Re: [RFC][PATCH] new byteorder primitives - ..._{replace,get}_bits()

Jakub Kicinski
2017-12-13T01:35:38Z

On Wed, 13 Dec 2017 01:30:56 +0000, Al Viro wrote:
> On Tue, Dec 12, 2017 at 05:04:37PM -0800, Jakub Kicinski wrote:
> > On Wed, 13 Dec 2017 00:36:59 +0000, Al Viro wrote:
> > > On Tue, Dec 12, 2017 at 03:59:33PM -0800, Jakub Kicinski wrote:
> > > > > +static __always_inline __##type type##_replace_bits(__##type old, \
> > > > > + base val, base mask) \
> > > > > +{ \
> > > > > + __##type m = to(mask); \
> > > > > + if (__builtin_constant_p(val) && \
> > > >
> > > > Is the lack of a __builtin_constant_p(mask) test intentional? Sometimes
> > > > the bitfield is a packed array and people may have a helper to which
> > > > only the mask is passed as non-constant and the value is implied by the
> > > > helper, thus constant.
> > >
> > > If the mask in non-constant, we probably shouldn't be using that at all;
> > > could you show a real-world example where that would be the case?
> >
> > FIELD_* macros explicitly forbid this, since the code would be...
> > suboptimal with the runtime ffsl. Real life examples are the hackish
> > macro NFP_ETH_SET_BIT_CONFIG() in
> >
> > drivers/net/ethernet/netronome/nfp/nfpcore/nfp_nsp_eth.c
>
> Why not simply make nfp_eth_set_bit_config() static inline and be
> done with that? It's not that large and there are only few callers,
> so...

It used to be __always_inline, but apparently LLVM/clang doesn't
propagate constants :(

4e59532541c8 ("nfp: don't depend on compiler constant propagation")

> > I remember there was also some Renesas code.. maybe this:
> >
> > https://patchwork.kernel.org/patch/9881279/
> >
> > it looks like cpg_z_clk_recalc_rate() and cpg_z2_clk_recalc_rate() only
> > differ in mask.
>
> *shrug*
>
> That thing would expand into "reg &= 15" in one case and "reg >>= 8; reg &= 15"
> in another. Either of which is cheaper than a function call, and definitely
> cheaper than any kind of dynamic calculation of shift, no matter how implemented.