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From: "christophe.leroy at csgroup dot eu" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c/97445] Some fonctions marked static inline in Linux kernel
 are not inlined
Date: Tue, 20 Oct 2020 13:28:19 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97445

--- Comment #38 from Christophe Leroy <christophe.leroy at csgroup dot eu> =
---
(In reply to Jan Hubicka from comment #32)
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97445
> >=20
> > --- Comment #31 from Segher Boessenkool <segher at gcc dot gnu.org> ---
> > (In reply to Jan Hubicka from comment #27)
> > > It is because --param inline-insns-single was reduced for -O2 from 200
> > > to 70.  GCC 10 has newly different set of parameters for -O2 and -O3 =
and
> > > enables auto-inlining at -O2.
> > >=20
> > > Problem with inlininig funtions declared inline is that C++ codebases
> > > tends to abuse this keyword for things that are really too large (and
> > > get_order would be such example if it did not have builtin_constant_p
> > > check which inliner does not understand well). So having same limit at
> > > -O2 and -O3 turned out to be problematic with respect to code size and
> > > especially with respect to LTO, where a lot more inlining oppurtuniti=
es
> > > appear.
> >=20
> > Do the heuristics account for that not inlining a "static inline" resul=
ts
> > in multiple copies?
>=20
> It prevents inlining only when there are multiple calls in the unit
> being compiled (there is no way to know that the same inline function is
> duplicated in other units).
> This is what happens here: there are multiple calls so inliner concludes
> inlining would cost too much of code size and later they are optimized
> away.
>=20
> get_order is a wrapper around ffs64.  This can be implemented w/o asm
> statement as follows:
> int
> my_fls64 (__u64 x)
> {
>   if (!x)
>       return 0;
>   return 64 - __builtin_clzl (x);
> }
>=20
> This results in longer assembly than the kernel asm implementation. If
> that matters I would replace builtin_constnat_p part of get_order by this
> implementation that is more transparent to the code size estimation and
> things will get inlined.
>=20

But on powerpc that's already the case and it doesn't solve the issue.

static inline int fls(unsigned int x)
{
        return 32 - __builtin_clz(x);
}

static inline int fls64(__u64 x)
{
        return 64 - __builtin_clzll(x);
}=