SSE2 - Differences Between MMX and SSE2

Differences Between MMX and SSE2

SSE2 extends MMX instructions to operate on XMM registers. Therefore, it is possible to convert all existing MMX code to an SSE2 equivalent. Since an XMM register is twice as long as an MMX register, loop counters and memory access may need to be changed to accommodate this. However, 8 byte loads and stores to XMM are available, so this is not strictly required.

Although one SSE2 instruction can operate on twice as much data as an MMX instruction, performance might not increase significantly. Two major reasons are: accessing SSE2 data in memory not aligned to a 16-byte boundary can incur significant penalty, and the throughput of SSE2 instructions in older x86 implementations was half that for MMX instructions. Intel addressed the first problem by adding an instruction in SSE3 to reduce the overhead of accessing unaligned data and improving the overall performance of misaligned loads, and the last problem by widening the execution engine in their Core microarchitecture in Core 2 Duo and later products.

Since MMX and x87 register files alias one another, using MMX will prevent x87 instructions from working as desired. Once MMX has been used, the programmer must use the emms instruction (C: _mm_empty) to restore operation to the x87 register file. On some operating systems, x87 is not used very much, but may still be used in some critical areas like pow where the extra precision is needed. In such cases, the corrupt floating-point state caused by failure to emit emms may go undetected for millions of instructions before ultimately causing the floating-point routine to fail, returning NaN. Since the problem is not locally apparent in the MMX code, the bug can be very time consuming to find and correct. As SSE2 does not have this problem, usually provides much better throughput and provides more registers in 64-bit code, it should be preferred for nearly all vectorization work.