Double word left shift optimisation

[Jon Beniston <jon dot beniston at ensilica dot com>]

Hi,

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;
}

ARM, MIPS etc expand to a fairly long sequence like:

        nor     $3,$0,$5
        sra     $2,$4,31
        srl     $7,$4,1
        srl     $7,$7,$3
        sll     $2,$2,$5
        andi    $6,$5,0x20
        sll     $3,$4,$5
        or      $2,$7,$2
        movn    $2,$3,$6
        movn    $3,$0,$6

I'd like to optimise this to something like:

 (long long) a * (1 << b)

Which should just be 3 or so instructions. I don't think this can be
sensibly done in the target backend as the generated pattern is too
complicated to match and am not familiar with the middle end. Any
suggestions as to where and how this should be best implemented?

Thanks,
Jon