[llvm-bugs] [Bug 25457] New: [AArch64] Aliases of WZR/XZR are not considered constant

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=25457

            Bug ID: 25457
           Summary: [AArch64] Aliases of WZR/XZR are not considered
                    constant
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: AArch64
          Assignee: unassignedbugs at nondot.org
          Reporter: mcrosier at codeaurora.org
                CC: haicheng at codeaurora.org, james.molloy at arm.com,
                    llvm-bugs at lists.llvm.org, t.p.northover at gmail.com
    Classification: Unclassified

The alias registers of WZR/XZR are WZR_W0, W30_WZR, LR_XZR, and XZR_X0.  None
of these aliases are marked as constant, so WZR/XZR is not considered as
constant.

The %wzr_w0 alias was added here:
http://reviews.llvm.org/rL238818

One practical implication is that in passes such as machine-cse and
machine-sink an instruction such as %v = copy %wzr cannot be moved because llvm
thinks %wzr is not a constant physical register.

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