Revert integer-to-string conversion optimizations pending more thorough analysis

PiperOrigin-RevId: 619261152
Change-Id: Id3409b326c52ace0fda42537e0b91dbb2d6a2287

6 files changed, 196 insertions, 755 deletions

diff --git a/absl/strings/numbers.cc b/absl/strings/numbers.cc
index 882c3a8b..b57d9e82 100644
--- a/absl/strings/numbers.cc
+++ b/absl/strings/numbers.cc
@@ -20,9 +20,7 @@
 #include <algorithm>
 #include <cassert>
 #include <cfloat>  // for DBL_DIG and FLT_DIG
-#include <climits>
 #include <cmath>   // for HUGE_VAL
-#include <cstddef>
 #include <cstdint>
 #include <cstdio>
 #include <cstdlib>
@@ -30,7 +28,6 @@
 #include <iterator>
 #include <limits>
 #include <system_error>  // NOLINT(build/c++11)
-#include <type_traits>
 #include <utility>
 
 #include "absl/base/attributes.h"
@@ -159,71 +156,28 @@ constexpr uint32_t kTwoZeroBytes = 0x0101 * '0';
 constexpr uint64_t kFourZeroBytes = 0x01010101 * '0';
 constexpr uint64_t kEightZeroBytes = 0x0101010101010101ull * '0';
 
-template <typename T>
-constexpr T Pow(T base, uint32_t n) {
-  // Exponentiation by squaring
-  return static_cast<T>((n > 1 ? Pow(base * base, n >> 1) : static_cast<T>(1)) *
-                        ((n & 1) ? base : static_cast<T>(1)));
-}
-
-// Given n, calculates C where the following holds for all 0 <= x < Pow(100, n):
-// x / Pow(10, n) == x * C / Pow(2, n * 10)
-// In other words, it allows us to divide by a power of 10 via a single
-// multiplication and bit shifts, assuming the input will be smaller than the
-// square of that power of 10.
-template <typename T>
-constexpr T ComputePowerOf100DivisionCoefficient(uint32_t n) {
-  if (n > 4) {
-    // This doesn't work for large powers of 100, due to overflow
-    abort();
-  }
-  T denom = 16 - 1;
-  T num = (denom + 1) - 10;
-  T gcd = 3;  // Greatest common divisor of numerator and denominator
-  denom = Pow(denom / gcd, n);
-  num = Pow(num / gcd, 9 * n);
-  T quotient = num / denom;
-  if (num % denom >= denom / 2) {
-    // Round up, since the remainder is more than half the denominator
-    ++quotient;
-  }
-  return quotient;
-}
-
-// * kDivisionBy10Mul / kDivisionBy10Div is a division by 10 for values from 0
-// to 99. It's also a division of a structure [k takes 2 bytes][m takes 2
-// bytes], then * kDivisionBy10Mul / kDivisionBy10Div will be [k / 10][m / 10].
-// It allows parallel division.
-constexpr uint64_t kDivisionBy10Mul =
-    ComputePowerOf100DivisionCoefficient<uint64_t>(1);
-static_assert(kDivisionBy10Mul == 103,
-              "division coefficient for 10 is incorrect");
+// * 103 / 1024 is a division by 10 for values from 0 to 99. It's also a
+// division of a structure [k takes 2 bytes][m takes 2 bytes], then * 103 / 1024
+// will be [k / 10][m / 10]. It allows parallel division.
+constexpr uint64_t kDivisionBy10Mul = 103u;
 constexpr uint64_t kDivisionBy10Div = 1 << 10;
 
-// * kDivisionBy100Mul / kDivisionBy100Div is a division by 100 for values from
-// 0 to 9999.
-constexpr uint64_t kDivisionBy100Mul =
-    ComputePowerOf100DivisionCoefficient<uint64_t>(2);
-static_assert(kDivisionBy100Mul == 10486,
-              "division coefficient for 100 is incorrect");
+// * 10486 / 1048576 is a division by 100 for values from 0 to 9999.
+constexpr uint64_t kDivisionBy100Mul = 10486u;
 constexpr uint64_t kDivisionBy100Div = 1 << 20;
 
-static_assert(ComputePowerOf100DivisionCoefficient<uint64_t>(3) == 1073742,
-              "division coefficient for 1000 is incorrect");
-
-// Same as `PrepareEightDigits`, but produces 2 digits for integers < 100.
-inline uint32_t PrepareTwoDigitsImpl(uint32_t i, bool reversed) {
-  assert(i < 100);
-  uint32_t div10 = (i * kDivisionBy10Mul) / kDivisionBy10Div;
-  uint32_t mod10 = i - 10u * div10;
-  return (div10 << (reversed ? 8 : 0)) + (mod10 << (reversed ? 0 : 8));
-}
-inline uint32_t PrepareTwoDigits(uint32_t i) {
-  return PrepareTwoDigitsImpl(i, false);
+// Encode functions write the ASCII output of input `n` to `out_str`.
+inline char* EncodeHundred(uint32_t n, absl::Nonnull<char*> out_str) {
+  int num_digits = static_cast<int>(n - 10) >> 8;
+  uint32_t div10 = (n * kDivisionBy10Mul) / kDivisionBy10Div;
+  uint32_t mod10 = n - 10u * div10;
+  uint32_t base = kTwoZeroBytes + div10 + (mod10 << 8);
+  base >>= num_digits & 8;
+  little_endian::Store16(out_str, static_cast<uint16_t>(base));
+  return out_str + 2 + num_digits;
 }
 
-// Same as `PrepareEightDigits`, but produces 4 digits for integers < 10000.
-inline uint32_t PrepareFourDigitsImpl(uint32_t n, bool reversed) {
+inline char* EncodeTenThousand(uint32_t n, absl::Nonnull<char*> out_str) {
   // We split lower 2 digits and upper 2 digits of n into 2 byte consecutive
   // blocks. 123 ->  [\0\1][\0\23]. We divide by 10 both blocks
   // (it's 1 division + zeroing upper bits), and compute modulo 10 as well "in
@@ -231,19 +185,22 @@ inline uint32_t PrepareFourDigitsImpl(uint32_t n, bool reversed) {
   // strip trailing zeros, add ASCII '0000' and return.
   uint32_t div100 = (n * kDivisionBy100Mul) / kDivisionBy100Div;
   uint32_t mod100 = n - 100ull * div100;
-  uint32_t hundreds =
-      (mod100 << (reversed ? 0 : 16)) + (div100 << (reversed ? 16 : 0));
+  uint32_t hundreds = (mod100 << 16) + div100;
   uint32_t tens = (hundreds * kDivisionBy10Mul) / kDivisionBy10Div;
   tens &= (0xFull << 16) | 0xFull;
-  tens = (tens << (reversed ? 8 : 0)) +
-         static_cast<uint32_t>((hundreds - 10ull * tens) << (reversed ? 0 : 8));
-  return tens;
-}
-inline uint32_t PrepareFourDigits(uint32_t n) {
-  return PrepareFourDigitsImpl(n, false);
-}
-inline uint32_t PrepareFourDigitsReversed(uint32_t n) {
-  return PrepareFourDigitsImpl(n, true);
+  tens += (hundreds - 10ull * tens) << 8;
+  ABSL_ASSUME(tens != 0);
+  // The result can contain trailing zero bits, we need to strip them to a first
+  // significant byte in a final representation. For example, for n = 123, we
+  // have tens to have representation \0\1\2\3. We do `& -8` to round
+  // to a multiple to 8 to strip zero bytes, not all zero bits.
+  // countr_zero to help.
+  // 0 minus 8 to make MSVC happy.
+  uint32_t zeroes = static_cast<uint32_t>(absl::countr_zero(tens)) & (0 - 8u);
+  tens += kFourZeroBytes;
+  tens >>= zeroes;
+  little_endian::Store32(out_str, tens);
+  return out_str + sizeof(tens) - zeroes / 8;
 }
 
