Re: [PATCH v2 01/18] lib/parity: Add __builtin_parity() fallback implementations

David Laight
2025-03-03T12:41:32Z

On Mon, 3 Mar 2025 10:47:20 +0800
Kuan-Wei Chiu <visitorckw@gmail.com> wrote:

> On Sun, Mar 02, 2025 at 07:09:54PM +0000, David Laight wrote:
> > On Mon, 3 Mar 2025 01:29:19 +0800
> > Kuan-Wei Chiu <visitorckw@gmail.com> wrote:
> >
> > > Hi Yury,
> > >
...
> > > #define parity(val) \
> > > ({ \
> > > __auto_type __v = (val); \
> > > bool __ret; \
> > > switch (BITS_PER_TYPE(val)) { \
> > > case 64: \
> > > __v ^= __v >> 16 >> 16; \
> > > fallthrough; \
> > > case 32: \
> > > __v ^= __v >> 16; \
> > > fallthrough; \
> > > case 16: \
> > > __v ^= __v >> 8; \
> > > fallthrough; \
> > > case 8: \
> > > __v ^= __v >> 4; \
> > > __ret = (0x6996 >> (__v & 0xf)) & 1; \
> > > break; \
> > > default: \
> > > BUILD_BUG(); \
> > > } \
> > > __ret; \
> > > })
> >
> > I'm seeing double-register shifts for 64bit values on 32bit systems.
> > And gcc is doing 64bit double-register maths all the way down.
> >
> > That is fixed by changing the top of the define to
> > #define parity(val) \
> > ({ \
> > unsigned int __v = (val); \
> > bool __ret; \
> > switch (BITS_PER_TYPE(val)) { \
> > case 64: \
> > __v ^= val >> 16 >> 16; \
> > fallthrough; \
> >
> > But it's need changing to only expand 'val' once.
> > Perhaps:
> > auto_type _val = (val);
> > u32 __ret = val;
> > and (mostly) s/__v/__ret/g
> >
> I'm happy to make this change, though I'm a bit confused about how much
> we care about the code generated by gcc. So this is the macro expected
> in v3:

There is 'good', 'bad' and 'ugly' - it was in the 'bad' to 'ugly' area.

>
> #define parity(val) \
> ({ \
> __auto_type __v = (val); \
> u32 __ret = val; \
> switch (BITS_PER_TYPE(val)) { \
> case 64: \
> __ret ^= __v >> 16 >> 16; \
> fallthrough; \
> case 32: \
> __ret ^= __ret >> 16; \
> fallthrough; \
> case 16: \
> __ret ^= __ret >> 8; \
> fallthrough; \
> case 8: \
> __ret ^= __ret >> 4; \
> __ret = (0x6996 >> (__ret & 0xf)) & 1; \
> break; \
> default: \
> BUILD_BUG(); \
> } \
> __ret; \
> })

That looks like it will avoid double-register shifts on 32bit archs.
arm64 can do slightly better (a couple of instructions) because of its
barrel shifter.
x86 can do a lot better because of the cpu 'parity' flag.
But maybe it is never used anywhere that really matters.

David