On Fri, Oct 4, 2024 at 11:20 AM Jonathan Wakely <jwak...@redhat.com> wrote: > > On Fri, 4 Oct 2024 at 10:19, Jonathan Wakely <jwak...@redhat.com> wrote: > > > > 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 > > Oh actually in the benchmark I used (!(1 <= n && n < 8)) because 1 <= > n is always true too.
Yes, expose the tightest range to the compiler. Of course this might trigger opportunistic optimiztation like complete peeling. Richard.
