On Wed, 2 Oct 2024 at 19:16, Jonathan Wakely <jwak...@redhat.com> wrote:
>
> On Wed, 2 Oct 2024 at 19:15, Dmitry Ilvokhin <d...@ilvokhin.com> wrote:
> >
> > Instead of looping over every byte of the tail, unroll loop manually
> > using switch statement, then compilers (at least GCC and Clang) will
> > generate a jump table [1], which is faster on a microbenchmark [2].
> >
> > [1]: https://godbolt.org/z/aE8Mq3j5G
> > [2]: https://quick-bench.com/q/ylYLW2R22AZKRvameYYtbYxag24
> >
> > libstdc++-v3/ChangeLog:
> >
> > * libstdc++-v3/libsupc++/hash_bytes.cc (load_bytes): unroll
> > loop using switch statement.
> >
> > Signed-off-by: Dmitry Ilvokhin <d...@ilvokhin.com>
> > ---
> > libstdc++-v3/libsupc++/hash_bytes.cc | 27 +++++++++++++++++++++++----
> > 1 file changed, 23 insertions(+), 4 deletions(-)
> >
> > diff --git a/libstdc++-v3/libsupc++/hash_bytes.cc
> > b/libstdc++-v3/libsupc++/hash_bytes.cc
> > index 3665375096a..294a7323dd0 100644
> > --- a/libstdc++-v3/libsupc++/hash_bytes.cc
> > +++ b/libstdc++-v3/libsupc++/hash_bytes.cc
> > @@ -50,10 +50,29 @@ namespace
> > load_bytes(const char* p, int n)
> > {
> > std::size_t result = 0;
> > - --n;
> > - do
> > - result = (result << 8) + static_cast<unsigned char>(p[n]);
> > - while (--n >= 0);
>
> Don't we still need to loop, for the case where n >= 8? Otherwise we
> only hash the first 8 bytes.
Ah, but it's only ever called with load_bytes(end, len & 0x7)
>
> > + switch(n & 7)
> > + {
> > + case 7:
> > + result |= std::size_t(p[6]) << 48;
> > + [[gnu::fallthrough]];
> > + case 6:
> > + result |= std::size_t(p[5]) << 40;
> > + [[gnu::fallthrough]];
> > + case 5:
> > + result |= std::size_t(p[4]) << 32;
> > + [[gnu::fallthrough]];
> > + case 4:
> > + result |= std::size_t(p[3]) << 24;
> > + [[gnu::fallthrough]];
> > + case 3:
> > + result |= std::size_t(p[2]) << 16;
> > + [[gnu::fallthrough]];
> > + case 2:
> > + result |= std::size_t(p[1]) << 8;
> > + [[gnu::fallthrough]];
> > + case 1:
> > + result |= std::size_t(p[0]);
> > + };
> > return result;
> > }
> >
> > --
> > 2.43.5
> >
