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: 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? 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