Re: GNAT Optimization of Constant Expressions

[Duncan Sands <baldrick@free.fr>]

Hi Randy,

> That doesn't match the OP's original description of the problem. He said
> that declaring constants in front of the expression made it run fast, even
> if they were *not used* in the expression in the loop. That could only
> happen if the compiler did a common subexpression elimination in that case,
> and a common subexpression elimination is *only* valid on a function call if
> it is Pure. (Other functions could have significant side effects that
> eliminating the extra calls would make the program incorrect, for example if
> the function is a random number generator.)
> 
> I think it is more likely that it didn't recognize the functions as being
> worth hoisting (there is only one instance of each in the original program)
> or it didn't do so because of other concerns. In thinking about it now, the
> former is probably more likely.

while what you say is very logical, unfortunately it seems to be wrong: I
checked and these functions are not being marked pure.  So how come declaring
a constant helps?  I don't know - I didn't look into it yet.

Ciao,

Duncan.