2013/3/13 Mark Marshall <[email protected]> > Hi. > ... > > PS. > We're not trying to create instruction sets to make writing assembly > programs nice and easy. We're creating instruction sets to make the > compilers life easy. I don't know how easy it is to get GCC to issue a MUX > instruction, but I bet it's much harder than the conditional move, which > almost all instruction sets have these days. >
Instruction in pipeline could be slow down by dependencies between register. One way of doing fast instruction execution is to reduce read after write dependancies, so MOV is faster than MUX. That why 3 registers instruction operation are fashionable, the "dsp instruction", because 2 registers instruction as in x86 instroduce more (false) dependancies between registers. Linus have wrote a comment about the use of CMOV compare to a branch. It prefer a branch because a branch could be easly predicted, which is not the case for a data value. The advantage of branch is seen in wide pipelined superscalar design. Nicolas
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