[libc++] Improve std::to_chars for base != 10.

This improves the speed of `to_chars` for bases 2, 8, and 16. These bases are common and used in `<format>`. This change uses a lookup table, like done in base 10 and causes an increase in code size. The change has a small overhead for the other bases. ``` Benchmark Time CPU Time Old Time New CPU Old CPU New ------------------------------------------------------------------------------------------------------------------ BM_to_chars_good/2 -0.9476 -0.9476 252 13 252 13 BM_to_chars_good/3 +0.0018 +0.0018 145 145 145 145 BM_to_chars_good/4 +0.0108 +0.0108 104 105 104 105 BM_to_chars_good/5 +0.0159 +0.0160 89 91 89 91 BM_to_chars_good/6 +0.0162 +0.0162 80 81 80 81 BM_to_chars_good/7 +0.0117 +0.0117 72 73 72 73 BM_to_chars_good/8 -0.8643 -0.8643 64 9 64 9 BM_to_chars_good/9 +0.0095 +0.0095 60 60 60 60 BM_to_chars_good/10 +0.0540 +0.0540 6 6 6 6 BM_to_chars_good/11 +0.0299 +0.0299 55 57 55 57 BM_to_chars_good/12 +0.0060 +0.0060 48 49 49 49 BM_to_chars_good/13 +0.0102 +0.0102 48 48 48 48 BM_to_chars_good/14 +0.0184 +0.0185 47 48 47 48 BM_to_chars_good/15 +0.0269 +0.0269 44 45 44 45 BM_to_chars_good/16 -0.8207 -0.8207 37 7 37 7 BM_to_chars_good/17 +0.0241 +0.0241 37 38 37 38 BM_to_chars_good/18 +0.0221 +0.0221 37 38 37 38 BM_to_chars_good/19 +0.0222 +0.0223 37 38 37 38 BM_to_chars_good/20 +0.0317 +0.0317 38 39 38 39 BM_to_chars_good/21 +0.0342 +0.0341 38 39 38 39 BM_to_chars_good/22 +0.0336 +0.0336 36 38 36 38 BM_to_chars_good/23 +0.0222 +0.0222 34 35 34 35 BM_to_chars_good/24 +0.0185 +0.0185 31 32 31 32 BM_to_chars_good/25 +0.0157 +0.0157 32 32 32 32 BM_to_chars_good/26 +0.0181 +0.0181 32 32 32 32 BM_to_chars_good/27 +0.0153 +0.0153 32 32 32 32 BM_to_chars_good/28 +0.0179 +0.0179 32 32 32 32 BM_to_chars_good/29 +0.0189 +0.0189 32 33 32 33 BM_to_chars_good/30 +0.0212 +0.0212 32 33 32 33 BM_to_chars_good/31 +0.0221 +0.0221 32 33 32 33 BM_to_chars_good/32 +0.0292 +0.0292 32 33 32 33 BM_to_chars_good/33 +0.0319 +0.0319 32 33 32 33 BM_to_chars_good/34 +0.0411 +0.0410 33 34 33 34 BM_to_chars_good/35 +0.0515 +0.0515 33 34 33 34 BM_to_chars_good/36 +0.0502 +0.0502 32 34 32 34 BM_to_chars_bad/2 -0.8752 -0.8752 40 5 40 5 BM_to_chars_bad/3 +0.1952 +0.1952 21 26 21 26 BM_to_chars_bad/4 +0.3626 +0.3626 16 22 16 22 BM_to_chars_bad/5 +0.2267 +0.2268 17 21 17 21 BM_to_chars_bad/6 +0.3560 +0.3559 14 19 14 19 BM_to_chars_bad/7 +0.4599 +0.4600 12 18 12 18 BM_to_chars_bad/8 -0.5074 -0.5074 11 5 11 5 BM_to_chars_bad/9 +0.4814 +0.4814 10 15 10 15 BM_to_chars_bad/10 +0.7761 +0.7761 2 4 2 4 BM_to_chars_bad/11 +0.3948 +0.3948 12 16 12 16 BM_to_chars_bad/12 +0.3203 +0.3203 10 13 10 13 BM_to_chars_bad/13 +0.3067 +0.3067 