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Vol. 2A 3-13
INSTRUCTION SET REFERENCE, A-M
• The __m128i data type can hold sixteen byte, eight word, or four doubleword, or
two quadword integer values.
The compiler aligns __m128, __m128d, and __m128i local and global data to
16-byte boundaries on the stack. To align integer, float, or double arrays, use the
declspec statement as described in Intel C/C++ compiler documentation. See
http://www.intel.com/support/performancetools/.
The __m128, __m128d, and __m128i data types are not basic ANSI C data types
and therefore some restrictions are placed on its usage:
• Use __m128, __m128d, and __m128i only on the left-hand side of an
assignment, as a return value, or as a parameter. Do not use it in other arithmetic
expressions such as “+” and “>>.”
• Do not initialize __m128, __m128d, and __m128i with literals; there is no way to
express 128-bit constants.
• Use __m128, __m128d, and __m128i objects in aggregates, such as unions (for
example, to access the float elements) and structures. The address of these
objects may be taken.
• Use __m128, __m128d, and __m128i data only with the intrinsics described in
this user’s guide. See Appendix C, “InteL® C/C++ Compiler Intrinsics and
Functional Equivalents,” in the Intel® 64 and IA-32 Architectures Software
Developer’s Manual, Volume 2B, for more information on using intrinsics.
The compiler aligns __m128, __m128d, and __m128i local data to 16-byte bound-
aries on the stack. Global __m128 data is also aligned on 16-byte boundaries. (To
align float arrays, you can use the alignment declspec described in the following
section.) Because the new instruction set treats the SIMD floating-point registers in
the same way whether you are using packed or scalar data, there is no __m32 data
type to represent scalar data as you might expect. For scalar operations, you should
use the __m128 objects and the “scalar” forms of the intrinsics; the compiler and the
processor implement these operations with 32-bit memory references.
The suffixes ps and ss are used to denote “packed single” and “scalar single” preci-
sion operations. The packed floats are represented in right-to-left order, with the
lowest word (right-most) being used for scalar operations: [z, y, x, w]. To explain
how memory storage reflects this, consider the following example.
The operation:
float a[4] ← { 1.0, 2.0, 3.0, 4.0 };
__m128 t ← _mm_load_ps(a);
Produces the same result as follows:
__m128 t ← _mm_set_ps(4.0, 3.0, 2.0, 1.0);
In other words:
t ← [ 4.0, 3.0, 2.0, 1.0 ]
Where the “scalar” element is 1.0.