[llvm-bugs] [Bug 42300] New: [llvm-mca] How to model "result bus bubbles"?

via llvm-bugs llvm-bugs at lists.llvm.org
Mon Jun 17 08:41:11 PDT 2019


            Bug ID: 42300
           Summary: [llvm-mca] How to model "result bus bubbles"?
           Product: tools
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: llvm-mca
          Assignee: unassignedbugs at nondot.org
          Reporter: lebedev.ri at gmail.com
                CC: andrea.dibiagio at gmail.com, llvm-bugs at lists.llvm.org,
                    matthew.davis at sony.com

"AMD Software Optimization Guide for AMD Family 10h and 12h Processors", page
250 of PDF:

"Multiplication operations always issue to integer pipe 0, and the issue logic
creates result bus bubbles for the multiplier in integer pipes 0 and 1 by
preventing non-multiply micro-ops from issuing at the appropriate time."

So in other words, as far as i understand it when "Integer Multiplier" is
no microops may be issued to the pipe 0 and/or 1, and ALU/AGU units in those
are unoccupied.

I'm not quite sure how to model that correctly.
I did see comments for "BufferSize = 0", but i'm not sure how to limit that to
certain pipes/ProcResource's, since the pipe3 (it's alu/agu units) and fpu
should not be affected.

Any hints, please? :)

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