I wonder if it is really a 'problem'. What kind of mormal program does this amount of branches back to where it just came from, messing up the entire pipeline. Is it efficient to optimize a z13 for this kind of programs? Isn't it better to optimize for the millions of other programs who are executed much more often than this one?
Kees. -----Original Message----- From: IBM Mainframe Discussion List [mailto:IBM-MAIN@LISTSERV.UA.EDU] On Behalf Of David Crayford Sent: 29 April, 2016 14:41 To: IBM-MAIN@LISTSERV.UA.EDU Subject: An explanation for branch performance? We're doing some performance work on our assembler code and one of my colleagues ran the following test which was surprising. Unconditional branching can add significant overhead. I always believed that conditional branches were expensive because the branch predictor needed to do more work and unconditional branches were easy to predict. Does anybody have an explanation for this. Our machine is z114. It appears that it's even worse on a z13. Here's the code. I wrote a simple program - it tight loops 1 billion times L R4,=A(1*1000*1000*1000) LTR R4,R4 J LOOP * LOOP DS 0D .LOOP START B NEXT NEXT JCT R4,LOOP The loop starts with a branch ... I tested it twice - when the CC is matched (branch happens) and when it is not matched (falls through) 1. When the CC is matched and branching happens, CPU TIME=2.94 seconds 2. When the CC is not matched the code falls through, CPU TIME=1.69 seconds - a reduction of 42% ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to lists...@listserv.ua.edu with the message: INFO IBM-MAIN ******************************************************** For information, services and offers, please visit our web site: http://www.klm.com. This e-mail and any attachment may contain confidential and privileged material intended for the addressee only. If you are not the addressee, you are notified that no part of the e-mail or any attachment may be disclosed, copied or distributed, and that any other action related to this e-mail or attachment is strictly prohibited, and may be unlawful. If you have received this e-mail by error, please notify the sender immediately by return e-mail, and delete this message. Koninklijke Luchtvaart Maatschappij NV (KLM), its subsidiaries and/or its employees shall not be liable for the incorrect or incomplete transmission of this e-mail or any attachments, nor responsible for any delay in receipt. Koninklijke Luchtvaart Maatschappij N.V. (also known as KLM Royal Dutch Airlines) is registered in Amstelveen, The Netherlands, with registered number 33014286 ******************************************************** ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to lists...@listserv.ua.edu with the message: INFO IBM-MAIN
