Re: [tip:core/types] bitops: Add sign_extend8(), 16 and 64 functions

[Guenter Roeck]

On 02/04/2015 11:17 PM, Ingo Molnar wrote:
> * H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>
> > On 01/19/2015 02:04 AM, Peter Zijlstra wrote:
> > > On Mon, Jan 19, 2015 at 07:54:22AM +1200, Linus Torvalds wrote:
> > > > Why?
> > > >
> > > > The 8- and 16- bit versions are the same as the 32-bit one.
> > > > This seems pointless. If you want something where the sign
> > > > is in bit 3, they all return the same value, just the return
> > > > type differs, but that's really a *caller* thing, no?
> > >
> > > Even for the 8bit ones? Since we have the *H and *L register
> > > we have more 8 bit regs than we have 16/32 bit regs and it
> > > might just be worth it.
> >
> > Fewer, actually.  gcc doesn't really use the H registers much,
>
> Is that true for other compilers as well?
>
> > and instead considers 8-bit values to occupy the whole
> > register, but that means only four are available in 32-bit
> > mode.
>
> So where are we with this? Should I consider:
>
>    7e9358073d3f ("bitops: Add sign_extend8(), 16 and 64 functions")
>
> NAK-ed due to having marginal benefits, or due to having no
> benefits whatsoever?
>
> How about the two patch series from Martin Keppling - that does
> seem to be both beneficial and correct, agreed?
Do you mean the two patches improving the documentation of
sign_extend32 and adding sign_extend64 ? I thought those
would be valuable.

The discussion resulted in sign_extend32() being used for non-32-bit
operations, so that by itself was quite useful.

Guenter

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