Re: 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