11 14 11 14 BM_to_chars_bad/14 +0.2235 +0.2235 12 14 12 14 BM_to_chars_bad/15 +0.2675 +0.2675 11 14 11 14 BM_to_chars_bad/16 -0.1801 -0.1801 7 5 7 5 BM_to_chars_bad/17 +0.5651 +0.5651 7 11 7 11 BM_to_chars_bad/18 +0.5407 +0.5406 7 11 7 11 BM_to_chars_bad/19 +0.5593 +0.5593 8 12 8 12 BM_to_chars_bad/20 +0.5823 +0.5823 8 13 8 13 BM_to_chars_bad/21 +0.6032 +0.6032 9 15 9 15 BM_to_chars_bad/22 +0.6407 +0.6408 9 14 9 14 BM_to_chars_bad/23 +0.6292 +0.6292 7 12 7 12 BM_to_chars_bad/24 +0.5784 +0.5784 6 10 6 10 BM_to_chars_bad/25 +0.5784 +0.5784 6 10 6 10 BM_to_chars_bad/26 +0.5713 +0.5713 7 10 7 10 BM_to_chars_bad/27 +0.5969 +0.5969 7 11 7 11 BM_to_chars_bad/28 +0.6131 +0.6131 7 11 7 11 BM_to_chars_bad/29 +0.6937 +0.6937 7 11 7 11 BM_to_chars_bad/30 +0.7655 +0.7656 7 12 7 12 BM_to_chars_bad/31 +0.8939 +0.8939 6 12 6 12 BM_to_chars_bad/32 +1.0157 +1.0157 6 13 6 13 BM_to_chars_bad/33 +1.0279 +1.0279 7 14 7 14 BM_to_chars_bad/34 +1.0388 +1.0388 7 14 7 14 BM_to_chars_bad/35 +1.0990 +1.0990 7 15 7 15 BM_to_chars_bad/36 +1.1503 +1.1503 7 15 7 15 ``` Reviewed By: ldionne, #libc Differential Revision: https://reviews.llvm.org/D97705
llvm · May 14, 2022 · 6c11aeb · 6c11aeb
1 parent 92eea11
commit 6c11aeb
Showing 1 changed file with 201 additions and 1 deletion.
diff --git a/libcxx/include/charconv b/libcxx/include/charconv
@@ -319,6 +319,197 @@ __to_chars_integral(char* __first, char* __last, _Tp __value, int __base,
     return __to_chars_integral(__first, __last, __x, __base, false_type());
 }
 
+namespace __itoa {
+
+static constexpr char __base_2_lut[64] = {
+  '0', '0', '0', '0',
+  '0', '0', '0', '1',
+  '0', '0', '1', '0',
+  '0', '0', '1', '1',
+  '0', '1', '0', '0',
+  '0', '1', '0', '1',
+  '0', '1', '1', '0',
+  '0', '1', '1', '1',
+  '1', '0', '0', '0',
+  '1', '0', '0', '1',
+  '1', '0', '1', '0',
+  '1', '0', '1', '1',
+  '1', '1', '0', '0',
+  '1', '1', '0', '1',
+  '1', '1', '1', '0',
+  '1', '1', '1', '1'
+};
+
+static constexpr char __base_8_lut[128] = {
+  '0', '0', '0', '1', '0', '2', '0', '3', '0', '4', '0', '5', '0', '6', '0', '7',
+  '1', '0', '1', '1', '1', '2', '1', '3', '1', '4', '1', '5', '1', '6', '1', '7',
+  '2', '0', '2', '1', '2', '2', '2', '3', '2', '4', '2', '5', '2', '6', '2', '7',
+  '3', '0', '3', '1', '3', '2', '3', '3', '3', '4', '3', '5', '3', '6', '3', '7',
+  '4', '0', '4', '1', '4', '2', '4', '3', '4', '4', '4', '5', '4', '6', '4', '7',
+  '5', '0', '5', '1', '5', '2', '5', '3', '5', '4', '5', '5', '5', '6', '5', '7',
+  '6', '0', '6', '1', '6', '2', '6', '3', '6', '4', '6', '5', '6', '6', '6', '7',
+  '7', '0', '7', '1', '7', '2', '7', '3', '7', '4', '7', '5', '7', '6', '7', '7'
+};
+
