Vectorization and Loops

Intel® C++ Compiler Classic Developer Guide and Reference

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Date 3/31/2023

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Vectorization and Loops

This topic provides more information on the interaction between the auto-vectorizer and loops.

Interactions with Loop Parallelization

Combine the [Q]parallel and [Q]x options to instruct the Intel® C++ Compiler to attempt both Automatic Parallelization and automatic loop vectorization in the same compilation.

NOTE:

Using this option enables parallelization for both Intel® microprocessors and non-Intel microprocessors. The resulting executable may get additional performance gain on Intel® microprocessors than on non-Intel microprocessors. The parallelization can also be affected by certain options, such as /arch (Windows), -m (Linux and macOS), or [Q]x.

NOTE:

This option enables vectorization at default optimization levels for both Intel® microprocessors and non-Intel microprocessors. Vectorization may call library routines that can result in additional performance gain on Intel® microprocessors than on non-Intel microprocessors. The vectorization can also be affected by certain options, such as /arch (Windows), -m (Linux and macOS), or [Q]x.

In most cases, the compiler will consider outermost loops for parallelization and innermost loops for vectorization. If deemed profitable, however, the compiler may even apply loop parallelization and vectorization to the same loop.

See Programming with Auto-parallelization and Programming Guidelines for Vectorization.

In some rare cases, a successful loop parallelization (either automatically or by means of OpenMP directives) may affect the messages reported by the compiler for a non-vectorizable loop in a non-intuitive way; for example, in the cases where the /Qopt-report:2 /Qopt-report-phase:vec (Windows) or -qopt-report=2 -qopt-report-phase=vec (Linux and macOS) options indicate that loops were not successfully vectorized.

Types of Vectorized Loops

For integer loops, the 128-bit Intel® Streaming SIMD Extensions (Intel® SSE) and the Intel® Advanced Vector Extensions (Intel® AVX) provide SIMD instructions for most arithmetic and logical operators on 32-bit, 16-bit, and 8-bit integer data types, with limited support for the 64-bit integer data type.

Vectorization may proceed if the final precision of integer wrap-around arithmetic is preserved. A 32-bit shift-right operator, for instance, is not vectorized in 16-bit mode if the final stored value is a 16-bit integer. Also, note that because the Intel® SSE and the Intel® AVX instruction sets are not fully orthogonal (shifts on byte operands, for instance, are not supported), not all integer operations can actually be vectorized.

For loops that operate on 32-bit single-precision and 64-bit double-precision floating-point numbers, Intel® SSE provides SIMD instructions for the following arithmetic operators:

Addition (+)
Subtraction (-)
Multiplication (*)
Division (/)

Additionally, Intel® SSE provide SIMD instructions for the binary MIN and MAX and unary SQRT operators. SIMD versions of several other mathematical operators (like the trigonometric functions SIN, COS, and TAN) are supported in software in a vector mathematical run-time library that is provided with the compiler.

To be vectorizable, loops must be:

Countable: The loop trip count must be known at entry to the loop at runtime, though it need not be known at compile time (that is, the trip count can be a variable but the variable must remain constant for the duration of the loop). This implies that exit from the loop must not be data-dependent.

Single entry and single exit: as is implied by stating that the loop must be countable. Consider the following example of a loop that is not vectorizable, due to a second, data-dependent exit:

void no_vec(float a[], float b[], float c[]){
  int i = 0.;
  while (i < 100) {
    a[i] = b[i] * c[i];
    //  this is a data-dependent exit condition:
    if (a[i] < 0.0)
    break;
    ++i;
  } 
}

Which results in the following message when the code is compiled:

> icc -c -O2 -qopt-report=2 -qopt-report-phase=vec two_exits.cpp 
two_exits.cpp(4) (col. 9): remark: loop was not vectorized: nonstandard loop is not a vectorization candidate.

