[fpc-devel] Const optimization is a serious bug

[Thaddy]

On 9-7-2011 3:29, Martin wrote:
> On 09/07/2011 02:14, Chad Berchek wrote:
>> Specifically, the way const is now "defined" (or not...) is 
>> disturbing because it leaves out important details. In C++, you can 
>> pass by value or reference, or by pointer which is a value but is 
>> used to make references. But what's important is you _always_ know 
>> what it's going to do. If you pass something by reference and modify 
>> that instance via another reference, they're all going to change, 
>> because they are the same instance. And if you pass by value, it's 
>> the opposite. The point is, though, you always know what it's going 
>> to be.
>>
>> Imagine if we took C++ and redefined the pass by reference syntax as 
>> "the compiler might pass by value or reference, depending what it 
>> feels like". Result? Boom. Tons of carefully written code would 
>> suddenly break.
>>
> ...
>> If we *knew* that const meant it would be by reference, then that 
>> immediately eliminates the confusion in the case of ShortString and 
>> records; modifying one instance through several references affects 
>> them all, as expected. What the programmer says happens; there can be 
>> no "bug", except in the programmer's own knowledge.
>>
>
> We do know:
> http://www.freepascal.org/docs-html/ref/refsu58.html#x135-14500011.4.4
>> A constant argument is passed by reference if its size is larger than 
>> a pointer. It is passed by value if the size is equal or is less then 
>> the size of a native pointer.
>
> Of course if you want to rely on this, you must ensure the size of 
> your type does ot change (a records definition can be chnaged, and 
> that would have consequences for "const" param). But then, if you rely 
> on it, but some compiler $IF and $FAIL in the code, to ensure it only 
> compiles if your assumptions are met.
>
> Personally I thing this page would be a good place to document the 
> properties of "const" param with regards to ref-counted types...
>
>
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Isn't it time to cut this discussion short? const means do not modify! 
full stop! whatever by reference or value. The rest is bovine extrement.