+static constexpr char __base_16_lut[512] = {
+  '0', '0', '0', '1', '0', '2', '0', '3', '0', '4', '0', '5', '0', '6', '0', '7', '0', '8', '0', '9', '0', 'a', '0', 'b', '0', 'c', '0', 'd', '0', 'e', '0', 'f',
+  '1', '0', '1', '1', '1', '2', '1', '3', '1', '4', '1', '5', '1', '6', '1', '7', '1', '8', '1', '9', '1', 'a', '1', 'b', '1', 'c', '1', 'd', '1', 'e', '1', 'f',
+  '2', '0', '2', '1', '2', '2', '2', '3', '2', '4', '2', '5', '2', '6', '2', '7', '2', '8', '2', '9', '2', 'a', '2', 'b', '2', 'c', '2', 'd', '2', 'e', '2', 'f',
+  '3', '0', '3', '1', '3', '2', '3', '3', '3', '4', '3', '5', '3', '6', '3', '7', '3', '8', '3', '9', '3', 'a', '3', 'b', '3', 'c', '3', 'd', '3', 'e', '3', 'f',
+  '4', '0', '4', '1', '4', '2', '4', '3', '4', '4', '4', '5', '4', '6', '4', '7', '4', '8', '4', '9', '4', 'a', '4', 'b', '4', 'c', '4', 'd', '4', 'e', '4', 'f',
+  '5', '0', '5', '1', '5', '2', '5', '3', '5', '4', '5', '5', '5', '6', '5', '7', '5', '8', '5', '9', '5', 'a', '5', 'b', '5', 'c', '5', 'd', '5', 'e', '5', 'f',
+  '6', '0', '6', '1', '6', '2', '6', '3', '6', '4', '6', '5', '6', '6', '6', '7', '6', '8', '6', '9', '6', 'a', '6', 'b', '6', 'c', '6', 'd', '6', 'e', '6', 'f',
+  '7', '0', '7', '1', '7', '2', '7', '3', '7', '4', '7', '5', '7', '6', '7', '7', '7', '8', '7', '9', '7', 'a', '7', 'b', '7', 'c', '7', 'd', '7', 'e', '7', 'f',
+  '8', '0', '8', '1', '8', '2', '8', '3', '8', '4', '8', '5', '8', '6', '8', '7', '8', '8', '8', '9', '8', 'a', '8', 'b', '8', 'c', '8', 'd', '8', 'e', '8', 'f',
+  '9', '0', '9', '1', '9', '2', '9', '3', '9', '4', '9', '5', '9', '6', '9', '7', '9', '8', '9', '9', '9', 'a', '9', 'b', '9', 'c', '9', 'd', '9', 'e', '9', 'f',
+  'a', '0', 'a', '1', 'a', '2', 'a', '3', 'a', '4', 'a', '5', 'a', '6', 'a', '7', 'a', '8', 'a', '9', 'a', 'a', 'a', 'b', 'a', 'c', 'a', 'd', 'a', 'e', 'a', 'f',
+  'b', '0', 'b', '1', 'b', '2', 'b', '3', 'b', '4', 'b', '5', 'b', '6', 'b', '7', 'b', '8', 'b', '9', 'b', 'a', 'b', 'b', 'b', 'c', 'b', 'd', 'b', 'e', 'b', 'f',
+  'c', '0', 'c', '1', 'c', '2', 'c', '3', 'c', '4', 'c', '5', 'c', '6', 'c', '7', 'c', '8', 'c', '9', 'c', 'a', 'c', 'b', 'c', 'c', 'c', 'd', 'c', 'e', 'c', 'f',
+  'd', '0', 'd', '1', 'd', '2', 'd', '3', 'd', '4', 'd', '5', 'd', '6', 'd', '7', 'd', '8', 'd', '9', 'd', 'a', 'd', 'b', 'd', 'c', 'd', 'd', 'd', 'e', 'd', 'f',
+  'e', '0', 'e', '1', 'e', '2', 'e', '3', 'e', '4', 'e', '5', 'e', '6', 'e', '7', 'e', '8', 'e', '9', 'e', 'a', 'e', 'b', 'e', 'c', 'e', 'd', 'e', 'e', 'e', 'f',
+  'f', '0', 'f', '1', 'f', '2', 'f', '3', 'f', '4', 'f', '5', 'f', '6', 'f', '7', 'f', '8', 'f', '9', 'f', 'a', 'f', 'b', 'f', 'c', 'f', 'd', 'f', 'e', 'f', 'f'
+};
+
+template <unsigned _Base>
