Developer Guide and Reference

Contents

Using Automatic Vectorization

Automatic vectorization is supported on Intel® 64
(for
C++, DPC++, and
Fortran)
architectures. The information below will guide you in setting up the auto-vectorizer.

Vectorization Speed-up

Where does the vectorization speedup come from? Consider the following sample code fragment, where
a
,
b
and
c
are integer arrays:
Sample Code Fragment
for (I=0;i<=MAX;i++) c[i]=a[i]+b[i];
If vectorization is not enabled, that is, you compile using the
O1
-no-vec-
(Linux*) or
/Qvec-
(Windows*) option, for each iteration, the compiler processes the code such that there is a lot of unused space in the SIMD registers, even though each of the registers could hold three additional integers. If vectorization is enabled (compiled using
O2
or higher options), the compiler may use the additional registers to perform four additions in a single instruction. The compiler looks for vectorization opportunities whenever you compile at default optimization (
O2
) or higher.
Using 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.
To get details about the type of loop transformations and optimizations that took place, use the
[Q]opt-report-phase
option by itself or along with the
[Q]opt-report
option.
How significant is the performance enhancement? To evaluate performance enhancement yourself, run
vec_samples
:
  1. Open an Intel®
    oneAPI DPC++/C++
    Compiler command-line window.
    • On Windows*:
      Under the
      Start
      menu item for your Intel product, select an icon under
      Intel oneAPI 2021
      Intel oneAPI Command Prompt
      for oneAPI Compilers.
    • On Linux*:
      Source an environment script such as
      vars.sh
      in the
      <installdir>
      directory and use the attribute appropriate for the architecture.
  2. Navigate to the
    <installdir>\Samples\<locale>\C++\
    (for C++) or
    <installdir>\Samples\<locale>\DPC++\
    (for DPC++)
    directory. On Windows, unzip the sample project
    vec_samples.zip
    to a writable directory. This small application multiplies a vector by a matrix using the following loop:
    Example: Vector Matrix Multiplication
    for (j = 0;j < size2; j++) { b[i] += a[i][j] * x[j]; }
  3. Build and run the application, first without enabling auto-vectorization. The default
    O2
    optimization enables vectorization, so you need to disable it with a separate option. Note the time taken for the application to run.
    Example: Building and Running an Application without Auto-vectorization
    // (Linux) icx -O2 -no-vec Multiply.c -o NoVectMult ./NoVectMult
    // (Windows) icx /O2 /Qvec- Multiply.c /FeNoVectMult NoVectMult
  4. Now build and run the application, this time with auto-vectorization. Note the time taken for the application to run.
    Example: Building and Running an Application with Auto-vectorization
    // (Linux) vicc -O2 -qopt-report=1 -qopt-report-phase=vec Multiply.c -o VectMult ./VectMult
    // (Windows for C++) icx /O2 /Qopt-report:1 /Qopt-report-phase:vec Multiply.c /FeVectMult VectMult
    // (Windows for DPC++) dpcpp /O2 /Qopt-report:1 /Qopt-report-phase:vec Multiply.c /FeVectMult VectMult
When you compare the timing of the two runs, you may see that the vectorized version runs faster. The time for the non-vectorized version is only slightly faster than would be obtained by compiling with the
O1
option.

