On Fri, 4 Oct 2024 at 07:53, Richard Biener <richard.guent...@gmail.com> wrote:
>
> On Wed, Oct 2, 2024 at 8:26 PM Jonathan Wakely <jwak...@redhat.com> wrote:
> >
> > 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)
>
> The compiler should do such transforms - you probably want to tell
> it that n < 8 though, it likely doesn't (always) know.
e.g. like this?
if ((n & 7) != n)
__builtin_unreachable();
For the microbenchmark that seems to make things consistently worse:
https://quick-bench.com/q/2yCEqzFS8R8ueJ0-Gs-sZ6uWWEw
