jbs@WATSON.IBM.COM (05/17/91)
Herman Rubin says (in connection with the Risc System 6000): As for (2), it would reduce the cost of the hybrid operations down to 1/3 their present cost at least, and in some cases more. For unsigned integer to floating conversion, at present this takes an integer store, a floating load, and a floating add, assuming that the preparations have been made. Other situations are even worse. Things are not this simple. The 3 instructions can be over- lapped with instructions of the opposite type (it turns out a float- ing load is of integer type). A proposed integer->float instruction would presumedly be of both types. This would make the ratio 3:2. Herman Rubin also says (in connection with potential gains from easier communication between the fixed and float units): I believe that the current elementary function library for the RS/6000 does quite a bit of this, and this might give an indication. I agree that easier communication between the units would help in computing elementary functions. However the gains would not be spectacular (I guess 10% or so) and elementary functions themselves don't seem all that important (anybody have numbers on this?). There are also other perhaps easier ways of achieving comparable gains. For example: 1.) Add an instruction which gives the integer part of a floating number as a floating number. 2.) Drop the requirement that the functions work for all round- ing modes. 3.) Write functions which compute more than 1 value at a time. James B. Shearer