 // Helper function to produce an ASCII representation of `i`.
@@ -259,309 +216,126 @@ inline uint32_t PrepareFourDigitsReversed(uint32_t n) {
 //  // Note two leading zeros:
 //  EXPECT_EQ(absl::string_view(ascii, 8), "00102030");
 //
-// If `Reversed` is set to true, the result becomes reversed to "03020100".
-//
 // Pre-condition: `i` must be less than 100000000.
-inline uint64_t PrepareEightDigitsImpl(uint32_t i, bool reversed) {
+inline uint64_t PrepareEightDigits(uint32_t i) {
   ABSL_ASSUME(i < 10000'0000);
   // Prepare 2 blocks of 4 digits "in parallel".
   uint32_t hi = i / 10000;
   uint32_t lo = i % 10000;
-  uint64_t merged = (uint64_t{hi} << (reversed ? 32 : 0)) |
-                    (uint64_t{lo} << (reversed ? 0 : 32));
+  uint64_t merged = hi | (uint64_t{lo} << 32);
   uint64_t div100 = ((merged * kDivisionBy100Mul) / kDivisionBy100Div) &
                     ((0x7Full << 32) | 0x7Full);
   uint64_t mod100 = merged - 100ull * div100;
-  uint64_t hundreds =
-      (mod100 << (reversed ? 0 : 16)) + (div100 << (reversed ? 16 : 0));
+  uint64_t hundreds = (mod100 << 16) + div100;
   uint64_t tens = (hundreds * kDivisionBy10Mul) / kDivisionBy10Div;
   tens &= (0xFull << 48) | (0xFull << 32) | (0xFull << 16) | 0xFull;
-  tens = (tens << (reversed ? 8 : 0)) +
-         ((hundreds - 10ull * tens) << (reversed ? 0 : 8));
+  tens += (hundreds - 10ull * tens) << 8;
   return tens;
 }
-inline uint64_t PrepareEightDigits(uint32_t i) {
-  return PrepareEightDigitsImpl(i, false);
-}
-inline uint64_t PrepareEightDigitsReversed(uint32_t i) {
-  return PrepareEightDigitsImpl(i, true);
-}
 
