Re: [PATCH] Merging Cilk Plus into Trunk (Patch 1 of approximately 22)

[Gabriel Dos Reis <gdr at integrable-solutions dot net>]

On Thu, Sep 6, 2012 at 11:11 AM, Iyer, Balaji V <balaji.v.iyer@intel.com> wrote:

>>> On Wed, Sep 5, 2012 at 5:09 PM, Iyer, Balaji V <balaji.v.iyer@intel.com> wrote:
>>>> Let's say we have two for loops like this:
>>>>
>>>> int my_func (int x, int y);
>>>>
>>>> For (ii = 0; ii < 10000; ii++)
>>>>         X[ii] = my_func (Y[ii], Z[ii]);
>>
>>I assume X, Y and Z are __restrict pointers (or something the compiler can detect
>>doesn't alias).
>
> Yes, the compiler must detect that.

Exactly what do you mean by detect?
That the user has supplied somewhere a declaration saying the pointers
are restricted? (Note that C++11 does not have restrict).  Or that the
compiler must perform an alias analysis?

>>> 2. Considering this example, won't you get the same behaviour
>>>     if my_func was declared with "pure" attribute?  If not, why?
>>
>>AFAIU, my_func is defined in a separate library and because of the attribute on
>>the definition, it will actually export overloads:
>>int myfunc(int,int);
>>v2si myfunc(v2si,v2si);
>>v4si myfunc(v4si,v4si);
>>etc (where does it stop? seems problematic if the library is compiled for
>>sse4 and I then compile and link an avx program)
>
> The user can provide at most 1 vector length and the compiler will map it to appropriate vector value. If the user omits the vectorlength clause then the compiler picks a vectorlength based on the architecture's vector units and the data width. So, it will stop at 2 (1 scalar and 1 vector) :-).

Is that part of the ABI and the function declaration?
Note that in C++11 (and I suppose in C++1y), attributes are supposed to be
semantics-neutral, in the sense that if a program compiles with attributes, then
ignoring those attributes should also lead to a well-formed program with the
same observable behaviour.

This brings us to the question: do you expect your proposal to the C++ committee
to be adopted as is, or do you anticipate or expect changes based on committee
feedback?  If you expect changes, what policy to propose for changes that would
reflect any feedback you would get from WG21?  The reason why this is important
is because WG21 has its own schedule, independent of GCC, and GCC has to
deal with forward/backward compatibility.

-- Gaby