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From: "hubicka at ucw dot cz" <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 11:12:52 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97445

--- Comment #34 from Jan Hubicka <hubicka at ucw dot cz> ---
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97445
>=20
> --- Comment #33 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
> (In reply to Jan Hubicka from comment #32)
> > 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 th=
is
> > implementation that is more transparent to the code size estimation and
> > things will get inlined.
>=20
> Better __builtin_clzll so that it works also on 32-bit arches.
> Anyway, if kernel's fls64 results in better code than the my_fls64, we sh=
ould
> look at GCC's code generation for that case.

Original asm is:

__attribute__ ((noinline))
int fls64(__u64 x)
{
 int bitpos =3D -1;
 asm("bsrq %1,%q0"
     : "+r" (bitpos)
     : "rm" (x));
 return bitpos + 1;
}

There seems to be bug in bsr{q} pattern.  I can make GCC produce same
code with:

__attribute__ ((noinline))
int
my_fls64 (__u64 x)
{
  asm volatile ("movl $-1, %eax");
  return (__builtin_clzll (x) ^ 63) + 1;
}

But obviously the volatile asm should not be needed.  I think bsrq is
incorrectly modelled as returning full register

(define_insn "bsr_rex64"
  [(set (match_operand:DI 0 "register_operand" "=3Dr")
        (minus:DI (const_int 63)
                  (clz:DI (match_operand:DI 1 "nonimmediate_operand" "rm"))=
))
   (clobber (reg:CC FLAGS_REG))]
  "TARGET_64BIT"
  "bsr{q}\t{%1, %0|%0, %1}"
  [(set_attr "type" "alu1")
   (set_attr "prefix_0f" "1")
   (set_attr "znver1_decode" "vector")
   (set_attr "mode" "DI")])=