[llvm-bugs] [Bug 48110] New: Incorrect Cost Calculations for Shuffle Ports on Icelake-client / Icelake-server

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=48110

            Bug ID: 48110
           Summary: Incorrect Cost Calculations for Shuffle Ports on
                    Icelake-client / Icelake-server
           Product: tools
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: llvm-mca
          Assignee: unassignedbugs at nondot.org
          Reporter: dweber at gmail.com
                CC: andrea.dibiagio at gmail.com, llvm-bugs at lists.llvm.org,
                    matthew.davis at sony.com

Hi there,

It looks like llvm-mca is treating icelake-client and icelake-server as having
only one shuffle port.  This causes incorrect cost calculations for any
operation that would use said shuffle ports.  A block diagram showing the
architecture can be found here:
https://en.wikichip.org/wiki/intel/microarchitectures/sunny_cove (sunny cove is
the core used by both icelake-server and icelake-client).  Similarly, you can
find the uops usage for any given shuffle here: 
https://uops.info/table.html?search=shuf&cb_lat=on&cb_tp=on&cb_uops=on&cb_ports=on&cb_SKL=on&cb_ICL=on&cb_measurements=on&cb_base=on&cb_avx=on

If the code generating the costs for LLVM-MCA is used by anything else in the
toolchain, it's likely this will yield a performance benefit for other users
targeting icelake-server and icelake-client.

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