From 7259a9d577b05735ccbb8bf808d14a702bf6d8c2 Mon Sep 17 00:00:00 2001 From: Antony Polukhin Date: Fri, 30 Aug 2019 16:25:24 +0000 Subject: [PATCH] Optimize std::to_chars Bunch of micro optimizations for std::to_chars: * For base == 8 replacing the lookup in __digits table with arithmetic computations leads to a same CPU cycles for a loop (exchanges two movzx with 3 bit ops). However this saves 129 bytes of data and totally avoids a chance of cache misses on __digits. * For base == 16 replacing the lookup in __digits table with arithmetic computations leads to a few additional instructions, but totally avoids a chance of cache misses on __digits (- ~9 cache misses for worst case) and saves 513 bytes of const data. * Replacing __first[pos] and __first[pos - 1] with __first[1] and __first[0] on final iterations saves ~2% of code size. * Removing trailing '\0' from arrays of digits allows the linker to merge the symbols (so that "0123456789abcdefghijklmnopqrstuvwxyz" and "0123456789abcdef" could share the same address). This improves data locality and reduces binary sizes. * Using __detail::__to_chars_len_2 instead of a generic __detail::__to_chars_len makes the operation O(1) instead of O(N). It also makes the code two times shorter. In sum: this significantly reduces the size of a binary (for about 4KBs only for base-8 conversion), deals with latency (CPU cache misses) without changing the iterations count and without adding costly instructions into the loops. 2019-08-30 Antony Polukhin * include/std/charconv (__detail::__to_chars_8) __detail::__to_chars_16): Replace array of precomputed digits with arithmetic operations to avoid CPU cache misses. Remove zero termination from array of digits to allow symbol merge with generic implementation of __detail::__to_chars. Replace final offsets with constants. Use __detail::__to_chars_len_2 instead of a generic __detail::__to_chars_len. (__detail::__to_chars): Remove zero termination from array of digits. (__detail::__to_chars_2): Leading digit is always '1'. From-SVN: r275205 --- libstdc++-v3/ChangeLog | 12 ++++++ libstdc++-v3/include/std/charconv | 71 ++++++++++++++++--------------- 2 files changed, 49 insertions(+), 34 deletions(-) diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog index 3bc86cb190c..65ef1c0bc1b 100644 --- a/libstdc++-v3/ChangeLog +++ b/libstdc++-v3/ChangeLog @@ -1,3 +1,15 @@ +2019-08-30 Antony Polukhin + + * include/std/charconv (__detail::__to_chars_8) + __detail::__to_chars_16): Replace array of precomputed digits with + arithmetic operations to avoid CPU cache misses. Remove zero + termination from array of digits to allow symbol merge with generic + implementation of __detail::__to_chars. Replace final offsets with + constants. Use __detail::__to_chars_len_2 instead of a generic + __detail::__to_chars_len. + (__detail::__to_chars): Remove zero termination from array of digits. + (__detail::__to_chars_2): Leading digit is always '1'. + 2019-08-30 Jonathan Wakely * testsuite/23_containers/vector/cons/89164_c++17.cc: Fix errors. diff --git a/libstdc++-v3/include/std/charconv b/libstdc++-v3/include/std/charconv index 53aa63ea277..4e94c39656d 100644 --- a/libstdc++-v3/include/std/charconv +++ b/libstdc++-v3/include/std/charconv @@ -131,7 +131,7 @@ namespace __detail : 1u; } else - return __to_chars_len(__value, 8); + return (__to_chars_len_2(__value) + 2) / 3; } // Generic implementation for arbitrary bases. @@ -155,8 +155,12 @@ namespace __detail unsigned __pos = __len - 1; - static constexpr char __digits[] - = "0123456789abcdefghijklmnopqrstuvwxyz"; + static constexpr char __digits[] = { + '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', + 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', + 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', + 'u', 'v', 'w', 'x', 'y', 'z' + }; while (__val >= __base) { @@ -181,7 +185,7 @@ namespace __detail to_chars_result __res; - const unsigned __len = __to_chars_len(__val, 0x10); + const unsigned __len = (__to_chars_len_2(__val) + 3) / 4; if (__builtin_expect((__last - __first) < __len, 0)) { @@ -190,32 +194,30 @@ namespace __detail return __res; } - static constexpr char __digits[513] = - "000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f" - "202122232425262728292a2b2c2d2e2f303132333435363738393a3b3c3d3e3f" - "404142434445464748494a4b4c4d4e4f505152535455565758595a5b5c5d5e5f" - "606162636465666768696a6b6c6d6e6f707172737475767778797a7b7c7d7e7f" - "808182838485868788898a8b8c8d8e8f909192939495969798999a9b9c9d9e9f" - "a0a1a2a3a4a5a6a7a8a9aaabacadaeafb0b1b2b3b4b5b6b7b8b9babbbcbdbebf" - "c0c1c2c3c4c5c6c7c8c9cacbcccdcecfd0d1d2d3d4d5d6d7d8d9dadbdcdddedf" - "e0e1e2e3e4e5e6e7e8e9eaebecedeeeff0f1f2f3f4f5f6f7f8f9fafbfcfdfeff"; + static constexpr char __digits[] = { + '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', + 'a', 'b', 'c', 'd', 'e', 'f' + }; unsigned __pos = __len - 1; while (__val >= 0x100) { - auto const __num = (__val % 0x100) * 2; - __val /= 0x100; - __first[__pos] = __digits[__num + 1]; + auto __num = __val & 0xF; + __val >>= 4; + __first[__pos] = __digits[__num]; + __num = __val & 0xF; + __val >>= 4; __first[__pos - 1] = __digits[__num]; __pos -= 2; } if (__val >= 0x10) { - auto const __num = __val * 2; - __first[__pos] = __digits[__num + 1]; - __first[__pos - 1] = __digits[__num]; + const auto __num = __val & 0xF; + __val >>= 4; + __first[1] = __digits[__num]; + __first[0] = __digits[__val]; } else - __first[__pos] = "0123456789abcdef"[__val]; + __first[0] = __digits[__val]; __res.ptr = __first + __len; __res.ec = {}; return __res; @@ -263,28 +265,26 @@ namespace __detail return __res; } - static constexpr char __digits[129] = - "00010203040506071011121314151617" - "20212223242526273031323334353637" - "40414243444546475051525354555657" - "60616263646566677071727374757677"; unsigned __pos = __len - 1; while (__val >= 0100) { - auto const __num = (__val % 0100) * 2; - __val /= 0100; - __first[__pos] = __digits[__num + 1]; - __first[__pos - 1] = __digits[__num]; + auto __num = __val & 7; + __val >>= 3; + __first[__pos] = '0' + __num; + __num = __val & 7; + __val >>= 3; + __first[__pos - 1] = '0' + __num; __pos -= 2; } if (__val >= 010) { - auto const __num = __val * 2; - __first[__pos] = __digits[__num + 1]; - __first[__pos - 1] = __digits[__num]; + auto const __num = __val & 7; + __val >>= 3; + __first[1] = '0' + __num; + __first[0] = '0' + __val; } else - __first[__pos] = '0' + __val; + __first[0] = '0' + __val; __res.ptr = __first + __len; __res.ec = {}; return __res; @@ -315,7 +315,10 @@ namespace __detail __first[__pos--] = '0' + (__val & 1); __val >>= 1; } - *__first = '0' + (__val & 1); + // First digit is always '1' because __to_chars_len_2 skips + // leading zero bits and std::to_chars handles zero values + // directly. + __first[0] = '1'; __res.ptr = __first + __len; __res.ec = {}; -- 2.30.2