-template <typename T, typename BackwardIt>
-class FastUIntToStringConverter {
-  static_assert(
-      std::is_same<T, decltype(+std::declval<T>())>::value,
-      "to avoid code bloat, only instantiate this for int and larger types");
-  static_assert(std::is_unsigned<T>::value,
-                "this class is only for unsigned types");
-
- public:
-  // Outputs the given number backward (like with std::copy_backward),
-  // starting from the end of the string.
-  // The number of digits in the number must have been already measured and
-  // passed *exactly*, otherwise the behavior is undefined.
-  // (This is an optimization, as calculating the number of digits again would
-  // slow down the hot path.)
-  // Returns an iterator to the start of the suffix that was appended.
-  static BackwardIt FastIntToBufferBackward(T v, BackwardIt end) {
-    // THIS IS A HOT FUNCTION with a very deliberate structure to exploit branch
-    // prediction and shorten the critical path for smaller numbers.
-    // Do not move around the if/else blocks or attempt to simplify it
-    // without benchmarking any changes.
-
-    if (v < 10) {
-      goto AT_LEAST_1 /* NOTE: mandatory for the 0 case */;
-    }
-    if (v < 1000) {
-      goto AT_LEAST_10;
-    }
-    if (v < 10000000) {
-      goto AT_LEAST_1000;
-    }
-
-    if (v >= 100000000 / 10) {
-      if (v >= 10000000000000000 / 10) {
-        DoFastIntToBufferBackward<8>(v, end);
-      }
-      DoFastIntToBufferBackward<8>(v, end);
-    }
-
-    if (v >= 10000 / 10) {
-    AT_LEAST_1000:
-      DoFastIntToBufferBackward<4>(v, end);
-    }
-
-    if (v >= 100 / 10) {
-    AT_LEAST_10:
-      DoFastIntToBufferBackward<2>(v, end);
-    }
-
-    if (v >= 10 / 10) {
-    AT_LEAST_1:
-      end = DoFastIntToBufferBackward(v, end, std::integral_constant<int, 1>());
-    }
-    return end;
+inline ABSL_ATTRIBUTE_ALWAYS_INLINE absl::Nonnull<char*> EncodeFullU32(
+    uint32_t n, absl::Nonnull<char*> out_str) {
+  if (n < 10) {
+    *out_str = static_cast<char>('0' + n);
+    return out_str + 1;
   }
-
- private:
-  // Only assume pointers are contiguous for now. String and vector iterators
-  // could be special-cased as well, but there's no need for them here.
-  // With C++20 we can probably switch to std::contiguous_iterator_tag.
-  static constexpr bool kIsContiguousIterator =
-      std::is_pointer<BackwardIt>::value;
-
-  template <int Exponent>
-  static void DoFastIntToBufferBackward(T& v, BackwardIt& end) {
-    constexpr T kModulus = Pow<T>(10, Exponent);
-    T remainder = static_cast<T>(v % kModulus);
-    v = static_cast<T>(v / kModulus);
-    end = DoFastIntToBufferBackward(remainder, end,
-                                    std::integral_constant<int, Exponent>());
-  }
-
-  static BackwardIt DoFastIntToBufferBackward(const T&, BackwardIt end,
-                                              std::integral_constant<int, 0>) {
-    return end;
-  }
-
-  static BackwardIt DoFastIntToBufferBackward(T v, BackwardIt end,
-                                              std::integral_constant<int, 1>) {
-    *--end = static_cast<char>('0' + v);
-    return DoFastIntToBufferBackward(v, end, std::integral_constant<int, 0>());
-  }
-
-  static BackwardIt DoFastIntToBufferBackward(T v, BackwardIt end,
-                                              std::integral_constant<int, 4>) {
-    if (kIsContiguousIterator) {
-      const uint32_t digits =
-          PrepareFourDigits(static_cast<uint32_t>(v)) + kFourZeroBytes;
-      end -= sizeof(digits);
-      little_endian::Store32(&*end, digits);
-    } else {
-      uint32_t digits =
-          PrepareFourDigitsReversed(static_cast<uint32_t>(v)) + kFourZeroBytes;
-      for (size_t i = 0; i < sizeof(digits); ++i) {
-        *--end = static_cast<char>(digits);
-        digits >>= CHAR_BIT;
-      }
-    }
-    return end;
+  if (n < 100'000'000) {
+    uint64_t bottom = PrepareEightDigits(n);
+    ABSL_ASSUME(bottom != 0);
+    // 0 minus 8 to make MSVC happy.
+    uint32_t zeroes =
+        static_cast<uint32_t>(absl::countr_zero(bottom)) & (0 - 8u);
+    little_endian::Store64(out_str, (bottom + kEightZeroBytes) >> zeroes);
+    return out_str + sizeof(bottom) - zeroes / 8;
   }
-
-  static BackwardIt DoFastIntToBufferBackward(T v, BackwardIt end,
-                                              std::integral_constant<int, 8>) {
-    if (kIsContiguousIterator) {
-      const uint64_t digits =
-          PrepareEightDigits(static_cast<uint32_t>(v)) + kEightZeroBytes;
-      end -= sizeof(digits);
-      little_endian::Store64(&*end, digits);
-    } else {
-      uint64_t digits = PrepareEightDigitsReversed(static_cast<uint32_t>(v)) +
-                        kEightZeroBytes;
-      for (size_t i = 0; i < sizeof(digits); ++i) {
-        *--end = static_cast<char>(digits);
-        digits >>= CHAR_BIT;
-      }
-    }
-    return end;
-  }
-
-  template <int Digits>
-  static BackwardIt DoFastIntToBufferBackward(
-      T v, BackwardIt end, std::integral_constant<int, Digits>) {
-    constexpr int kLogModulus = Digits - Digits / 2;
-    constexpr T kModulus = Pow(static_cast<T>(10), kLogModulus);
-    bool is_safe_to_use_division_trick = Digits <= 8;
-    T quotient, remainder;
-    if (is_safe_to_use_division_trick) {
-      constexpr uint64_t kCoefficient =
-          ComputePowerOf100DivisionCoefficient<uint64_t>(kLogModulus);
-      quotient = (v * kCoefficient) >> (10 * kLogModulus);
-      remainder = v - quotient * kModulus;
-    } else {
-      quotient = v / kModulus;
-      remainder = v % kModulus;
-    }
-    end = DoFastIntToBufferBackward(remainder, end,
-                                    std::integral_constant<int, kLogModulus>());
-    return DoFastIntToBufferBackward(
-        quotient, end, std::integral_constant<int, Digits - kLogModulus>());
-  }
-};
-
-// Returns an iterator to the start of the suffix that was appended
-template <typename T, typename BackwardIt>
-std::enable_if_t<std::is_unsigned<T>::value, BackwardIt>
-DoFastIntToBufferBackward(T v, BackwardIt end, uint32_t digits) {
-  using PromotedT = std::decay_t<decltype(+v)>;
-  using Converter = FastUIntToStringConverter<PromotedT, BackwardIt>;
-  (void)digits;
-  return Converter().FastIntToBufferBackward(v, end);
+  uint32_t div08 = n / 100'000'000;
+  uint32_t mod08 = n % 100'000'000;
+  uint64_t bottom = PrepareEightDigits(mod08) + kEightZeroBytes;
+  out_str = EncodeHundred(div08, out_str);
+  little_endian::Store64(out_str, bottom);
+  return out_str + sizeof(bottom);
 }
 
-template <typename T, typename BackwardIt>
-std::enable_if_t<std::is_signed<T>::value, BackwardIt>
-DoFastIntToBufferBackward(T v, BackwardIt end, uint32_t digits) {
-  if (absl::numbers_internal::IsNegative(v)) {
-    // Store the minus sign *before* we produce the number itself, not after.
-    // This gets us a tail call.
-    end[-static_cast<ptrdiff_t>(digits) - 1] = '-';
+inline ABSL_ATTRIBUTE_ALWAYS_INLINE char* EncodeFullU64(uint64_t i,
+                                                        char* buffer) {
+  if (i <= std::numeric_limits<uint32_t>::max()) {
+    return EncodeFullU32(static_cast<uint32_t>(i), buffer);
   }
-  return DoFastIntToBufferBackward(
-      absl::numbers_internal::UnsignedAbsoluteValue(v), end, digits);
-}
-
-template <class T>
-std::enable_if_t<std::is_integral<T>::value, int>
-GetNumDigitsOrNegativeIfNegativeImpl(T v) {
-  const auto /* either bool or std::false_type */ is_negative =
-      absl::numbers_internal::IsNegative(v);
-  const int digits = static_cast<int>(absl::numbers_internal::Base10Digits(
-      absl::numbers_internal::UnsignedAbsoluteValue(v)));
-  return is_negative ? ~digits : digits;
+  uint32_t mod08;
+  if (i < 1'0000'0000'0000'0000ull) {
+    uint32_t div08 = static_cast<uint32_t>(i / 100'000'000ull);
+    mod08 =  static_cast<uint32_t>(i % 100'000'000ull);
+    buffer = EncodeFullU32(div08, buffer);
+  } else {
+    uint64_t div08 = i / 100'000'000ull;
+    mod08 =  static_cast<uint32_t>(i % 100'000'000ull);
+    uint32_t div016 = static_cast<uint32_t>(div08 / 100'000'000ull);
+    uint32_t div08mod08 = static_cast<uint32_t>(div08 % 100'000'000ull);
+    uint64_t mid_result = PrepareEightDigits(div08mod08) + kEightZeroBytes;
+    buffer = EncodeTenThousand(div016, buffer);
+    little_endian::Store64(buffer, mid_result);
+    buffer += sizeof(mid_result);
+  }
+  uint64_t mod_result = PrepareEightDigits(mod08) + kEightZeroBytes;
+  little_endian::Store64(buffer, mod_result);
+  return buffer + sizeof(mod_result);
 }
 
