[PATCH] Optimize to_chars

Antony Polukhin antoshkka@gmail.com
Fri Aug 30 14:27:00 GMT 2019


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 https://godbolt.org/z/RTui7m ). 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 ( https://godbolt.org/z/Peq_PG)
.

In sum: this significantly reduces the size of a binary (for about
4KBs only for base-8 conversion https://godbolt.org/z/WPKijS ), deals
with latency (CPU cache misses) without changing the iterations count
and without adding costly instructions into the loops.

Changelog:
    * 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.
    * include/std/charconv (__detail::__to_chars): Remove
    zero termination from array of digits.
    * include/std/charconv (__detail::__to_chars_2): Leading digit
    is always '1'.

-- 
Best regards,
Antony Polukhin
-------------- next part --------------
diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index eaa6f74..35706d0 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,17 @@
+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.
+	* include/std/charconv (__detail::__to_chars): Remove
+	zero termination from array of digits.
+	* include/std/charconv (__detail::__to_chars_2): Leading digit
+	is always '1'.
+
 2019-08-29  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR libstdc++/91067
diff --git a/libstdc++-v3/include/std/charconv b/libstdc++-v3/include/std/charconv
index 53aa63e..4e94c39 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 = {};


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