On Sat, 10 Mar 2012 19:27:05 -0600, Manu <turkey...@gmail.com> wrote:
On 11 March 2012 00:25, Sean Cavanaugh <worksonmymach...@gmail.com> wrote:
On 3/10/2012 4:37 AM, Manu wrote:
If I pass a structure TO a function by value, I know what happens, a
copy is written to the stack which the function expects to find there.
This is only true if the compiler is forced to use the ABI, when inlining
is impossible, or the type being passed is too complex. Structs of pods,
most compilers do magical things to provided you don't actively work
against the code gen (virtual methods, dllexports etc), too many separate
.obj units in C++ etc.
Talking about the typical case here, obviously not inlined, calling through
the ABI, struct may be simple, just 2-3 values, can you show me a case
where C++ is able to to anything particularly fancy?
I've never seen the compiler do wildly unexpected things unless whole
program optimisation is enabled, which I don't imagine D will be able to
support any time real soon?
...and even then, relying on WPO for the language to generate good code in
certain circumstances is a really really bad idea. This makes the task of
implementing an efficient compiler for the language extremely difficult.
In most cases, making concise expression of the operation you want to
perform possible in the language will generate better results anyway,
without depending on an aggressive optimiser. It will also make the code
more explicit and readable.
Manu, please go read the D ABI (http://dlang.org/abi.html). Remember, your
example of returning two values using Tuple vs 'real' MRV? The D ABI states
that those values will be returned via registers. Returning something larger?
Then the NVRO kicks in which gives you a zero copy approach. On x86-64 these
limits are different, since you have more registers to play with, but the
concept is the same. In fact, returning arguments has always been more
efficient than passing arguments.