 }  // namespace
 
 void numbers_internal::PutTwoDigits(uint32_t i, absl::Nonnull<char*> buf) {
-  little_endian::Store16(
-      buf, static_cast<uint16_t>(PrepareTwoDigits(i) + kTwoZeroBytes));
+  assert(i < 100);
+  uint32_t base = kTwoZeroBytes;
+  uint32_t div10 = (i * kDivisionBy10Mul) / kDivisionBy10Div;
+  uint32_t mod10 = i - 10u * div10;
+  base += div10 + (mod10 << 8);
+  little_endian::Store16(buf, static_cast<uint16_t>(base));
 }
 
 absl::Nonnull<char*> numbers_internal::FastIntToBuffer(
-    uint32_t i, absl::Nonnull<char*> buffer) {
-  const uint32_t digits = absl::numbers_internal::Base10Digits(i);
-  buffer += digits;
-  *buffer = '\0';  // We're going backward, so store this first
-  FastIntToBufferBackward(i, buffer, digits);
-  return buffer;
+    uint32_t n, absl::Nonnull<char*> out_str) {
+  out_str = EncodeFullU32(n, out_str);
+  *out_str = '\0';
+  return out_str;
 }
 
 absl::Nonnull<char*> numbers_internal::FastIntToBuffer(
     int32_t i, absl::Nonnull<char*> buffer) {
-  buffer += static_cast<int>(i < 0);
-  uint32_t digits = absl::numbers_internal::Base10Digits(
-      absl::numbers_internal::UnsignedAbsoluteValue(i));
-  buffer += digits;
-  *buffer = '\0';  // We're going backward, so store this first
-  FastIntToBufferBackward(i, buffer, digits);
+  uint32_t u = static_cast<uint32_t>(i);
+  if (i < 0) {
+    *buffer++ = '-';
+    // We need to do the negation in modular (i.e., "unsigned")
+    // arithmetic; MSVC++ apparently warns for plain "-u", so
+    // we write the equivalent expression "0 - u" instead.
+    u = 0 - u;
+  }
+  buffer = EncodeFullU32(u, buffer);
+  *buffer = '\0';
   return buffer;
 }
 
 absl::Nonnull<char*> numbers_internal::FastIntToBuffer(
     uint64_t i, absl::Nonnull<char*> buffer) {
-  uint32_t digits = absl::numbers_internal::Base10Digits(i);
-  buffer += digits;
-  *buffer = '\0';  // We're going backward, so store this first
-  FastIntToBufferBackward(i, buffer, digits);
+  buffer = EncodeFullU64(i, buffer);
+  *buffer = '\0';
   return buffer;
 }
 
 absl::Nonnull<char*> numbers_internal::FastIntToBuffer(
     int64_t i, absl::Nonnull<char*> buffer) {
-  buffer += static_cast<int>(i < 0);
-  uint32_t digits = absl::numbers_internal::Base10Digits(
-      absl::numbers_internal::UnsignedAbsoluteValue(i));
-  buffer += digits;
-  *buffer = '\0';  // We're going backward, so store this first
-  FastIntToBufferBackward(i, buffer, digits);
+  uint64_t u = static_cast<uint64_t>(i);
+  if (i < 0) {
+    *buffer++ = '-';
+    // We need to do the negation in modular (i.e., "unsigned")
+    // arithmetic; MSVC++ apparently warns for plain "-u", so
+    // we write the equivalent expression "0 - u" instead.
+    u = 0 - u;
+  }
+  buffer = EncodeFullU64(u, buffer);
+  *buffer = '\0';
   return buffer;
 }
 
-absl::Nonnull<char*> numbers_internal::FastIntToBufferBackward(
-    uint32_t i, absl::Nonnull<char*> buffer_end, uint32_t exact_digit_count) {
-  return DoFastIntToBufferBackward(i, buffer_end, exact_digit_count);
-}
-
-absl::Nonnull<char*> numbers_internal::FastIntToBufferBackward(
-    int32_t i, absl::Nonnull<char*> buffer_end, uint32_t exact_digit_count) {
-  return DoFastIntToBufferBackward(i, buffer_end, exact_digit_count);
-}
-
-absl::Nonnull<char*> numbers_internal::FastIntToBufferBackward(
-    uint64_t i, absl::Nonnull<char*> buffer_end, uint32_t exact_digit_count) {
-  return DoFastIntToBufferBackward(i, buffer_end, exact_digit_count);
-}
-
-absl::Nonnull<char*> numbers_internal::FastIntToBufferBackward(
-    int64_t i, absl::Nonnull<char*> buffer_end, uint32_t exact_digit_count) {
-  return DoFastIntToBufferBackward(i, buffer_end, exact_digit_count);
-}
-
-int numbers_internal::GetNumDigitsOrNegativeIfNegative(signed char v) {
-  return GetNumDigitsOrNegativeIfNegativeImpl(v);
-}
-int numbers_internal::GetNumDigitsOrNegativeIfNegative(unsigned char v) {
-  return GetNumDigitsOrNegativeIfNegativeImpl(v);
-}
-int numbers_internal::GetNumDigitsOrNegativeIfNegative(short v) {  // NOLINT
-  return GetNumDigitsOrNegativeIfNegativeImpl(v);
-}
-int numbers_internal::GetNumDigitsOrNegativeIfNegative(
-    unsigned short v) {  // NOLINT
-  return GetNumDigitsOrNegativeIfNegativeImpl(v);
-}
-int numbers_internal::GetNumDigitsOrNegativeIfNegative(int v) {
-  return GetNumDigitsOrNegativeIfNegativeImpl(v);
-}
-int numbers_internal::GetNumDigitsOrNegativeIfNegative(unsigned int v) {
-  return GetNumDigitsOrNegativeIfNegativeImpl(v);
-}
-int numbers_internal::GetNumDigitsOrNegativeIfNegative(long v) {  // NOLINT
-  return GetNumDigitsOrNegativeIfNegativeImpl(v);
-}
-int numbers_internal::GetNumDigitsOrNegativeIfNegative(
-    unsigned long v) {  // NOLINT
-  return GetNumDigitsOrNegativeIfNegativeImpl(v);
-}
-int numbers_internal::GetNumDigitsOrNegativeIfNegative(long long v) {  // NOLINT
-  return GetNumDigitsOrNegativeIfNegativeImpl(v);
-}
-int numbers_internal::GetNumDigitsOrNegativeIfNegative(
-    unsigned long long v) {  // NOLINT
-  return GetNumDigitsOrNegativeIfNegativeImpl(v);
-}
-
 // Given a 128-bit number expressed as a pair of uint64_t, high half first,
 // return that number multiplied by the given 32-bit value.  If the result is
 // too large to fit in a 128-bit number, divide it by 2 until it fits.
diff --git a/absl/strings/numbers.h b/absl/strings/numbers.h
index ad4e66b6..739dbb28 100644
--- a/absl/strings/numbers.h
+++ b/absl/strings/numbers.h
@@ -32,7 +32,6 @@
 #endif
 