Obstacles to Vectorization

The following do not always prevent vectorization, but frequently either prevent it or cause the compiler to decide that vectorization would not be worthwhile.
  • Non-contiguous memory access:
    Four consecutive integers or floating-point values, or two consecutive doubles, may be loaded directly from memory in a single SSE instruction. But if the four integers are not adjacent, they must be loaded separately using multiple instructions, which is considerably less efficient. The most common examples of non-contiguous memory access are loops with non-unit stride or with indirect addressing, as in the examples below. The compiler rarely vectorizes such loops, unless the amount of computational work is large compared to the overhead from non-contiguous memory access.
    Example: Non-contiguous Memory Access
    // arrays accessed with stride 2 for (int I=0; i<SIZE; I+=2) b[i] += a[i] * x[i]; // inner loop accesses a with stride SIZE for (int j=0; j<SIZE; j++) { for (int I=0; i<SIZE; I++) b[i] += a[i][j] * x[j]; } // indirect addressing of x using index array for (int I=0; i<SIZE; I+=2) b[i] += a[i] * x[index[i]];
    The typical message from the vectorization report is:
    vectorization possible but seems inefficient
    , although indirect addressing may also result in the following report:
    Existence of vector dependence.
  • Data dependencies:
    Vectorization entails changes in the order of operations within a loop, since each SIMD instruction operates on several data elements at once. Vectorization is only possible if this change of order does not change the results of the calculation.
    • The simplest case is when data elements that are written (stored to) do not appear in any other iteration of the individual loop. In this case, all the iterations of the original loop are independent of each other, and can be executed in any order, without changing the result. The loop may be safely executed using any parallel method, including vectorization. All the examples considered so far fall into this category.
    • When a variable is written in one iteration and read in a subsequent iteration, there is a “read-after-write” dependency, also known as a flow dependency, as in this example:
      Example: Flow Dependency
      A[0]=0; for (j=1; j<MAX; j++) A[j]=A[j-1]+1; // this is equivalent to: A[1]=A[0]+1; A[2]=A[1]+1; A[3]=A[2]+1; A[4]=A[3]+1;
      So the value of j gets propagated to all
      A[j]
      . This cannot safely be vectorized: if the first two iterations are executed simultaneously by a SIMD instruction, the value of
      A[1]
      is used by the second iteration before it has been calculated by the first iteration.
    • When a variable is read in one iteration and written in a subsequent iteration, this is a
      write-after-read
      dependency, also known as an
      anti-dependency
      , as in the following example:
      Example: Write-after-read Dependency
      for (j=1; j<MAX; j++) A[j-1]=A[j]+1; // this is equivalent to: A[0]=A[1]+1; A[1]=A[2]+1; A[2]=A[3]+1; A[3]=A[4]+1;
      This write-after-read dependency is not safe for general parallel execution, since the iteration with the write may execute before the iteration with the read. However, for vectorization, no iteration with a higher value of
      j
      can complete before an iteration with a lower value of
      j
      , and so vectorization is safe (that is, it gives the same result as non-vectorized code) in this case. The following example, however, may not be safe, since vectorization might cause some elements of
      A
      to be overwritten by the first SIMD instruction before being used for the second SIMD instruction.
      Example: Unsafe Vectorization
      for (j=1; j<MAX; j++) { A[j-1]=A[j]+1; B[j]=A[j]*2; } // this is equivalent to: A[0]=A[1]+1; A[1]=A[2]+1; A[2]=A[3]+1; A[3]=A[4]+1;
    • Read-after-read situations are not really dependencies, and do not prevent vectorization or parallel execution. If a variable is unwritten, it does not matter how often it is read.
    • Write-after-write, or ‘output’, dependencies, where the same variable is written to in more than one iteration, are in general unsafe for parallel execution, including vectorization.
    • One important exception, that apparently contains all of the above types of dependency:
      Example: Dependency Exception
      sum=0; for (j=1; j<MAX; j++) sum = sum + A[j]*B[j]
      Although
      sum
      is both read and written in every iteration, the compiler recognizes such reduction idioms, and is able to vectorize them safely. The loop in the first example was another example of a reduction, with a loop-invariant array element in place of a scalar.
      These types of dependencies between loop iterations are sometimes known as loop-carried dependencies.
      The above examples are of proven dependencies. The compiler cannot safely vectorize a loop if there is even a potential dependency. Consider the following example:
      Example: Potential Dependency
      for (I = 0; I < size; I++) { c[i] = a[i] * b[i]; }
      In the above example, the compiler needs to determine whether, for some iteration
      I
      ,
      c[i]
      might refer to the same memory location as
      a[i]
      or
      b[i]
      for a different iteration. Such memory locations are sometimes said to be
      aliased
      . For example, if
      a[i]
      pointed to the same memory location as
      c[i-1]
      , there would be a read-after-write dependency as in the earlier example. If the compiler cannot exclude this possibility, it will not vectorize the loop unless you provide the compiler with hints.

