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8.1.1 x86 extensions

GCC can take advantage of the additional instructions in the MMX, SSE, SSE2, SSE3 and 3dnow extensions of recent Intel and AMD processors. The options -mmmx, -msse, -msse2, -msse3 and -m3dnow enable the use of these extra instructions, allowing multiple words of data to be processed in parallel. The resulting executables will only run on processors supporting the appropriate extensions--on other systems they will crash with an Illegal instruction error (or similar).⁽²³⁾

The option -mfpmath=sse instructs GCC to use the SSE extensions for floating-point arithmetic where possible. For this option to take effect, the SSE or SSE2 extensions must first be enabled with -msse or -msse2.

Note that the plain SSE extensions only support single precision operations--double precision arithmetic is part of SSE2. Since most C and C++ programs declare floating-point variables as double rather than float, the combined options -msse2 -mfpmath=sse are usually needed. On 64-bit processors these options are enabled by default.