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  <antoshkka@gmail.com>

	* 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

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index 3bc86cb190c39f270384039b241c3e134d396427..65ef1c0bc1ba0fee0abc96822060d220d3215e73 100644 (file)
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,15 @@
+2019-08-30  Antony Polukhin  <antoshkka@gmail.com>
+
+       * 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  <jwakely@redhat.com>
 
        * 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 53aa63ea27792a1914ab89644c160bf79eca2288..4e94c39656de27073b0b0b2d88d90c3b243bf6f4 100644 (file)
--- 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 = {};