The Streaming SIMD Extensions intrinsics provide access to ItaniumŪ instructions for Streaming SIMD Extensions. To provide source compatibility with the IA-32 architecture, these intrinsics are equivalent both in name and functionality to the set of IA-32-based Streaming SIMD Extensions intrinsics.
To write programs with the intrinsics, you should be familiar with the hardware features provided by the Streaming SIMD Extensions. Keep the following issues in mind:
The new data type __m128 is used with the Streaming SIMD Extensions intrinsics. It represents a 128-bit quantity composed of four single-precision FP values. This corresponds to the 128-bit IA-32 Streaming SIMD Extensions register.
The compiler aligns __m128 local data to 16-byte boundaries on the stack. Global data of these types is also 16 byte-aligned. To align integer, float, or double arrays, you can use the declspec alignment.
Because Itanium instructions treat the Streaming SIMD Extensions registers in the same way whether you are using packed or scalar data, there is no __m32 data type to represent scalar data. For scalar operations, use the __m128 objects and the "scalar" forms of the intrinsics; the compiler and the processor implement these operations with 32-bit memory references. But, for better performance the packed form should be substituting for the scalar form whenever possible.
The address of a __m128 object may be taken.
For more information, see Intel Architecture Software Developer's Manual, Volume 2: Instruction Set Reference Manual, Intel Corporation, doc. number 243191.
Implementation on Itanium-based systems
Streaming SIMD Extensions intrinsics are defined for the __m128 data type, a 128-bit quantity consisting of four single-precision FP values. SIMD instructions for Itanium-based systems operate on 64-bit FP register quantities containing two single-precision floating-point values. Thus, each __m128 operand is actually a pair of FP registers and therefore each intrinsic corresponds to at least one pair of Itanium instructions operating on the pair of FP register operands.
Many of the Streaming SIMD Extensions intrinsics for Itanium-based systems were created for compatibility with existing IA-32 intrinsics and not for performance. In some situations, intrinsic usage that improved performance on IA-32 will not do so on Itanium-based systems. One reason for this is that some intrinsics map nicely into the IA-32 instruction set but not into the Itanium instruction set. Thus, it is important to differentiate between intrinsics which were implemented for a performance advantage on Itanium-based systems, and those implemented simply to provide compatibility with existing IA-32 code.
The following intrinsics are likely to reduce performance and should only be used to initially port legacy code or in non-critical code sections:
If the inaccuracy is acceptable, the SIMD reciprocal and reciprocal square root approximation intrinsics (rcp and rsqrt) are much faster than the true div and sqrt intrinsics.