A. There is no answer to the question. Instructions no longer have "timing" in isolation. Because of pipelining and out-of-order execution, with any luck one of those non-storage instructions takes no time at all. Really, literally no time at all. You could benchmark a loop, stick an SR into the middle of it, and have it still run at exactly the same speed.
B. Considering pipelining and so forth, you *might* save more time not by picking one of the below but rather by giving the CPU a little break between register references. In other words, do *not* do the usual SR/XR R3,R3 ICM R3,... But rather do SR/XR R3,R3 Do some other stuff that does not use R3 ICM R3,... That's what the compilers do. (Unless of course, readability and maintainability are important to you -- in which case the latter sequence is a lot more likely to get mucked up by a future maintainer -- a problem that compilers do not have.) C. LHI has the advantage that it (1) does not require the arithmetic unit and (2) does not affect the condition code, which might make the pipeline logic a little easier on the CPU. (That's why I tend to use LHI -- but I don't go out of my way. I would not change a working SR into an LHI.) D. Modern CPUs all have a "shortcut" in the decoder that recognizes the common methods and treats them all equivalently as "slam a zero into Rn." Charles -----Original Message----- From: IBM Mainframe Assembler List [mailto:ASSEMBLER-LIST@LISTSERV.UGA.EDU] On Behalf Of Schmitt, Michael Sent: Monday, August 10, 2020 12:13 PM To: ASSEMBLER-LIST@LISTSERV.UGA.EDU Subject: Clearing a register There are many ways to set a register to binary zero. My question is, which is the most efficient? 1. XR Rn,Rn 2. SR Rn,RN 3. SRA Rn,Rn 4. LHI Rn,0 5. Other instruction 6. Depends on hardware 7. Depends on other factors. (What?) 8. They are have equal performance 9. Method X has worse performance but other advantages. (What?) ____________________________________ Michael Schmitt | DXC.technology (469) 808-4403 | michael.schm...@dxc.com DXC Technology Company - Headquarters: 1775 Tysons Boulevard, Tysons, Virginia 22102, USA. DXC Technology Company -- This message is transmitted to you by or on behalf of DXC Technology Company or one of its affiliates. It is intended exclusively for the addressee. The substance of this message, along with any attachments, may contain proprietary, confidential or privileged information or information that is otherwise legally exempt from disclosure. Any unauthorized review, use, disclosure or distribution is prohibited. If you are not the intended recipient of this message, you are not authorized to read, print, retain, copy or disseminate any part of this message. If you have received this message in error, please destroy and delete all copies and notify the sender by return e-mail. Regardless of content, this e-mail shall not operate to bind DXC Technology Company or any of its affiliates to any order or other contract unless pursuant to explicit written agreement or government initiative expressly permitting the use of e-mail for such purpose. --.
