C++ Classes and SIMD Operations
Use of C++ classes for SIMD operations allows for operating on arrays or vectors of data in a single operation. Consider the addition of two vectors,
A
and
B
, where each vector contains four elements. Using an integer vector class, the elements
A[i]
and
B[i]
from each array are summed as shown in the following example.
Typical Method of Adding Elements Using a Loop
int a[4], b[4], c[4];
for (i=0; i<4; i++) /* needs four iterations */
c[i] = a[i] + b[i]; /* computes c[0], c[1], c[2], c[3] */The following example shows the same results using one operation with an integer class.
SIMD Method of Adding Elements Using Ivec Classes
Is16vec4 ivecA, ivecB, ivec C; /*needs one iteration*/
ivecC = ivecA + ivecB; /*computes ivecC0, ivecC1, ivecC2, ivecC3 */Available Classes
The Intel® C++ SIMD classes provide parallelism, which is not easily implemented using typical mechanisms of C++. The following table shows how the Intel® C++ classes use the SIMD classes and libraries.
SIMD Vector Classes
Instruction Set
| Class
| Signedness
| Data Type
| Size
| Elements
| Header File
|
|---|---|---|---|---|---|---|
MMX™ technology
| I64vec1 | unspecified
| __m64 | 64
| 1
| ivec.h |
I32vec2 | unspecified
| int | 32
| 2
| ivec.h | |
Is32vec2 | signed
| int | 32
| 2
| ivec.h | |
Iu32vec2 | unsigned
| int | 32
| 2
| ivec.h | |
I16vec4 | unspecified
| short | 16
| 4
| ivec.h | |
Is16vec4 | signed
| short | 16
| 4
| ivec.h | |
Iu16vec4 | unsigned
| short | 16
| 4
| ivec.h | |
I8vec8 | unspecified
| char | 8
| 8
| ivec.h | |
Is8vec8 | signed
| char | 8
| 8
| ivec.h | |
Iu8vec8 | unsigned
| char | 8
| 8
| ivec.h | |
Intel® SSE
| F32vec4 | unspecified
| float | 32
| 4
| fvec.h |
F32vec1 | unspecified
| float | 32
| 1
| fvec.h | |
Intel® SSE2
| F64vec2 | unspecified
| double | 64
| 2
| dvec.h |
I128vec1 | unspecified
| __m128i | 128
| 1
| dvec.h | |
I64vec2 | unspecified
| long int | 64
| 2
| dvec.h | |
I32vec4 | unspecified
| int | 32
| 4
| dvec.h | |
Is32vec4 | signed
| int | 32
| 4
| dvec.h | |
Iu32vec4 | unsigned
| int | 32
| 4
| dvec.h | |
I16vec8 | unspecified
| int | 16
| 8
| dvec.h | |
Is16vec8 | signed
| int | 16
| 8
| dvec.h | |
Iu16vec8 | unsigned
| int | 16
| 8
| dvec.h | |
I8vec16 | unspecified
| char | 8
| 16
| dvec.h | |
Is8vec16 | signed
| char | 8
| 16
| dvec.h | |
Iu8vec16 | unsigned
| char | 8
| 16
| dvec.h | |
Intel® AVX
| F32vec8 | unspecified
| float
| 32
| 8
| dvec.h |
F64vec4 | unspecified
| double
| 64
| 4
| dvec.h | |
Intel® AVX-512 Foundation
| F32vec16
| unspecified
| float
| 32
| 16
| dvec.h |
F64vec8
| unspecified
| double
| 64
| 8
| dvec.h | |
M512vec
| unspecified
| __m512i
| 512
| 1
| dvec.h | |
I32vec16
| unspecified
| int
| 32
| 16
| dvec.h | |
Is32vec16
| signed
| int
| 32
| 16
| dvec.h | |
Iu32vec16
| unsigned
| int
| 32
| 16
| dvec.h | |
I64vec8
| unspecified
| long int
| 64
| 8
| dvec.h | |
Is64vec8
| signed
| long int
| 64
| 8
| dvec.h | |
Iu64vec8
| unsigned
| long int
| 64
| 8
| dvec.h | |
Intel® AVX-512 Byte and Word
| I16vec32
| unspecified
| int
| 16
| 32
| dvec.h |
Is16vec32 | signed
| int
| 16
| 32
| dvec.h | |
Iu16vec32 | unsigned
| int
| 16
| 32
| dvec.h | |
I8vec64 | unspecified
| int
| 8
| 64
| dvec.h | |
Is8vec64 | signed
| int
| 8
| 64
| dvec.h | |
Iu8vec64 | unsigned
| int
| 8
| 64
| dvec.h |
Most classes contain similar functionality for all data types and are represented by all available intrinsics. However, some capabilities do not translate from one data type to another without suffering from poor performance, and are therefore excluded from individual classes.
Intrinsics that take immediate values and cannot be expressed easily in classes are not implemented. For example:
- _mm_shuffle_ps
- _mm_shuffle_pi16
- _mm_shuffle_ps
- _mm_extract_pi16
- _mm_insert_pi16
Access to Classes Using Header Files
The required class header files are installed in the include directory with the
directive in your program file as shown in the table that follows.
Intel® oneAPI
. To enable the classes, use the
DPC++/C++
Compiler#include
Include Directives for Enabling Classes
Instruction Set Extension
| Include Directive
|
|---|---|
MMX™ Technology
| #include < ivec.h > |
Intel® SSE
| #include < fvec.h > |
Intel® SSE 2
| #include < dvec.h > |
Intel® SSE 3
| #include < dvec.h > |
Intel® SSE 4
| #include < dvec.h > |
Intel® AVX
| #include < dvec.h > |
Each succeeding file from the top down includes the preceding class. You only need to include
fvec.h
if you want to use both the
Ivec
and
Fvec
classes. Similarly, to use all the classes including those for Intel® Streaming SIMD Extensions 2, you only need to include the
dvec.h
file.
Usage Precautions
When using the C++ classes, you should follow some general guidelines. More detailed usage rules for each class are listed in
Integer Vector Classes, and
Floating-point Vector Classes.
Clear MMX Registers
If you use both the
Ivec
and
Fvec
classes at the same time, your program could mix Intel® MMX™ instructions, called by
Ivec
classes, with Intel® architecture floating-point instructions, called by
Fvec
classes. x87 floating-point instructions exist in the following
Fvec
functions:
- fvecconstructors
- debug functions (coutand element access)
- rsqrt_nr
Intel® MMX™ technology registers are aliased on the floating-point registers, so you should clear the MMX state with the EMMS instruction intrinsic before issuing an x87 floating-point instruction, as in the following example.
ivecA = ivecA & ivecB; | Ivec logical operation that uses MMX instructions
|
empty (); | clear state
|
cout << f32vec4a; | F32vec4 operation that uses x87 floating-point instructions
|
Failure to clear the Intel® MMX™ technology registers can result in incorrect execution or poor performance due to an incorrect register state.