View mode: basic / threaded / horizontal-split · Log in · Help
November 09, 2012
Settling rvalue to (const) ref parameter binding once and for all
Hi guys,

I hope you don't mind that I'm starting yet another thread about 
this tedious issue, but I think the other threads are too clogged.

Let me summarize my (final, I guess) proposal. I think it makes 
sense to compare it to C++ in order to anticipate and hopefully 
invalidate (mainly Andrei's) objections.

     parameter type     |   lvalue    |    rvalue
                        | C++     D   | C++     D
------------------------|-------------|------------
T                       | copy   copy | copy   move
T& / ref T              | ref    ref  | n/a    n/a
out T (D only)          |        ref  |        n/a
T&& (C++ only)          | n/a         | move
auto ref T (D only) (*) |        ref  |        ref
------------------------|-------------|------------
const T                 | copy   copy | copy   move
const T& / const ref T  | ref    ref  | ref    ref (*)
const T&& (C++ only)    | n/a         | move

(*): proposed additions

For lvalues in both C++ and D, there are 2 options: either copy 
the argument (pass-by-value) or pass it by ref. There's no real 
difference between both languages except for D's additional 'out' 
keyword and, with the proposed 'auto ref' syntax, an (imo 
negligible) ambiguity between 'ref T' and 'auto ref T' in D.

Rvalues are a different topic though. There are 3 possibilites in 
general: copy, move and pass by ref. Copying rvalue arguments 
does not make sense - the argument won't be used by the caller 
after the invokation, so a copy is redundant and hurts 
performance. D corrects this design flaw of C++ (which had to 
introduce rvalue refs to add move semantics on top of the default 
copy semantics) and therefore only supports moving instead. C++ 
additionally supports pass-by-ref of rvalues to const refs, but 
not to mutable refs. I propose to allow pass-by-ref to both const 
(identical syntax as C++, it's perfectly safe and logical) and 
mutable refs (new syntax with 'auto ref' to emphasize that the 
parameter may be an rvalue reference, with related consequences 
such as potentially missing side effects).

Regarding the required overloading priorities for the proposed 
additions to work properly, I propose:
1) lvalues: prefer pass-by-ref
   so: ref/out T -> auto ref T (*) -> const ref T -> (const) T
   - const lvalues:   const ref T -> (const) T
   - mutable lvalues: ref/out T -> auto ref T (*) -> const ref T 
->
                      (const) T
2) rvalues: prefer pass-by-value (moving: argument allocated
   directly on callee's stack (parameter) vs. pointer/reference
   indirection implied by pass-by-ref)
   so: (const) T -> auto ref T (*) -> const ref T (*)

Finally, regarding templates, I'm in favor of dropping the 
current 'auto ref' semantics and propose to simply adopt the 
proposed semantics for consistency and simplicity and to avoid 
excessive code bloating. That shouldn't break existing code I 
hope (unless parameters have been denoted with 'const auto ref 
T', which would need to be changed to 'const ref T').

---

Before posting concerns about a perceived unsafety of binding 
rvalues to 'const ref' parameters, please try to find a plausible 
argument as to why the following is currently allowed:

void foo(const ref T x);
if (condition)
{
    T tmp;
    foo(tmp);
} // destruction of tmp

but the following shortcut, eliminating 3 lines (depending on 
code formatting preferences ;)) and avoiding the pollution of the 
local namespace with a 'tmp' variable, shouldn't be allowed:

if (condition)
    foo(T()); // rvalue destructed immediately after the call

---

Let me also illustrate a deterministic allocation/destruction 
scheme for the compiler implementation/language specification:

void foo(auto/const ref T a, auto/const ref T b);

foo(T(), T());
/* order:
   1) allocate argument a on caller's stack
   2) allocate argument b on caller's stack
   3) invoke foo() and pass the argument addresses (refs)
   4) destruct b
   5) destruct a
*/

I guess something like that is covered by the C++ specification 
for binding rvalues to const refs.

---

Now please go ahead and shoot. :)
November 10, 2012
Re: Settling rvalue to (const) ref parameter binding once and for all
Hear hear!
I have dreams at night that look exactly like this proposal! :)
I think I had one just last night, and woke up with a big grin on my face...

> 2) rvalues: prefer pass-by-value (moving: argument allocated
  directly on callee's stack (parameter) vs. pointer/reference
  indirection implied by pass-by-ref)

Is this actually possible?
Does the C/C++ ABI support such an action? GDC and LDC use the C ABI
verbatim, so can this work, or will they have to, like usual, allocate on
the caller's stack, and pass the ref through. I don't really see a
significant disadvantage to that regardless.