Contain straight-line code: SIMD instruction perform the same operation on data elements from multiple iterations of the original loop, therefore, it is not possible for different iterations to have different control flow; that is, they must not branch. It follows that switch statements are not allowed. However, if statements are allowed if they can be implemented as masked assignments, which is usually the case. The calculation is performed for all data elements but the result is stored only for those elements for which the mask evaluates to true. Consider the following example that may be vectorized:
```
#include <math.h> 
void quad(int length, float *a, float *b, float *c, float *restrict x1, float *restrict x2) { 
  for (int i=0; i<length; i++) {
    float s = b[i]*b[i] - 4*a[i]*c[i];
    if ( s >= 0 ) {
      s = sqrt(s) ;
      x2[i] = (-b[i]+s)/(2.*a[i]);
      x1[i] = (-b[i]-s)/(2.*a[i]);
    } else {
      x2[i] = 0.;
      x1[i] = 0.;
    } 
  } 
} 
```
Which results in the following message when the code is compiled:
```
> icc -c -restrict -qopt-report=2 -qopt-report-phase=vec quad.cpp 
quad5.cpp(5) (col. 3): remark: LOOP WAS VECTORIZED.
```
Innermost loop of a nest: The only exception is if an original outer loop is transformed into an inner loop as a result of some other prior optimization phase, such as unrolling, loop collapsing or interchange, or an original outermost loop is transformed to an innermost loop due to loop materialization.
Without function calls: Even a print statement is sufficient to prevent a loop from getting vectorized. The vectorization report message is typically: non-standard loop is not a vectorization candidate. The two major exceptions are for intrinsic math functions and for functions that may be inlined.

Intrinsic math functions are allowed, because the compiler runtime library contains vectorized versions of these functions. See below for a list of these functions; most exist in both float and double versions:

acos
acosh
asin
asinh
atan
atan2
atanh
cbrt
ceil
cos
cosh
erf
erfc
erfinv
exp
exp2
fabs
floor
fmax
fmin
log
log2
log10
pow
round
sin
sinh
sqrt
tan
tanh
trunc

The loop in the following example may be vectorized because sqrt() is vectorizable and func() gets inlined. Inlining is enabled at default optimization for functions in the same source file. An inlining report may be obtained by setting the options /Qopt-report:2 /Qopt-report-phase:ipo (Windows) or -qopt-report=2 -qopt-report-phase=ipo (Linux).

float func(float x, float y, float xp, float yp) {
  float denom;
  denom = (x-xp)*(x-xp) + (y-yp)*(y-yp);
  denom = 1./sqrtf(denom);
  return denom; 
} 

float trap_int(float y, float x0, float xn, int nx, float xp, float yp) {
  float x, h, sumx;
  int i;
  h = (xn-x0) / nx;
  sumx = 0.5*( func(x0,y,xp,yp) + func(xn,y,xp,yp) );
  for (i=1;i<nx;i++) {
    x = x0 + i*h;
    sumx = sumx + func(x,y,xp,yp);
  }
  sumx = sumx * h;
  return sumx; 
}

Which results in the following message when the code is compiled:

> icc -c -qopt-report=2 -qopt-report-phase=vec trap_integ.c 
trap_int.c(16) (col. 3): remark: LOOP WAS VECTORIZED.

Statements in the Loop Body

The vectorizable operations are different for floating-point and integer data.

Integer Array Operations

The statements within the loop body may contain char, unsigned char, short, unsigned short, int, and unsigned int. Calls to functions such as sqrt and fabs are also supported. Arithmetic operations are limited to addition, subtraction, bitwise AND, OR, and XOR operators, division (via run-time library call), multiplication, min, and max. You can mix data types but this may potentially cost you in terms of lowering efficiency. Some example operators where you can mix data types are multiplication, shift, or unary operators.

Other Operations

No statements other than the preceding floating-point and integer operations are allowed. In particular, note that the special __m64__m128, and __m256 data types are not vectorizable. The loop body cannot contain any function calls. Use of Intel® SSE intrinsics ( for example, _mm_add_ps) or Intel® AVX intrinsics (for example, _mm256_add_ps) are not allowed.

Product and Performance Information
Performance varies by use, configuration and other factors. Learn more at www.Intel.com/PerformanceIndex. Notice revision #20201201

Product and Performance Information

Performance varies by use, configuration and other factors. Learn more at www.Intel.com/PerformanceIndex.

Notice revision #20201201

Parent topic: Automatic Vectorization

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Vectorization and Loops

Interactions with Loop Parallelization

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