On 03/11/2012 09:30 PM, Robert Jacques wrote:
On Sun, 11 Mar 2012 12:39:56 -0500, Manu <turkey...@gmail.com> wrote:
On 11 March 2012 18:50, Robert Jacques <sandf...@jhu.edu> wrote:
On Sun, 11 Mar 2012 05:57:05 -0500, Manu <turkey...@gmail.com> wrote:
On 11 March 2012 04:35, Sean Cavanaugh <worksonmymach...@gmail.com>
wrote:
On 3/10/2012 8:08 PM, Mantis wrote:
[snip]
And Walter has talked about using the XMM registers to return floating
point data for exactly this reason. But those optimization apply to all
structs and all data types. There's nothing special about MRV. It has to
return a set of data in a structured manner; this is identical to the
case
of returning a struct.
Well you can't build these sorts of tightly packed structs in XMM
registers... and even for the GPR's, it's very inefficient to do so. I
haven't done tests, but I suspect this feature is probably a
de-optimisation in all cases other than an int-pair struct (ranges,
delegates? I suspect that's why this was invented). Structure
packing/unpacking code will likely be slower than a store/load.
We just need proper multiple return values, then this can go away. And
the
earlier any ABI breaking changes are implemented, the better, while there
are still few(/none?) closed source, binary distributed D libraries.
Manu, why are you assuming that the struct is returned in any sort of
fixed memory layout?
Because that is what the ABI says.
* 1, 2 and 4 byte structs are returned in EAX.
* 8 byte structs are returned in EDX,EAX, where EDX gets the most
significant half.
* For other struct sizes, the return value is stored through a hidden
pointer passed as an argument to the function.
You can _not_ take the memory address, etc of a
register so maintaining the layout of a struct is a) impossible and b)
not necessary, if you're returning it via registers. The compiler is
free to use XMM registers for floats/doubles and GPRs for pointers/ints.
And there's no need for the caller to place those struct register values
on the stack if they're never used. Furthermore, even on the stack, the
struct's memory layout is only ever valid immediately before (and
forever after due to pointer escapes) a non-inlined function call or
address taking. At a low level, MRV is a structured set of value, ergo
it's a struct.
Multiple function arguments are a structured set of values too.