On 9 November 2012 20:05, martin <kinke@libero.it> wrote:

> Hi guys,
>
> I hope you don't mind that I'm starting yet another thread about this
> tedious issue, but I think the other threads are too clogged.
>
> Let me summarize my (final, I guess) proposal. I think it makes sense to
> compare it to C++ in order to anticipate and hopefully invalidate (mainly
> Andrei's) objections.
>
>      parameter type     |   lvalue    |    rvalue
>                         | C++     D   | C++     D
> ------------------------|-----**--------|------------
> T                       | copy   copy | copy   move
> T& / ref T              | ref    ref  | n/a    n/a
> out T (D only)          |        ref  |        n/a
> T&& (C++ only)          | n/a         | move
> auto ref T (D only) (*) |        ref  |        ref
> ------------------------|-----**--------|------------
> const T                 | copy   copy | copy   move
> const T& / const ref T  | ref    ref  | ref    ref (*)
> const T&& (C++ only)    | n/a         | move
>
> (*): proposed additions
>
> For lvalues in both C++ and D, there are 2 options: either copy the
> argument (pass-by-value) or pass it by ref. There's no real difference
> between both languages except for D's additional 'out' keyword and, with
> the proposed 'auto ref' syntax, an (imo negligible) ambiguity between 'ref
> T' and 'auto ref T' in D.
>
> Rvalues are a different topic though. There are 3 possibilites in general:
> copy, move and pass by ref. Copying rvalue arguments does not make sense -
> the argument won't be used by the caller after the invokation, so a copy is
> redundant and hurts performance. D corrects this design flaw of C++ (which
> had to introduce rvalue refs to add move semantics on top of the default
> copy semantics) and therefore only supports moving instead. C++
> additionally supports pass-by-ref of rvalues to const refs, but not to
> mutable refs. I propose to allow pass-by-ref to both const (identical
> syntax as C++, it's perfectly safe and logical) and mutable refs (new
> syntax with 'auto ref' to emphasize that the parameter may be an rvalue
> reference, with related consequences such as potentially missing side
> effects).
>
> Regarding the required overloading priorities for the proposed additions
> to work properly, I propose:
> 1) lvalues: prefer pass-by-ref
>    so: ref/out T -> auto ref T (*) -> const ref T -> (const) T
>    - const lvalues:   const ref T -> (const) T
>    - mutable lvalues: ref/out T -> auto ref T (*) -> const ref T ->
>                       (const) T
> 2) rvalues: prefer pass-by-value (moving: argument allocated
>    directly on callee's stack (parameter) vs. pointer/reference
>    indirection implied by pass-by-ref)
>    so: (const) T -> auto ref T (*) -> const ref T (*)
>
> Finally, regarding templates, I'm in favor of dropping the current 'auto
> ref' semantics and propose to simply adopt the proposed semantics for
> consistency and simplicity and to avoid excessive code bloating. That
> shouldn't break existing code I hope (unless parameters have been denoted
> with 'const auto ref T', which would need to be changed to 'const ref T').
>
> ---
>
> Before posting concerns about a perceived unsafety of binding rvalues to
> 'const ref' parameters, please try to find a plausible argument as to why
> the following is currently allowed:
>
> void foo(const ref T x);
> if (condition)
> {
>     T tmp;
>     foo(tmp);
> } // destruction of tmp
>
> but the following shortcut, eliminating 3 lines (depending on code
> formatting preferences ;)) and avoiding the pollution of the local
> namespace with a 'tmp' variable, shouldn't be allowed:
>
> if (condition)
>     foo(T()); // rvalue destructed immediately after the call
>
> ---
>
> Let me also illustrate a deterministic allocation/destruction scheme for
> the compiler implementation/language specification:
>
> void foo(auto/const ref T a, auto/const ref T b);
>
> foo(T(), T());
> /* order:
>    1) allocate argument a on caller's stack
>    2) allocate argument b on caller's stack
>    3) invoke foo() and pass the argument addresses (refs)
>    4) destruct b
>    5) destruct a
> */
>
> I guess something like that is covered by the C++ specification for
> binding rvalues to const refs.
>
> ---
>
> Now please go ahead and shoot. :)
>
November 11, 2012
Re: Settling rvalue to (const) ref parameter binding once and for all
On Saturday, 10 November 2012 at 18:20:56 UTC, Manu wrote:
> Hear hear!
> I have dreams at night that look exactly like this proposal! :)
> I think I had one just last night, and woke up with a big grin 
> on my face...

I'm glad I'm not alone on this. :)

>> 2) rvalues: prefer pass-by-value (moving: argument allocated
>    directly on callee's stack (parameter) vs. pointer/reference
>    indirection implied by pass-by-ref)
>
> Is this actually possible?

It surely is and would only require the compiler to map the 
location of the parameter in the callee's future stack frame to 
the caller's stack frame. I think this is how DMD implements it.

> Does the C/C++ ABI support such an action? GDC and LDC use the 
> C ABI verbatim, so can this work, or will they have to, like 
> usual, allocate on the caller's stack, and pass the ref 
> through. I don't
> really see a significant disadvantage to that regardless.

I'm not sure how C++ does it, I was wondering about that too 
actually after posting this. It quite possibly uses real 
references for '(const) T&&' (otherwise how would you be able to 
transform a passed lvalue reference to an rvalue reference via 
std::move() without copying it?), so my table may be incorrect 
for these 2 cells, although that wouldn't change anything really 
for the proposal, unless someone showed a use case for 'const 
T&&' where 'const T&' doesn't fit (I certainly can't think of 
any).
Top | Discussion index | About this forum | D home