Double word left shift optimisation

[Segher Boessenkool]

Hi Jon,

> I'd like to try to optimise double word left shifts of sign/zero  
> extended
> operands if a widening multiply instruction is available. For the  
> following
> code:
>
> long long f(long a, long b)
> {
>   return (long long)a << b;
> }

> I'd like to optimise this to something like:
>
>  (long long) a * (1 << b)
>
> Which should just be 3 or so instructions.

Only if b < bits_per_long; it is quite a bit more otherwise
(you then need 1LL << b).

If your pattern actually works for b <= bits_per_long, you
can do   b1 = b >> 1; b2 = b - b1; return (a << b1) << b2;
reasonably cheaply.


Segher