[fpc-pascal] Optimisation : X+X or 2*X or X*2 ?

[Sven Barth]

On 19.01.2014 20:47, Ched wrote:
> Dear Freepascalers,
>
> The multiplication in floating arthmetic units is usualy know as being
> slower than the addition. So, have I interest to code X+X, 2*X or X*2
> when using reals (mainly double and extended) ?And with integers ?

Can't really comment on floats, but for integers the fasted to multiply 
by 2 should be a shift left.

> Can the compiler optimise this operation choosing the fastest one -
> probably depending on the architecture x86, arm, aso.

It can if someone codes support for it (maybe some code generators 
already have such simplifications). I thought I've at least seen such 
code for integers (that exactly converts *2 to a shl), but I can't find 
it currently...

Regards,
Sven