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

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.

Note

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 Intel® C++ Compiler. To enable the classes, use the #include directive in your program file as shown in the table that follows.

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. 87 floating-point instructions exist in the following Fvec functions:

fvec constructors
debug functions (cout and element access)
rsqrt_nr

Note

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

Caution

Failure to clear the Intel® MMX™ technology registers can result in incorrect execution or poor performance due to an incorrect register state.