> XR over SR seems to be a persistent myth. I have been told again and again > from time to time that XR was faster than SR. There is no reason it should > be, and the evidence is that it is not.
Response from hardware designer: Here are some thoughts on the best way to zero a register. First, on the z196 and prior generations, all of the common instructions for zeroing a register are actually executed in the fixed-point execution unit. Starting with the zEC12 generation the following instructions are actually "executed" in the register renaming stage of the pipeline (where "R" is the same register number for a given instruction): XR R,R; SR R,R; SLR R,R; LA R,0(0,0); LHI R,0 (plus other small constant values); LGHI R,0 (plus other small constant values); SGR R,R; SLGR R,R; XGR R,R; IIHF R,0; IILF R,0. On the z196 and earlier processors, since they are actually executed, use of an XR or SR may set up an unnecessary register dependency. For example: .... AL 5,MEM1 Using GR5 for some prior calculation ST 5,MEM2 ... XR 5,5 This instruction is dependent on the AL .... So although the XR will zero the register, regardless of its prior value, it still appears to be dependent on the AL so it can not execute early. Other points of interest...the XR/SR/SLR are 2-byte instructions so they have a smaller instruction footprint, but this is a second or third order performance effect in most cases. The IILF/IIHF are 6 byte instructions, so there would be little reason to use them. To conclude, the LHI and LGHI are probably the best overall instructions for clearing a register on any generation processor. I can not imagine we would ever design a machine where these instructions were not at least as fast as XR/SR/SLR for clearing a register, and they have some benefits on prior models. Jim Mulder z/OS System Test IBM Corp. Poughkeepsie, NY ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