Helping the
Intel® oneAPI
DPC++/C++
Compiler
to Vectorize

Sometimes the compiler has insufficient information to decide to vectorize a loop. There are several ways to provide additional information to the compiler:
  • Pragmas:
    • #pragma ivdep:
      may be used to tell the compiler that it may safely ignore any potential data dependencies. (The compiler will not ignore proven dependencies). Use of this pragma when there are dependencies may lead to incorrect results.
      There are cases where the compiler cannot tell by a static dependency analysis that it is safe to vectorize. Consider the following loop:
      Loop Example
      void copy(char *cp_a, char *cp_b, int n) { for (int I = 0; I < n; I++) { cp_a[i] = cp_b[i]; } }
      Without more information, a vectorizing compiler must conservatively assume that the memory regions accessed by the pointer variables
      cp_a
      and
      cp_b
      may (partially) overlap, which gives rise to potential data dependencies that prohibit straightforward conversion of this loop into SIMD instructions. At this point, the compiler may decide to keep the loop serial or, as done by the
      Intel® oneAPI
      DPC++/C++
      Compiler
      , generate a run-time test for overlap, where the loop in the true-branch can be converted into SIMD instructions:
      Example: True-branch Loop
      if (cp_a + n < cp_b || cp_b + n < cp_a) /* vector loop */ for (int I = 0; I < n; I++) cp_a[i] = cp_b [I]; else /* serial loop */ for (int I = 0; I < n; I++) cp_a[i] = cp_b[i];
      Run-time data-dependency testing provides a generally effective way to exploit implicit parallelism in C or C++ code at the expense of a slight increase in code size and testing overhead. If the function copy is only used in specific ways, however, you can assist the vectorizing compiler as follows:
      • If the function is mainly used for small values of n or for overlapping memory regions, you can simply prevent vectorization and, hence, the corresponding run-time overhead by inserting a
        #pragma novector
        hint before the loop.
      • Conversely, if the loop is guaranteed to operate on non-overlapping memory regions, you can provide this information to the compiler by means of a
        #pragma ivdep
        hint before the loop, which informs the compiler that conservatively assumed data dependencies that prevent vectorization can be ignored. This results in vectorization of the loop without run-time data-dependency testing.
        Example: Ignoring Data Dependencies with
        #pragma ivdep
        #pragma ivdep void copy(char *cp_a, char *cp_b, int n) { for (int I = 0; I < n; I++) { cp_a[i] = cp_b[i]; } }
      You can also use the
      restrict
      keyword.
    • #pragma loop count (n):
      may be used to advise the compiler of the typical trip count of the loop. This may help the compiler to decide whether vectorization is worthwhile, or whether or not it should generate alternative code paths for the loop.
    • #pragma vector always:
      asks the compiler to vectorize the loop if it is safe to do so, whether or not the compiler thinks that will improve performance.
    • #pragma vector align:
      asserts that data within the following loop is aligned (to a 16-byte boundary, for Intel® SSE instruction sets).
    • #pragma novector:
      asks the compiler not to vectorize a particular loop.
    • #pragma vector nontemporal:
      gives a hint to the compiler that data will not be reused, and therefore to use streaming stores that bypass cache.
  • Keywords:
    The
    restrict
    keyword may be used to assert that the memory referenced by a pointer is not aliased, i.e. that it is not accessed in any other way. The keyword requires the use of the
    [Q]
    std=c99
    compiler option. The example under
    #pragma ivdep
    above can also be handled using the
    restrict
    keyword.
    You may use the
    restrict
    keyword in the declarations of
    cp_a
    and
    cp_b
    , as shown below, to inform the compiler that each pointer variable provides exclusive access to a certain memory region. The
    restrict
    qualifier in the argument list lets the compiler know that there are no other aliases to the memory to which the pointers point. In other words, the pointer for which it is used provides the only means of accessing the memory in question in the scope in which the pointers live. Even if the code gets vectorized without the
    restrict
    keyword, the compiler checks for aliasing at run-time, if the
    restrict
    keyword was used.
    Example:
    Restrict
    Keyword
    void copy(char * __restrict cp_a, char * __restrict cp_b, int n) { for (int I = 0; I < n; I++) cp_a[i] = cp_b[i]; }
    This method is convenient in case the exclusive access property holds for pointer variables that are used in a large portion of code with many loops because it avoids the need to annotate each of the vectorizable loops individually. Note, however, that both the loop-specific
    #pragma ivdep
    hint, as well as the pointer variable-specific
    restrict
    hint must be used with care because incorrect usage may change the semantics intended in the original program.
    Another example is the following loop that may also not get vectorized because of a potential aliasing problem between pointers
    a
    ,
    b
    and
    c
    :
    Example: Potential Unsupported Loop Structure
    void add(float *a, float *b, float *c) { for (int I=0; i<SIZE; I++) { c[i] += a[i] + b[i]; } }
    If the
    restrict
    keyword is added to the parameters, the compiler will trust you, that you will not access the memory in question with any other pointer and vectorize the code properly:
    Example: Using Pointers with the
    Restrict
    Keyword
    // let the compiler know, the pointers are safe with restrict void add(float * __restrict a, float * __restrict b, float * __restrict c) { for (int I=0; i<SIZE; I++) { c[i] += a[i] + b[i]; } }
    The down-side of using
    restrict
    is that not all compilers support this keyword, so your source code may lose portability.
  • Options/switches:
    You can use options to enable different levels of optimizations to achieve automatic vectorization:
    • Interprocedural optimization (IPO):
      Enable IPO using the
      [Q]ipo
      option across source files. You provide the compiler with additional information (trip counts, alignment, or data dependencies) about a loop. Enabling IPO may also allow inlining of function calls.
    • High-level optimizations (HLO):
      Enable HLO with option
      O3
      . This will enable additional loop optimizations that make it easier for the compiler to vectorize the transformed loops.

Product and Performance Information

1

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