[llvm-bugs] [Bug 35072] New: [AArch64] cmpxchg not using Armv8.1-a instructions at -O0

via llvm-bugs llvm-bugs at lists.llvm.org
Wed Oct 25 06:08:06 PDT 2017


            Bug ID: 35072
           Summary: [AArch64] cmpxchg not using Armv8.1-a instructions at
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: Christof.Douma at arm.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 19337
  --> https://bugs.llvm.org/attachment.cgi?id=19337&action=edit

The Armv8.1-a target has a compare-and-swap (CAS) instruction which is used to
implement cmpxchg operation of LLVM IR. However, at -O0 the code generator is
still emitting a long code sequence to emulate this instruction.

LLVM should use CAS even at -O0.

A reproducer that show the behaviour is attached. Compile using:
  llc -mattr=+v8.1a  cmpxchg.ll -O[0|2]

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