[llvm-bugs] [Bug 36667] New: [llvm-mca] Teach the tool how to identify memory fence instructions.

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=36667

            Bug ID: 36667
           Summary: [llvm-mca] Teach the tool how to identify memory fence
                    instructions.
           Product: new-bugs
           Version: unspecified
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: andrea.dibiagio at gmail.com
                CC: llvm-bugs at lists.llvm.org

This could be fixed by using an extra flag in MCInstrDesc.

Currently, the LSUnit in llvm-mca (LSUnit.h) doesn't know about
memory-barriers. It conservatively assumes that an instruction with both
'mayLoad' and 'unmodeledSideEffects' behaves like a "soft" load barrier.
Similarly, an instruction with flag 'mayStore' and 'unmodeledSideEffects' is
treated like a "soft" store barrier. That means, memory operations are
serialized without forcing a flush (of the load/store queue).
This is inaccurate, and currently it leads to potentially incorrect analysis.

If we teach llvm-mca how to correctly identify fence instructions, then the
analysis becomes more accurate (and we don't need to guess anymore).

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