Windows X64 Calling conventions

[Tofu Ninja via Digitalmars-d-learn]

On Thursday, 20 April 2017 at 08:36:32 UTC, kinke wrote:
> On Thursday, 20 April 2017 at 01:16:11 UTC, Tofu Ninja wrote:
>> My question is, why is it passed twice, both in xmm0 and rcx? 
>> The MSDN docs say floating point are passed in xmm registers, 
>> why is it also copied in into rcx? Is it necessary for 
>> anything?
>
> That is only required for variadics, quoting MSDN 
> [https://msdn.microsoft.com/en-us/library/dd2wa36c.aspx]:
>> For floating-point values only, both the integer and the
>> floating-point register will contain the float value in case
>> the callee expects the value in the integer registers.

Oh thanks, that answers my question, but it seem odd that dmd 
does it no matter what even when variadics are not used.