pow function working unexpectedly

[Nicolas Blais]

On Tuesday 11 April 2006 19:24, Colin Percival wrote:
> Andy Reitz wrote:
> > So, clearly, something is optimizing the pow() function away when the
> > arguments are hard-coded lvalues, instead of varibles.
> >
> > Now, what that thing *is*, I don't know.
>
> The C compiler precomputes constant expressions; your "pow(2,3)" is
> being rewritten to "8" by the compiler.  Similarly, if you write
> "1 + 2 / 3 + 4 * 5 - 6", the C compiler will turn this into "15"
> rather than producing a series of instructions which computes the
> expression.
>
> When you reference variables, this optimization isn't possible, since
> those variables might be modified before you reach the line where
> they are used.  (Obviously this doesn't happen in your program, but
> the compiler isn't smart enough to figure that out.)
>
> Colin Percival

+1. The compiler sees constants differently from variables (which reside in a 
memory space). With the constant, the optimizer can rewrite the equation with 
its answer.

Anyhow, it can't be seen as a bug because you should include the math library 
whenever you're using a function from it (see man pow). Therefore, if you 
would have included the library in the first place, you would have never 
found this 'oddity'.

Nicolas.

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