On Thu, Jul 26, 2012 at 8:57 AM, Jon Beniston <jon.benis...@ensilica.com> wrote:
>
> 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?
It seems to me that you could just add an ashldi3 pattern.
Ian
