Valgrind didn't recognize certain older i386 AMD athlon processors had support for the mmxext (integer sse) instruction set (a subset of the SSE1 instruction set). In particular it didn't recognize these processors supported the PREFETCH instruction.
Running an application under valgrind on such a processor that did use an mmxext instruction (in particular the PREFETCH instruction) would case an error from valgrind about using an unsupported instruction.
Valgrind now understands which processors implement the mmxext (integer sse) instruction set.
Programs now run on such processors when they use such instructions.