Intel® 64 and IA-32 Architectures Optimization Reference Manual

CODING FOR SIMD ARCHITECTURES4.4.3 Data Alignment for MMX TechnologyMany compilers enable alignment of variables using controls. This aligns variable bitlengths to the appropriate boundaries. If some of the variables are not appropriatelyaligned as specified, you can align them using the C algorithm in Example 4-11.Example 4-11. C Algorithm for 64-bit Data Alignment/* Make newp a pointer to a 64-bit aligned array of NUM_ELEMENTS 64-bit elements. */double *p, *newp;p = (double*)malloc (sizeof(double)*(NUM_ELEMENTS+1));newp = (p+7) & (~0x7);The algorithm in Example 4-11 aligns an array of 64-bit elements on a 64-bitboundary. The constant of 7 is derived from one less than the number of bytes in a64-bit element, or 8-1. Aligning data in this manner avoids the significant performancepenalties that can occur when an access crosses a cache line boundary.Another way to improve data alignment is to copy the data into locations that arealigned on 64-bit boundaries. When the data is accessed frequently, this can providea significant performance improvement.4.4.4 Data Alignment for 128-bit dataData must be 16-byte aligned when loading to and storing from the 128-bit XMMregisters used by SSE/SSE2/SSE3/SSSE3. This must be done to avoid severe performancepenalties and, at worst, execution faults.There are MOVE instructions (and intrinsics) that allow unaligned data to be copied toand out of XMM registers when not using aligned data, but such operations are muchslower than aligned accesses. If data is not 16-byte-aligned and the programmer orthe compiler does not detect this and uses the aligned instructions, a fault occurs. Sokeep data 16-byte-aligned. Such alignment also works for MMX technology code,even though MMX technology only requires 8-byte alignment.The following describes alignment techniques for Pentium 4 processor as implementedwith the Intel C++ Compiler.4.4.4.1 Compiler-Supported AlignmentThe Intel C++ Compiler provides the following methods to ensure that the data isaligned.Alignment by F32vec4 or __m128 Data TypesWhen the compiler detects F32VEC4 or __M128 data declarations or parameters, itforces alignment of the object to a 16-byte boundary for both global and local data,as well as parameters. If the declaration is within a function, the compiler also aligns4-16