectorizer which vector length/factor to use when generating the main vector loop.

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. 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.

CAUTION:

If you specify aligned as an argument, you must be sure that the loop is vectorizable using this pragma. Otherwise, the compiler generates incorrect code.

Using the always keyword

When the always argument keyword is used, the pragma will ignore compiler efficiency heuristics for the subsequent loop. When assert is added, the compiler will generate a diagnostic message if the loop cannot be vectorized for any reason.

Using the dynamic_align and nodynamic_align keywords

Dynamic alignment is an optimization the compiler attempts to perform by default. It involves peeling iterations from the vector loop into a scalar loop before the vector loop so that the vector loop aligns with a particular memory reference. The dynamic_align (var) form of the directive allows the user to provide a scalar or array variable name to align on. Specifying nodynamic_align with or without var does not guarantee the optimization is performed; the compiler still uses heuristics to determine feasibility of the operation.

Using the multiple_gather_scatter_by_shuffles and nomultiple_gather_scatter_by_shuffles keywords

These clauses do not affect loops nested in the specified loop.

Using the nontemporal and 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.

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.

Using the vectorlength keyword

n is an integer power of 2; the value must be 2, 4, 6, 8, 16, 32, or 64. If more than one value is specified, the vectorizer will choose one of the specified vector lengths based on a cost model decision.

NOTE:

The pragma vector should be used with care.

Overriding the efficiency heuristics of the compiler should only be done if the programmer is absolutely sure that vectorization will improve performance. Furthermore, instructing the compiler to implement all array references with aligned data movement instructions will cause a run-time exception in case some of the access patterns are actually unaligned.

Examples

Use the vector aligned pragma

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; 
}

Use the vector always pragma

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]; 
}

Use the vector multiple gather type pragma

float sum=0.0f;
#pragma omp simd reduction(+:sum)
for (i=0; i<N; i++){
   sum += A[3*i+0] + A[3*i+1] + A[3*i+2];
}

Use the vector nontemporal pragma

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

Use ASM code for the loop body

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

Use pragma vector nontemporal with variables for implementing streaming stores

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;
  } 
}

Parent topic: Intel-Specific Pragma Reference

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