Optimize count_digits
diff --git a/include/fmt/format.h b/include/fmt/format.h index 64adc27..30f54de 100644 --- a/include/fmt/format.h +++ b/include/fmt/format.h
@@ -995,13 +995,26 @@ #ifdef FMT_BUILTIN_CLZ // Optional version of count_digits for better performance on 32-bit platforms. FMT_CONSTEXPR20 inline auto count_digits(uint32_t n) -> int { - if (is_constant_evaluated()) { + if (is_constant_evaluated() || FMT_MSC_VER) { return count_digits_fallback(n); } - auto t = bsr2log10(FMT_BUILTIN_CLZ(n | 1) ^ 31); - constexpr const uint32_t zero_or_powers_of_10[] = {0, 0, - FMT_POWERS_OF_10(1U)}; - return t - (n < zero_or_powers_of_10[t]); + // An optimization by Kendall Willets from https://bit.ly/3uOIQrB. + // This increments the upper 32 bits (log10(T) - 1) when >= T is added. +# define FMT_INC(T) (((sizeof(# T) - 1) << 32) - T) + constexpr uint64_t table[] = { + FMT_INC(0), FMT_INC(0), FMT_INC(0), // 8 + FMT_INC(10), FMT_INC(10), FMT_INC(10), // 64 + FMT_INC(100), FMT_INC(100), FMT_INC(100), // 512 + FMT_INC(1000), FMT_INC(1000), FMT_INC(1000), // 4096 + FMT_INC(10000), FMT_INC(10000), FMT_INC(10000), // 32k + FMT_INC(100000), FMT_INC(100000), FMT_INC(100000), // 256k + FMT_INC(1000000), FMT_INC(1000000), FMT_INC(1000000), // 2048k + FMT_INC(10000000), FMT_INC(10000000), FMT_INC(10000000), // 16M + FMT_INC(100000000), FMT_INC(100000000), FMT_INC(100000000), // 128M + FMT_INC(1000000000), FMT_INC(1000000000), FMT_INC(1000000000), // 1024M + FMT_INC(1000000000), FMT_INC(1000000000) // 4B + }; + return static_cast<int>((n + table[FMT_BUILTIN_CLZ(n | 1) ^ 31]) >> 32); } #endif