Am 07.03.2013 20:45, schrieb Walter Bright:
On 3/7/2013 9:36 AM, Johannes Pfau wrote:
I'm sorry I have to pester you with this again, but I still have some
questions regarding POD types and I'd like to fix this in GDC.
So from last discussion:
>> Wouldn't it be legal to still pass non-PODs in registers when
calling functions and only copying them back to
>> the stack if the address is needed? As we pass structs by value
anyway, how could this be problematic?
>
> No, not allowed. Consider why there are copy constructors, and what
they do.
I compiled some test programs with dmd and dmd _does_ pass non-POD
values in registers as I suggested above.
See this example:
https://gist.github.com/jpf91/5064703 (D)
https://gist.github.com/jpf91/5064764 (ASM)
That's because objects with constructors are now regarded as POD.
This example uses a postblit to make sure the type is not a POD. It's
obvious in the ASM that the copy ctor is called, nevertheless the
non-POD is passed in registers. Add a __traits(isPOD, Date) test to the
example, it returns false.
I also don't understand how a copy ctor could break this.
Because a copy ctor executes arbitrary code, and this just does not
work in the general case if a value is in a register.
Yes, the struct value can't be passed _to the copy constructor_ in a
register - but the copy ctor itself is always called with a reference to
the value, i.e. it's declared as
__copyctor(ref Date this, ref Date b)
For all other functions I don't see why it can't be passed in a register.
--
Johannes Pfau
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