Re: GNAT 4.9 - missing optimization feature?

[Adam Beneschan <adambeneschan@gmail.com>]

On Wednesday, August 6, 2014 7:19:08 AM UTC-7, Victor Porton wrote:
> Victor Porton wrote:
> 
> > The below program, compiled with GNAT 4.9, calls Adjust two times when
> > copying a T1 object.
> 
> > But it does the same operations with a T2 object without calling Adjust.
> 
> > So calling Adjust on a T1 object is here redundant and can be optimized
> > away for greater performance. If I recall correctly, Ada Reference Manual
> > allows this kind of optimization.

> Is it true that this kind of optimization is legitimate accordingly ARM?

Yes.  See 7.6(17.1 - 17.4).  You're using a function call to initialize an object.  This section says that an object must be "built in place" if the type is limited, which is why there is no copying and no Adjust for T2.  (Limited_Controlled doesn't have an Adjust procedure, of course.)  7.6(17.4) says that when the two cases where an object *must* be built in place don't apply, "it is unspecified whether the anonymous object is built in place".  That means that the compiler could generate code to put the function result directly in T1 (the optimization), or it could generate code to put the function result in some other object and then copy.

                                 -- Adam