On Thu, Apr 13, 2017 at 11:16:08AM +0000, Wilco Dijkstra wrote:
> >On Wed, Apr 12, 2017 at 09:29:55AM +0000, Sudi Das wrote:
> > > Hi all
> > >
> > > This is a fix for PR 80131
> > > Currently the code A << (B - C) is not simplified.
> >> However at least a more specific case of 1U << (C -x) where C =
> >> precision(type) - 1 can be simplified to (1 << C) >> x.
> >
> > Is that always a win though?
>
> Yes assuming left and right shift have the same performance.
>
> > Some constants have higher costs than others on various targets, some
> > significantly higher. This change might be beneficial only
> > if if C is as expensive as 1, then you get rid of a one (typically cheap)
> > operation.
>
> Most targets can create the constant cheaply. Targets that can't would need 2
No. Some constants sometimes even 7 instructions (e.g. sparc64; not talking
in particular about 1ULL << 63 constant), or have one instruction
that is more expensive than normal small constant load. Compare say x86_64
movl/movq vs. movabsq, I think the latter has 3 times longer latency on many
CPUs. So no, I think it isn't an unconditional win.
Jakub