+struct _LIBCPP_HIDDEN __integral;
+
+template <>
+struct _LIBCPP_HIDDEN __integral<2> {
+  template <typename _Tp>
+  _LIBCPP_HIDE_FROM_ABI static constexpr int __width(_Tp __value) noexcept {
+    // If value == 0 still need one digit. If the value != this has no
+    // effect since the code scans for the most significant bit set. (Note
+    // that __libcpp_clz doesn't work for 0.)
+    return numeric_limits<_Tp>::digits - std::__libcpp_clz(__value | 1);
+  }
+
+  template <typename _Tp>
+  _LIBCPP_HIDE_FROM_ABI static to_chars_result __to_chars(char* __first, char* __last, _Tp __value) {
+    ptrdiff_t __cap = __last - __first;
+    int __n = __width(__value);
+    if (__n > __cap)
+      return {__last, errc::value_too_large};
+
+    __last = __first + __n;
+    char* __p = __last;
+    const unsigned __divisor = 16;
+    while (__value > __divisor) {
+      unsigned __c = __value % __divisor;
+      __value /= __divisor;
+      __p -= 4;
+      std::memcpy(__p, &__base_2_lut[4 * __c], 4);
+    }
+    do {
+      unsigned __c = __value % 2;
+      __value /= 2;
+      *--__p = "01"[__c];
+    } while (__value != 0);
+    return {__last, errc(0)};
+  }
+};
+
+template <>
+struct _LIBCPP_HIDDEN __integral<8> {
+  template <typename _Tp>
+  _LIBCPP_HIDE_FROM_ABI static constexpr int __width(_Tp __value) noexcept {
+    // If value == 0 still need one digit. If the value != this has no
+    // effect since the code scans for the most significat bit set. (Note
+    // that __libcpp_clz doesn't work for 0.)
+    return ((numeric_limits<_Tp>::digits - std::__libcpp_clz(__value | 1)) + 2) / 3;
+  }
+
+  template <typename _Tp>
+  _LIBCPP_HIDE_FROM_ABI static to_chars_result __to_chars(char* __first, char* __last, _Tp __value) {
+    ptrdiff_t __cap = __last - __first;
+    int __n = __width(__value);
+    if (__n > __cap)
+      return {__last, errc::value_too_large};
+
+    __last = __first + __n;
+    char* __p = __last;
+    unsigned __divisor = 64;
+    while (__value > __divisor) {
+      unsigned __c = __value % __divisor;
+      __value /= __divisor;
+      __p -= 2;
+      std::memcpy(__p, &__base_8_lut[2 * __c], 2);
+    }
+    do {
+      unsigned __c = __value % 8;
+      __value /= 8;
+      *--__p = "01234567"[__c];
+    } while (__value != 0);
+    return {__last, errc(0)};
+  }
+
+};
+
+template <>
+struct _LIBCPP_HIDDEN __integral<16> {
+  template <typename _Tp>
+  _LIBCPP_HIDE_FROM_ABI static constexpr int __width(_Tp __value) noexcept {
+    // If value == 0 still need one digit. If the value != this has no
+    // effect since the code scans for the most significat bit set. (Note
+    // that __libcpp_clz doesn't work for 0.)
