On Mon, 18 Apr 2022, 15:39 Patrick Palka via Libstdc++, <
libstd...@gcc.gnu.org> wrote:
> At the first iteration of __from_chars_pow2_base's main loop, we need
> to remember the value of the leading significant digit for sake of the
> overflow check at the end of the function (for bases other than 2).
>
> This patch manually unrolls this first iteration so as to not encumber
> the entire loop with logic that only the first iteration needs. This
> seems to significantly improve performance:
>
Great, I was going to suggest looking into that change.
> Base Before After (seconds, lower is better)
> 2 9.36 9.37
> 8 3.66 2.93
> 16 2.93 1.91
> 32 2.39 2.24
>
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
>
OK, thanks!
> libstdc++-v3/ChangeLog:
>
> * include/std/charconv (__from_chars_pow2_base): Manually
> unroll the first iteration of the main loop and simplify
> accordingly.
> ---
> libstdc++-v3/include/std/charconv | 30 +++++++++++++++++++++---------
> 1 file changed, 21 insertions(+), 9 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/charconv
> b/libstdc++-v3/include/std/charconv
> index dda4ec87779..cd7f52e2195 100644
> --- a/libstdc++-v3/include/std/charconv
> +++ b/libstdc++-v3/include/std/charconv
> @@ -469,25 +469,37 @@ namespace __detail
> while (__i < __len && __first[__i] == '0')
> ++__i;
> const ptrdiff_t __leading_zeroes = __i;
> + if (__i >= __len) [[__unlikely__]]
> + {
> + __first += __i;
> + return true;
> + }
> +
> + // Remember the leading significant digit value if necessary.
> + unsigned char __leading_c;
> + if (__base != 2)
> + {
> + __leading_c = __from_chars_alnum_to_val<_DecOnly>(__first[__i]);
> + // __glibcxx_assert(__leading_c != 0);
> + if (__leading_c >= __base) [[__unlikely__]]
> + {
> + __first += __i;
> + return true;
> + }
> + __val = __leading_c;
> + ++__i;
> + }
>
> - unsigned char __leading_c = 0;
> for (; __i < __len; ++__i)
> {
> const unsigned char __c =
> __from_chars_alnum_to_val<_DecOnly>(__first[__i]);
> if (__c >= __base)
> break;
> __val = (__val << __log2_base) | __c;
> -
> - if (__i == __leading_zeroes)
> - {
> - // At the first iteration, remember the leading significant
> digit.
> - // __glibcxx_assert(__leading_c == 0 && __c != 0);
> - __leading_c = __c;
> - }
> }
> __first += __i;
> auto __significant_bits = (__i - __leading_zeroes) * __log2_base;
> - if (__base != 2 && __leading_c != 0)
> + if (__base != 2)
> // Compensate for a leading significant digit that didn't use all
> // of its available bits.
> __significant_bits -= __log2_base - __bit_width(__leading_c);
> --
> 2.36.0.rc2.10.g1ac7422e39
>
>