 #include <cstddef>
-#include <cstdint>
 #include <cstdlib>
 #include <cstring>
 #include <ctime>
@@ -40,12 +39,10 @@
 #include <string>
 #include <type_traits>
 
-#include "absl/base/attributes.h"
 #include "absl/base/config.h"
 #include "absl/base/internal/endian.h"
 #include "absl/base/macros.h"
 #include "absl/base/nullability.h"
-#include "absl/base/optimization.h"
 #include "absl/base/port.h"
 #include "absl/numeric/bits.h"
 #include "absl/numeric/int128.h"
@@ -161,96 +158,6 @@ bool safe_strtou128_base(absl::string_view text,
 static const int kFastToBufferSize = 32;
 static const int kSixDigitsToBufferSize = 16;
 
-template <class T>
-std::enable_if_t<!std::is_unsigned<T>::value, bool> IsNegative(const T& v) {
-  return v < T();
-}
-
-template <class T>
-std::enable_if_t<std::is_unsigned<T>::value, std::false_type> IsNegative(
-    const T&) {
-  // The integer is unsigned, so return a compile-time constant.
-  // This can help the optimizer avoid having to prove bool to be false later.
-  return std::false_type();
-}
-
-template <class T>
-std::enable_if_t<std::is_unsigned<std::decay_t<T>>::value, T&&>
-UnsignedAbsoluteValue(T&& v ABSL_ATTRIBUTE_LIFETIME_BOUND) {
-  // The value is unsigned; just return the original.
-  return std::forward<T>(v);
-}
-
-template <class T>
-ABSL_ATTRIBUTE_CONST_FUNCTION
-    std::enable_if_t<!std::is_unsigned<T>::value, std::make_unsigned_t<T>>
-    UnsignedAbsoluteValue(T v) {
-  using U = std::make_unsigned_t<T>;
-  return IsNegative(v) ? U() - static_cast<U>(v) : static_cast<U>(v);
-}
-
-// Returns the number of base-10 digits in the given number.
-// Note that this strictly counts digits. It does not count the sign.
-// The `initial_digits` parameter is the starting point, which is normally equal
-// to 1 because the number of digits in 0 is 1 (a special case).
-// However, callers may e.g. wish to change it to 2 to account for the sign.
-template <typename T>
-std::enable_if_t<std::is_unsigned<T>::value, uint32_t> Base10Digits(
-    T v, const uint32_t initial_digits = 1) {
-  uint32_t r = initial_digits;
-  // If code size becomes an issue, the 'if' stage can be removed for a minor
-  // performance loss.
-  for (;;) {
-    if (ABSL_PREDICT_TRUE(v < 10 * 10)) {
-      r += (v >= 10);
-      break;
-    }
-    if (ABSL_PREDICT_TRUE(v < 1000 * 10)) {
-      r += (v >= 1000) + 2;
-      break;
-    }
-    if (ABSL_PREDICT_TRUE(v < 100000 * 10)) {
-      r += (v >= 100000) + 4;
-      break;
-    }
-    r += 6;
-    v = static_cast<T>(v / 1000000);
-  }
-  return r;
-}
-
-template <typename T>
-std::enable_if_t<std::is_signed<T>::value, uint32_t> Base10Digits(
-    T v, uint32_t r = 1) {
-  // Branchlessly add 1 to account for a minus sign.
-  r += static_cast<uint32_t>(IsNegative(v));
-  return Base10Digits(UnsignedAbsoluteValue(v), r);
-}
-
-// These functions return the number of base-10 digits, but multiplied by -1 if
-// the input itself is negative. This is handy and efficient for later usage,
-// since the bitwise complement of the result becomes equal to the number of
-// characters required.
-ABSL_ATTRIBUTE_CONST_FUNCTION int GetNumDigitsOrNegativeIfNegative(
-    signed char v);
-ABSL_ATTRIBUTE_CONST_FUNCTION int GetNumDigitsOrNegativeIfNegative(
-    unsigned char v);
-ABSL_ATTRIBUTE_CONST_FUNCTION int GetNumDigitsOrNegativeIfNegative(
-    short v);  // NOLINT
-ABSL_ATTRIBUTE_CONST_FUNCTION int GetNumDigitsOrNegativeIfNegative(
-    unsigned short v);  // NOLINT
-ABSL_ATTRIBUTE_CONST_FUNCTION int GetNumDigitsOrNegativeIfNegative(int v);
-ABSL_ATTRIBUTE_CONST_FUNCTION int GetNumDigitsOrNegativeIfNegative(
-    unsigned int v);
-ABSL_ATTRIBUTE_CONST_FUNCTION int GetNumDigitsOrNegativeIfNegative(
-    long v);  // NOLINT
-ABSL_ATTRIBUTE_CONST_FUNCTION int GetNumDigitsOrNegativeIfNegative(
-    unsigned long v);  // NOLINT
-ABSL_ATTRIBUTE_CONST_FUNCTION int GetNumDigitsOrNegativeIfNegative(
-    long long v);  // NOLINT
-ABSL_ATTRIBUTE_CONST_FUNCTION int GetNumDigitsOrNegativeIfNegative(
-    unsigned long long v);  // NOLINT
-
 // Helper function for fast formatting of floating-point values.
 // The result is the same as printf's "%g", a.k.a. "%.6g"; that is, six
 // significant digits are returned, trailing zeros are removed, and numbers
@@ -259,18 +166,24 @@ ABSL_ATTRIBUTE_CONST_FUNCTION int GetNumDigitsOrNegativeIfNegative(
 // Required buffer size is `kSixDigitsToBufferSize`.
 size_t SixDigitsToBuffer(double d, absl::Nonnull<char*> buffer);
 
-// All of these functions take an output buffer
+// WARNING: These functions may write more characters than necessary, because
+// they are intended for speed. All functions take an output buffer
 // as an argument and return a pointer to the last byte they wrote, which is the
 // terminating '\0'. At most `kFastToBufferSize` bytes are written.
-absl::Nonnull<char*> FastIntToBuffer(int32_t i, absl::Nonnull<char*> buffer);
-absl::Nonnull<char*> FastIntToBuffer(uint32_t i, absl::Nonnull<char*> buffer);
-absl::Nonnull<char*> FastIntToBuffer(int64_t i, absl::Nonnull<char*> buffer);
-absl::Nonnull<char*> FastIntToBuffer(uint64_t i, absl::Nonnull<char*> buffer);
+absl::Nonnull<char*> FastIntToBuffer(int32_t i, absl::Nonnull<char*> buffer)
+    ABSL_INTERNAL_NEED_MIN_SIZE(buffer, kFastToBufferSize);
+absl::Nonnull<char*> FastIntToBuffer(uint32_t n, absl::Nonnull<char*> out_str)
+    ABSL_INTERNAL_NEED_MIN_SIZE(out_str, kFastToBufferSize);
+absl::Nonnull<char*> FastIntToBuffer(int64_t i, absl::Nonnull<char*> buffer)
+    ABSL_INTERNAL_NEED_MIN_SIZE(buffer, kFastToBufferSize);
+absl::Nonnull<char*> FastIntToBuffer(uint64_t i, absl::Nonnull<char*> buffer)
+    ABSL_INTERNAL_NEED_MIN_SIZE(buffer, kFastToBufferSize);
 
