On 6/9/15 12:23 PM, kink wrote:
On Tuesday, 9 June 2015 at 15:08:07 UTC, Steven Schveighoffer wrote:
Because it's not moved. It's written in the stack where it will be
passed to the next function.
Hmm, you mean the callee's function parameters stack? That's not always
going to work, e.g., on Win64 the first 4 args are passed in registers,
always.
If that's what's required, that's what we do. I'm not strictly saying
that we will *always* do stack passing in spite of the ABI. But passing
a large rvalue by value does not involve any moving of data, and can be
abstracted to a pass by ref if needed. It's never less performing than
an explicit pass by ref.
And, as I said, that ABI doesn't support byval passing of types
> 64 bits (let's exclude vector types here), so rvalues > 64 bits can
sadly not be constructed in-place without violating the Win64 ABI -
they'll have to be passed byref.
I don't think we violate the Win64 ABI. So if it requires pass by ref,
it requires pass by ref. I am not an expert in this matter. You may want
to try some disassembly and see what D does.
-Steve
