[llvm-bugs] [Bug 28942] New: MachineScheduler: clusterNeighboringMemOps can lead to unstable output

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=28942

            Bug ID: 28942
           Summary: MachineScheduler: clusterNeighboringMemOps can lead to
                    unstable output
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: gberry at codeaurora.org
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

The code in BaseMemOpClusterMutation::clusterNeighboringMemOps() builds a
sorted list of MemOpInfos, and the ordering of this list can ultimately effect
the code generated (whether loads/stores get clustered when scheduling). 
std::sort is used to sort the entries, which is not stable w.r.t. the order of
equal elements.

Unstable output could be generated for the same input if two different
implementations of std::sort were used.  The clustering decision for equal
MemOpInfos could also be altered e.g. by the addition of extra MemOpInfos in
the list before or after the equal elements.  I ran into the latter case
refactoring the code for AArch64InstrInfo::getMemOpBaseRegImmOfs().

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