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Date: Thu, 05 Feb 2004 10:43:12 -0500
From: Hyman Rosen <hyrosen@mail.com>
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Subject: Re: In-Out Parameters for functions
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Date: 2004-02-05T10:43:12-05:00
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Robert I. Eachus wrote:
> 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.

Sigh. When I said "access to the body" I meant that the compiler has
no more information about what the function does than its declaration.
Obviously if a function is declared Pure then its return value is not
affected by order of execution, and the compiler can hoist away.

Think about it - if you write
     s := Sin(x) + Cos(y)
Ada requires that both arguments are evaluated, but doesn't specify the
order. Yet, with a pragma Pure in place, Ada need not evaluate the operands
each time in a loop, but can hoist the calculations out, as you say. Why
would specifying the order change this?