Editing SSE2 (section)

==Differences between x87 FPU and SSE2==
FPU (x87) instructions provide higher precision by calculating intermediate results with 80 bits of precision, by default,  to minimise [[roundoff error]] in numerically unstable algorithms (see [[IEEE 754#Design rationale|IEEE 754 design rationale]] and references therein).  However, the x87 FPU is a scalar unit only whereas SSE2 can process a small vector of operands in parallel.

If code designed for x87 is ported to the lower precision double precision SSE2 floating point, certain combinations of math operations or input datasets can result in measurable numerical deviation, which can be an issue in reproducible scientific computations, e.g. if the calculation results must be compared against results generated from a different machine architecture. A related issue is that, historically, language standards and compilers had been inconsistent in their handling of the x87 80-bit registers implementing double extended precision variables, compared with the double and single precision formats implemented in SSE2: the rounding of extended precision intermediate values to double precision variables was not fully defined and was dependent  on implementation details such as when registers were spilled to memory.