msvc abi incompatibility: classes with a deleted copy ctor can be returned in registers

[efriedma-quic]

struct Z { int z;  Z(const Z&) = delete;};
Z z(Z z) { return {z.z}; }

MSVC produces:

        mov     eax, DWORD PTR [rcx]
        ret     0

Clang targeting Windows produces:

        mov     rax, rcx
        mov     ecx, dword ptr [rdx]
        mov     dword ptr [rax], ecx
        ret

[llvmbot]

@llvm/issue-subscribers-clang-codegen

[efriedma-quic]

Interesting edge case on AArch64:

struct Z { double z;  Z(const Z&) = delete;};
Z z(Z z) { return {z.z}; }

Produces:

        ldr         x0,[x0]
        ret

So we return the value in registers, but only integer registers; it doesn't count as an HVA/HFA.

[efriedma-quic]

CC @rnk