+    return (numeric_limits<_Tp>::digits - std::__libcpp_clz(__value | 1) + 3) / 4;
+  }
+
+  template <typename _Tp>
+  _LIBCPP_HIDE_FROM_ABI static to_chars_result __to_chars(char* __first, char* __last, _Tp __value) {
+    ptrdiff_t __cap = __last - __first;
+    int __n = __width(__value);
+    if (__n > __cap)
+      return {__last, errc::value_too_large};
+
+    __last = __first + __n;
+    char* __p = __last;
+    unsigned __divisor = 256;
+    while (__value > __divisor) {
+      unsigned __c = __value % __divisor;
+      __value /= __divisor;
+      __p -= 2;
+      std::memcpy(__p, &__base_16_lut[2 * __c], 2);
+    }
+    if (__first != __last)
+      do {
+        unsigned __c = __value % 16;
+        __value /= 16;
+        *--__p = "0123456789abcdef"[__c];
+      } while (__value != 0);
+    return {__last, errc(0)};
+  }
+};
+
+} // namespace __itoa
+
+template <unsigned _Base, typename _Tp,
+          typename enable_if<(sizeof(_Tp) >= sizeof(unsigned)), int>::type = 0>
+_LIBCPP_HIDE_FROM_ABI int
+__to_chars_integral_width(_Tp __value) {
+  return __itoa::__integral<_Base>::__width(__value);
+}
+
+template <unsigned _Base, typename _Tp,
+          typename enable_if<(sizeof(_Tp) < sizeof(unsigned)), int>::type = 0>
+_LIBCPP_HIDE_FROM_ABI int
+__to_chars_integral_width(_Tp __value) {
+  return std::__to_chars_integral_width<_Base>(static_cast<unsigned>(__value));
+}
+
+template <unsigned _Base, typename _Tp,
+          typename enable_if<(sizeof(_Tp) >= sizeof(unsigned)), int>::type = 0>
+_LIBCPP_HIDE_FROM_ABI to_chars_result
+__to_chars_integral(char* __first, char* __last, _Tp __value) {
+  return __itoa::__integral<_Base>::__to_chars(__first, __last, __value);
+}
+
+template <unsigned _Base, typename _Tp,
+          typename enable_if<(sizeof(_Tp) < sizeof(unsigned)), int>::type = 0>
+_LIBCPP_HIDE_FROM_ABI to_chars_result
+__to_chars_integral(char* __first, char* __last, _Tp __value) {
+  return std::__to_chars_integral<_Base>(__first, __last, static_cast<unsigned>(__value));
+}
+
 template <typename _Tp>
 _LIBCPP_AVAILABILITY_TO_CHARS _LIBCPP_HIDE_FROM_ABI int
 __to_chars_integral_width(_Tp __value, unsigned __base) {
@@ -352,9 +543,18 @@ inline _LIBCPP_HIDE_FROM_ABI to_chars_result
 __to_chars_integral(char* __first, char* __last, _Tp __value, int __base,
                     false_type)
 {
-  if (__base == 10)
+  if (__base == 10) [[likely]]
     return __to_chars_itoa(__first, __last, __value, false_type());
 
+  switch (__base) {
+  case 2:
+    return __to_chars_integral<2>(__first, __last, __value);
+  case 8:
+    return __to_chars_integral<8>(__first, __last, __value);
+  case 16:
+    return __to_chars_integral<16>(__first, __last, __value);
+  }
+
   ptrdiff_t __cap = __last - __first;
   int __n = __to_chars_integral_width(__value, __base);
   if (__n > __cap)