Messages in this thread | | | Date | Thu, 05 Feb 2015 06:11:59 -0800 | From | Guenter Roeck <> | Subject | Re: [tip:core/types] bitops: Add sign_extend8(), 16 and 64 functions |
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On 02/04/2015 11:17 PM, Ingo Molnar wrote: > > * H. Peter Anvin <hpa@zytor.com> 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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