Well, the obvious solution is to code the eyecatcher literals before the entry point. It will be less obvious that the eyecatcher is part of the program (and not the end of the previous program) but as the technique become more widespread it should become more trusted.
On Fri, Apr 29, 2016 at 9:13 AM, David Crayford <[email protected]> wrote: > On 29/04/2016 10:09 PM, Mike Schwab wrote: >> >> The pipeline is optimized for running many instructions in a row. A >> branch is not recognized until through a good part of the pipeline. >> Meanwhile the data to be skipped is in the instruction pipeline. >> >> Results meet expectations. > > > So branching over eyecatchers is expected to be x2 slower on a z13 than a > z114? I was always lead to believe that new hardware always ran old code > faster unless it was doing nasty stuff like storing into the instruction > stream. > > >> >> On Fri, Apr 29, 2016 at 7:40 AM, David Crayford <[email protected]> >> wrote: >>> >>> 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 [email protected] with the message: INFO IBM-MAIN >> >> >> > > ---------------------------------------------------------------------- > For IBM-MAIN subscribe / signoff / archive access instructions, > send email to [email protected] with the message: INFO IBM-MAIN -- Mike A Schwab, Springfield IL USA Where do Forest Rangers go to get away from it all? ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
