<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/85417>85417</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            LLVM 17.0.6 Win64 calling convention wrong for Windows x64 ABI
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          Bob64375
      </td>
    </tr>
</table>

<pre>
    LLVM 17.0.6 generates incorrect return instructions on Windows x64 when a
composite is returned. Refer to the below example.

When calling `F` from Visual Studio x64 compiled code, CL passes in `rcx` the
address of `_rslt`. When `F` is compiled by LLVM, with `CallingConv::Win64`,
and, since the function returns a composite, `void` is passed as the return
type to `Function::Create` and the real (composite) return type is passed
through attribute `StructRet` via.
`function->addRetAttrs(AttrBuilder(cntxt).addStructRetAttr(the `Return` type))`,
see IR code below which reflects this, LLVM ignores the Windows x64 ABI
and generates instructions that return `a` and `b` in registers.

The issue is in `FunctionLoweringInfo.cpp` in method `set` around line 92:

GetReturnInfo(CC, Fn->getReturnType(), Fn->getAttributes(), Outs, *TLI,
 mf.getDataLayout());
CanLowerReturn =
      TLI->CanLowerReturn(CC, *MF, Fn->isVarArg(), Outs, Fn->getContext());

which sets `CanLowerReturn` true, and that is incorrect.
`CanLowerReturn` must be false, in which case, LLVM does indeed generate the
correct instructions that copy the return through the pointer in `rcx`.
A solution is to pass to `CanLowerReturn`
 `Fn->hasRetAttribute( Attribute::StructRet )` so that the
target `Win64` machine can decide not to lower, or, better still, pass `Fn` so
the target has more leeway to decide. Best of all would be if there were not
two different mechanisms for making the return type known to `Function`.

**************************************************************************

struct Return { long long a; long long b; };

*****Compiled by Visual Studio x64:

Return _rslt( F( nullptr, 0 ) );

***** LlVm 17.0.6: IR

Return F( void*, int )
{
  Return rtn = { 0, 1 };

 return rtn;
}

define dllexport win64cc sret({ i64, i64 }) void @"F@ "(ptr %"P", ptr %"Q") {
entry:
  %"P1" = alloca ptr, align 8
  %"Q2" = alloca ptr, align 8
  %"rtn" = alloca { i64, i64 }, align 8
  store ptr %"P", ptr %"P1", align 8
  store ptr %"Q", ptr %"Q2", align 8
  store { i64, i64 } { i64 0, i64 1 }, ptr %"rtn", align 8
  %"rtn" = load { i64, i64 }, ptr %"rtn", align 8
  ret { i64, i64 } %"rtn"
}

**************************************************************************
</pre>
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