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

Yury Norov
2025-03-03T15:43:31Z

On Mon, Mar 03, 2025 at 10:47:20AM +0800, Kuan-Wei Chiu wrote:
> > > #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:

We do care about code generated by any compiler. But we don't spread
hacks here and there just to make GCC happy. This is entirely broken
strategy. Things should work the other way: compiler people should
collect real-life examples and learn from them.

I'm not happy even with this 'v >> 16 >> 16' hack, I just think that
disabling Wshift-count-overflow is the worse option. Hacking the macro
to optimize parity64() on 32-bit arch case doesn't worth it entirely.

In your patchset, you have only 3 drivers using parity64(). For each
of them, please in the commit message refer that calling generic
parity() with 64-bit argument may lead to sub-optimal code generation
with a certain compiler against 32-bit arches. If you'll get a
feedback that it's a real problem for somebody, we'll think about
mitigating it.

> #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; \
> })