[llvm-bugs] [Bug 47611] New: [Windows][AArch64] Inconsistent handling of HFAs/HVAs

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=47611

            Bug ID: 47611
           Summary: [Windows][AArch64] Inconsistent handling of HFAs/HVAs
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: LLVM Codegen
          Assignee: unassignedclangbugs at nondot.org
          Reporter: Richard.Townsend at arm.com
                CC: llvm-bugs at lists.llvm.org, neeilans at live.com,
                    richard-llvm at metafoo.co.uk

Mentioned by Eli Friedman when the initial Clang work was done to support the
Windows on Arm ABI[1] Microsoft have confirmed how their HFA/HVA handling
works.[2] In the case of:

  struct Pod {
    double b[2];
  };
  struct NotAggregate {
    NotAggregate();
    double b[2];
  };
  struct NotPod {
    NotAggregate x;
  };
  Pod copy(Pod *x) { return *x; }  // MSVC: ldp d0,d1,[x0], Clang: ldp
d0,d1,[x0]
  NotAggregate copy(NotAggregate *x) { return *x; } // MSVC: stp x8,x9,[x0],
Clang: str q0,[x0]
  NotPod copy(NotPod *x) { return *x; } // MSVC: ldp x0,x1,[x8], Clang:
d0,d1,[x0]

Microsoft has clarified that NotPod is not considered HFA in this ABI, because
NotAggregate is also considered not to be HFA - all base classes and members
should have the same HFA type to be considered HFA.

[1] https://reviews.llvm.org/D60349 
[2]
https://developercommunity.visualstudio.com/content/problem/1179157/arm64-abi-inconsistent-handling-of-hfas.html

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