 // For enums and integer types that are not an exact match for the types above,
 // use templates to call the appropriate one of the four overloads above.
 template <typename int_type>
-absl::Nonnull<char*> FastIntToBuffer(int_type i, absl::Nonnull<char*> buffer) {
+absl::Nonnull<char*> FastIntToBuffer(int_type i, absl::Nonnull<char*> buffer)
+    ABSL_INTERNAL_NEED_MIN_SIZE(buffer, kFastToBufferSize) {
   static_assert(sizeof(i) <= 64 / 8,
                 "FastIntToBuffer works only with 64-bit-or-less integers.");
   // TODO(jorg): This signed-ness check is used because it works correctly
@@ -294,58 +207,6 @@ absl::Nonnull<char*> FastIntToBuffer(int_type i, absl::Nonnull<char*> buffer) {
   }
 }
 
-// These functions do NOT add any null-terminator.
-// They return a pointer to the beginning of the written string.
-// The digit counts provided must *exactly* match the number of base-10 digits
-// in the number, or the behavior is undefined.
-// (i.e. do NOT count the minus sign, or over- or under-count the digits.)
-absl::Nonnull<char*> FastIntToBufferBackward(int32_t i,
-                                             absl::Nonnull<char*> buffer_end,
-                                             uint32_t exact_digit_count);
-absl::Nonnull<char*> FastIntToBufferBackward(uint32_t i,
-                                             absl::Nonnull<char*> buffer_end,
-                                             uint32_t exact_digit_count);
-absl::Nonnull<char*> FastIntToBufferBackward(int64_t i,
-                                             absl::Nonnull<char*> buffer_end,
-                                             uint32_t exact_digit_count);
-absl::Nonnull<char*> FastIntToBufferBackward(uint64_t i,
-                                             absl::Nonnull<char*> buffer_end,
-                                             uint32_t exact_digit_count);
-
-// For enums and integer types that are not an exact match for the types above,
-// use templates to call the appropriate one of the four overloads above.
-template <typename int_type>
-absl::Nonnull<char*> FastIntToBufferBackward(int_type i,
-                                             absl::Nonnull<char*> buffer_end,
-                                             uint32_t exact_digit_count) {
-  static_assert(
-      sizeof(i) <= 64 / 8,
-      "FastIntToBufferBackward works only with 64-bit-or-less integers.");
-  // This signed-ness check is used because it works correctly
-  // with enums, and it also serves to check that int_type is not a pointer.
-  // If one day something like std::is_signed<enum E> works, switch to it.
-  // These conditions are constexpr bools to suppress MSVC warning C4127.
-  constexpr bool kIsSigned = static_cast<int_type>(1) - 2 < 0;
-  constexpr bool kUse64Bit = sizeof(i) > 32 / 8;
-  if (kIsSigned) {
-    if (kUse64Bit) {
-      return FastIntToBufferBackward(static_cast<int64_t>(i), buffer_end,
-                                     exact_digit_count);
-    } else {
-      return FastIntToBufferBackward(static_cast<int32_t>(i), buffer_end,
-                                     exact_digit_count);
-    }
-  } else {
-    if (kUse64Bit) {
-      return FastIntToBufferBackward(static_cast<uint64_t>(i), buffer_end,
-                                     exact_digit_count);
-    } else {
-      return FastIntToBufferBackward(static_cast<uint32_t>(i), buffer_end,
-                                     exact_digit_count);
-    }
-  }
-}
-
 // Implementation of SimpleAtoi, generalized to support arbitrary base (used
 // with base different from 10 elsewhere in Abseil implementation).
 template <typename int_type>
diff --git a/absl/strings/numbers_test.cc b/absl/strings/numbers_test.cc
index 1ceff70f..75c2dcf2 100644
--- a/absl/strings/numbers_test.cc
+++ b/absl/strings/numbers_test.cc
@@ -231,15 +231,10 @@ TEST(Numbers, TestFastPrints) {
   CheckInt32(INT_MIN);
   CheckInt32(INT_MAX);
   CheckInt64(LONG_MIN);
-  CheckInt64(uint64_t{10000000});
-  CheckInt64(uint64_t{100000000});
   CheckInt64(uint64_t{1000000000});
   CheckInt64(uint64_t{9999999999});
   CheckInt64(uint64_t{100000000000000});
   CheckInt64(uint64_t{999999999999999});
-  CheckInt64(uint64_t{1000000000000000});
-  CheckInt64(uint64_t{10000000000000000});
-  CheckInt64(uint64_t{100000000000000000});
   CheckInt64(uint64_t{1000000000000000000});
   CheckInt64(uint64_t{1199999999999999999});
   CheckInt64(int64_t{-700000000000000000});
@@ -251,8 +246,6 @@ TEST(Numbers, TestFastPrints) {
   CheckUInt64(uint64_t{999999999999999});
   CheckUInt64(uint64_t{1000000000000000000});
   CheckUInt64(uint64_t{1199999999999999999});
-  CheckUInt64(uint64_t{10000000000000000000u});
-  CheckUInt64(uint64_t{10200300040000500006u});
   CheckUInt64(std::numeric_limits<uint64_t>::max());
 
   for (int i = 0; i < 10000; i++) {
diff --git a/absl/strings/str_cat.cc b/absl/strings/str_cat.cc
index b637b551..c51c1373 100644
--- a/absl/strings/str_cat.cc
+++ b/absl/strings/str_cat.cc
@@ -22,13 +22,11 @@
 #include <initializer_list>
 #include <limits>
 #include <string>
-#include <type_traits>
 
