Via http://en.wikipedia.org/wiki/SSE5 ...

Compatibility

AMD's SSE5 extension bundle does not include the full set of Intel's SSE4 instructions making it a competitor to SSE4 rather than a successor.

This ambiguity complicates software development.

It is recommended practice for a program to test for the presence of instruction set extensions by means of the CPUID instruction before entering a code path which depends upon those instructions to function correctly.

For maximum portability, an optimized application will require three code paths: a base code path for compatibility with older processors (from either vendor), a separately optimized Intel code path exploiting SSE4 or AVX, and a separately optimized AMD code path exploiting SSE5.

Due to this proliferation, benchmarks between Intel and AMD processors increasingly reflect the cleverness or implementation quality of the divergent code paths rather than the strength of the underlying platform.