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The description of -march=i686/pentiumpro in the gcc manpage is:
Intel Pentium CPU with no MMX support.
Intel PentiumMMX CPU based on Pentium core with MMX instruction set support.
Intel PentiumPro CPU.
Same as "generic", but when used as "march" option, PentiumPro instruction set will be used, so the code will run on all i686 family chips.
In this context it is not clear whether i686 includes MMX support or not.
Checking Wikipedia suggests (though it is not clearly mentioned) that the PentiumPro contained no MMX support, which arrived only with the Pentium II.
Thus the description of "pentiumpro" should probably include "with no MMX support" as the description of "pentium" does.