 #include "absl/base/config.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/base/nullability.h"
 #include "absl/strings/internal/resize_uninitialized.h"
-#include "absl/strings/numbers.h"
 #include "absl/strings/string_view.h"
 
 namespace absl {
@@ -44,7 +42,8 @@ ABSL_NAMESPACE_BEGIN
 namespace {
 // Append is merely a version of memcpy that returns the address of the byte
 // after the area just overwritten.
-absl::Nonnull<char*> Append(absl::Nonnull<char*> out, const AlphaNum& x) {
+inline absl::Nonnull<char*> Append(absl::Nonnull<char*> out,
+                                   const AlphaNum& x) {
   // memcpy is allowed to overwrite arbitrary memory, so doing this after the
   // call would force an extra fetch of x.size().
   char* after = out + x.size();
@@ -54,6 +53,11 @@ absl::Nonnull<char*> Append(absl::Nonnull<char*> out, const AlphaNum& x) {
   return after;
 }
 
+inline void STLStringAppendUninitializedAmortized(std::string* dest,
+                                                  size_t to_append) {
+  strings_internal::AppendUninitializedTraits<std::string>::Append(dest,
+                                                                   to_append);
+}
 }  // namespace
 
 std::string StrCat(const AlphaNum& a, const AlphaNum& b) {
@@ -120,130 +124,6 @@ std::string StrCat(const AlphaNum& a, const AlphaNum& b, const AlphaNum& c,
 namespace strings_internal {
 
 // Do not call directly - these are not part of the public API.
-void STLStringAppendUninitializedAmortized(std::string* dest,
-                                           size_t to_append) {
-  strings_internal::AppendUninitializedTraits<std::string>::Append(dest,
-                                                                   to_append);
-}
-
-template <typename Integer>
-std::enable_if_t<std::is_integral<Integer>::value, std::string> IntegerToString(
-    Integer i) {
-  std::string str;
-  const auto /* either bool or std::false_type */ is_negative =
-      absl::numbers_internal::IsNegative(i);
-  const uint32_t digits = absl::numbers_internal::Base10Digits(
-      absl::numbers_internal::UnsignedAbsoluteValue(i));
-  absl::strings_internal::STLStringResizeUninitialized(
-      &str, digits + static_cast<uint32_t>(is_negative));
-  absl::numbers_internal::FastIntToBufferBackward(i, &str[str.size()], digits);
-  return str;
-}
-
-template <>
-std::string IntegerToString(long i) {  // NOLINT
-  if (sizeof(i) <= sizeof(int)) {
-    return IntegerToString(static_cast<int>(i));
-  } else {
-    return IntegerToString(static_cast<long long>(i));  // NOLINT
-  }
-}
-
-template <>
-std::string IntegerToString(unsigned long i) {  // NOLINT
-  if (sizeof(i) <= sizeof(unsigned int)) {
-    return IntegerToString(static_cast<unsigned int>(i));
-  } else {
-    return IntegerToString(static_cast<unsigned long long>(i));  // NOLINT
-  }
-}
-
-template <typename Float>
-std::enable_if_t<std::is_floating_point<Float>::value, std::string>
-FloatToString(Float f) {
-  std::string result;
-  strings_internal::STLStringResizeUninitialized(
-      &result, numbers_internal::kSixDigitsToBufferSize);
-  char* start = &result[0];
-  result.erase(numbers_internal::SixDigitsToBuffer(f, start));
-  return result;
-}
-
-std::string SingleArgStrCat(int x) { return IntegerToString(x); }
-std::string SingleArgStrCat(unsigned int x) { return IntegerToString(x); }
-// NOLINTNEXTLINE
-std::string SingleArgStrCat(long x) { return IntegerToString(x); }
-// NOLINTNEXTLINE
-std::string SingleArgStrCat(unsigned long x) { return IntegerToString(x); }
-// NOLINTNEXTLINE
-std::string SingleArgStrCat(long long x) { return IntegerToString(x); }
-// NOLINTNEXTLINE
-std::string SingleArgStrCat(unsigned long long x) { return IntegerToString(x); }
-std::string SingleArgStrCat(float x) { return FloatToString(x); }
-std::string SingleArgStrCat(double x) { return FloatToString(x); }
-
-template <class Integer>
-std::enable_if_t<std::is_integral<Integer>::value, void> AppendIntegerToString(
-    std::string& str, Integer i) {
-  const auto /* either bool or std::false_type */ is_negative =
-      absl::numbers_internal::IsNegative(i);
-  const uint32_t digits = absl::numbers_internal::Base10Digits(
-      absl::numbers_internal::UnsignedAbsoluteValue(i));
-  absl::strings_internal::STLStringAppendUninitializedAmortized(
-      &str, digits + static_cast<uint32_t>(is_negative));
-  absl::numbers_internal::FastIntToBufferBackward(i, &str[str.size()], digits);
-}
-
-template <>
-void AppendIntegerToString(std::string& str, long i) {  // NOLINT
-  if (sizeof(i) <= sizeof(int)) {
-    return AppendIntegerToString(str, static_cast<int>(i));
-  } else {
-    return AppendIntegerToString(str, static_cast<long long>(i));  // NOLINT
-  }
-}
-
-template <>
-void AppendIntegerToString(std::string& str,
-                           unsigned long i) {  // NOLINT
-  if (sizeof(i) <= sizeof(unsigned int)) {
-    return AppendIntegerToString(str, static_cast<unsigned int>(i));
-  } else {
-    return AppendIntegerToString(str,
-                                 static_cast<unsigned long long>(i));  // NOLINT
-  }
-}
-
-// `SingleArgStrAppend` overloads are defined here for the same reasons as with
-// `SingleArgStrCat` above.
-void SingleArgStrAppend(std::string& str, int x) {
-  return AppendIntegerToString(str, x);
-}
-
-void SingleArgStrAppend(std::string& str, unsigned int x) {
-  return AppendIntegerToString(str, x);
-}
-
-// NOLINTNEXTLINE
-void SingleArgStrAppend(std::string& str, long x) {
-  return AppendIntegerToString(str, x);
-}
-
-// NOLINTNEXTLINE
-void SingleArgStrAppend(std::string& str, unsigned long x) {
-  return AppendIntegerToString(str, x);
-}
-
-// NOLINTNEXTLINE
-void SingleArgStrAppend(std::string& str, long long x) {
-  return AppendIntegerToString(str, x);
-}
-
-// NOLINTNEXTLINE
-void SingleArgStrAppend(std::string& str, unsigned long long x) {
-  return AppendIntegerToString(str, x);
-}
-
 std::string CatPieces(std::initializer_list<absl::string_view> pieces) {
   std::string result;
   // Use uint64_t to prevent size_t overflow. We assume it is not possible for
@@ -287,7 +167,7 @@ void AppendPieces(absl::Nonnull<std::string*> dest,
     ASSERT_NO_OVERLAP(*dest, piece);
     to_append += piece.size();
   }
-  strings_internal::STLStringAppendUninitializedAmortized(dest, to_append);
+  STLStringAppendUninitializedAmortized(dest, to_append);
 
