[COLOR=royalblue]Therefore, any code optimization performed on a function that does not change the resulting value of the function for any argument, is uncontroversially considered a valid optimization. Therefore, techniques such as instruction selection, instruction scheduling, dead code elimination, & load/store reordering are all acceptable. These techniques change the performance profile of the function, without affecting its extensional meaning.
Optimization techniques which change your function into a function that extensionally differs from what you specified are generally not considered valid optimizations. These sorts of optimizations have occasionally been exposed, for example, in C++ compilers as features that programmers can optionally enable when they want the extra performance & are willing to accept that the meaning of their function is being changed but hopefully to a reasonable numeric approximation. 1 example of this is Visual C++'s "improve float consistency" option. Such non-extensional optimizations, in all sane programming systems, default to off.[/COLOR]
Would you like to know more? To apply this to the NVIDIA/ATi Drivers in 3D Mark 2003 what's he's saying is NVIDIA cheated, ATi optimized.