Re: In-Out Parameters for functions

["Robert I. Eachus" <rieachus@comcast.net>]

Hyman Rosen wrote:

> But you are now talking about finding an optimal implementation
> order for an expression involving more than one function call
> where the compiler has no access to the body of the functions.
> Do you really think that allowing the compiler to pick the order
> of the calls will significantly affect the efficiency?

Of course it will.  Consider for one example a Pure subprogram, say a 
call to Sin or Log from a math package inside a loop.  The pragma Pure 
rules allow the compiler to hoist the call outside the loop, if the 
parameter(s) are unchanged in successive calls.

YOU may think that is a contrived example, but it is not.  That sort of 
thing happens all the time in embedded flight dynamics systems where a 
set of expressions is iterated a couple of times for convergence.  And 
even if the expressions are not iterated, they often include common 
subexpressions that can be pulled out of line by the compiler.  Any call 
to a Pure function can be treated as a common subexpression, even if the 
subprogram itself is not inlined.

This is one of the nice features of Ada for safety-critical systems. 
The formulas for things like flight dynamics can be almost copied 
verbatum from the specification, and the compiler (and Verifier!) can do 
the grody work of finding an efficient implementation and showing that 
it is equivalent.

-- 
                                           Robert I. Eachus

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