   char* const begin = &(*dest)[0];
   char* out = begin + old_size;
@@ -306,7 +186,7 @@ void AppendPieces(absl::Nonnull<std::string*> dest,
 void StrAppend(absl::Nonnull<std::string*> dest, const AlphaNum& a) {
   ASSERT_NO_OVERLAP(*dest, a);
   std::string::size_type old_size = dest->size();
-  strings_internal::STLStringAppendUninitializedAmortized(dest, a.size());
+  STLStringAppendUninitializedAmortized(dest, a.size());
   char* const begin = &(*dest)[0];
   char* out = begin + old_size;
   out = Append(out, a);
@@ -318,8 +198,7 @@ void StrAppend(absl::Nonnull<std::string*> dest, const AlphaNum& a,
   ASSERT_NO_OVERLAP(*dest, a);
   ASSERT_NO_OVERLAP(*dest, b);
   std::string::size_type old_size = dest->size();
-  strings_internal::STLStringAppendUninitializedAmortized(dest,
-                                                          a.size() + b.size());
+  STLStringAppendUninitializedAmortized(dest, a.size() + b.size());
   char* const begin = &(*dest)[0];
   char* out = begin + old_size;
   out = Append(out, a);
@@ -333,8 +212,7 @@ void StrAppend(absl::Nonnull<std::string*> dest, const AlphaNum& a,
   ASSERT_NO_OVERLAP(*dest, b);
   ASSERT_NO_OVERLAP(*dest, c);
   std::string::size_type old_size = dest->size();
-  strings_internal::STLStringAppendUninitializedAmortized(
-      dest, a.size() + b.size() + c.size());
+  STLStringAppendUninitializedAmortized(dest, a.size() + b.size() + c.size());
   char* const begin = &(*dest)[0];
   char* out = begin + old_size;
   out = Append(out, a);
@@ -350,7 +228,7 @@ void StrAppend(absl::Nonnull<std::string*> dest, const AlphaNum& a,
   ASSERT_NO_OVERLAP(*dest, c);
   ASSERT_NO_OVERLAP(*dest, d);
   std::string::size_type old_size = dest->size();
-  strings_internal::STLStringAppendUninitializedAmortized(
+  STLStringAppendUninitializedAmortized(
       dest, a.size() + b.size() + c.size() + d.size());
   char* const begin = &(*dest)[0];
   char* out = begin + old_size;
diff --git a/absl/strings/str_cat.h b/absl/strings/str_cat.h
index 33355bfd..b98adc02 100644
--- a/absl/strings/str_cat.h
+++ b/absl/strings/str_cat.h
@@ -94,6 +94,7 @@
 #include <cstdint>
 #include <cstring>
 #include <initializer_list>
+#include <limits>
 #include <string>
 #include <type_traits>
 #include <utility>
@@ -453,36 +454,77 @@ std::string CatPieces(std::initializer_list<absl::string_view> pieces);
 void AppendPieces(absl::Nonnull<std::string*> dest,
                   std::initializer_list<absl::string_view> pieces);
 
-void STLStringAppendUninitializedAmortized(std::string* dest, size_t to_append);
+template <typename Integer>
+std::string IntegerToString(Integer i) {
+  // Any integer (signed/unsigned) up to 64 bits can be formatted into a buffer
+  // with 22 bytes (including NULL at the end).
+  constexpr size_t kMaxDigits10 = 22;
+  std::string result;
+  strings_internal::STLStringResizeUninitialized(&result, kMaxDigits10);
+  char* start = &result[0];
+  // note: this can be optimized to not write last zero.
+  char* end = numbers_internal::FastIntToBuffer(i, start);
+  auto size = static_cast<size_t>(end - start);
+  assert((size < result.size()) &&
+         "StrCat(Integer) does not fit into kMaxDigits10");
+  result.erase(size);
+  return result;
+}
+template <typename Float>
+std::string FloatToString(Float f) {
+  std::string result;
+  strings_internal::STLStringResizeUninitialized(
+      &result, numbers_internal::kSixDigitsToBufferSize);
+  char* start = &result[0];
+  result.erase(numbers_internal::SixDigitsToBuffer(f, start));
+  return result;
+}
 
 // `SingleArgStrCat` overloads take built-in `int`, `long` and `long long` types
 // (signed / unsigned) to avoid ambiguity on the call side. If we used int32_t
 // and int64_t, then at least one of the three (`int` / `long` / `long long`)
 // would have been ambiguous when passed to `SingleArgStrCat`.
-std::string SingleArgStrCat(int x);
-std::string SingleArgStrCat(unsigned int x);
-std::string SingleArgStrCat(long x);                // NOLINT
-std::string SingleArgStrCat(unsigned long x);       // NOLINT
-std::string SingleArgStrCat(long long x);           // NOLINT
-std::string SingleArgStrCat(unsigned long long x);  // NOLINT
-std::string SingleArgStrCat(float x);
-std::string SingleArgStrCat(double x);
-
-// `SingleArgStrAppend` overloads are defined here for the same reasons as with
-// `SingleArgStrCat` above.
-void SingleArgStrAppend(std::string& str, int x);
-void SingleArgStrAppend(std::string& str, unsigned int x);
-void SingleArgStrAppend(std::string& str, long x);                // NOLINT
-void SingleArgStrAppend(std::string& str, unsigned long x);       // NOLINT
-void SingleArgStrAppend(std::string& str, long long x);           // NOLINT
-void SingleArgStrAppend(std::string& str, unsigned long long x);  // NOLINT
-
-template <typename T,
-          typename = std::enable_if_t<std::is_arithmetic<T>::value &&
-                                      !std::is_same<T, char>::value &&
-                                      !std::is_same<T, bool>::value>>
+inline std::string SingleArgStrCat(int x) { return IntegerToString(x); }
+inline std::string SingleArgStrCat(unsigned int x) {
+  return IntegerToString(x);
+}
+// NOLINTNEXTLINE
+inline std::string SingleArgStrCat(long x) { return IntegerToString(x); }
+// NOLINTNEXTLINE
+inline std::string SingleArgStrCat(unsigned long x) {
+  return IntegerToString(x);
+}
+// NOLINTNEXTLINE
+inline std::string SingleArgStrCat(long long x) { return IntegerToString(x); }
+// NOLINTNEXTLINE
+inline std::string SingleArgStrCat(unsigned long long x) {
+  return IntegerToString(x);
+}
+inline std::string SingleArgStrCat(float x) { return FloatToString(x); }
+inline std::string SingleArgStrCat(double x) { return FloatToString(x); }
+
+// As of September 2023, the SingleArgStrCat() optimization is only enabled for
+// libc++. The reasons for this are:
+// 1) The SSO size for libc++ is 23, while libstdc++ and MSSTL have an SSO size
+// of 15. Since IntegerToString unconditionally resizes the string to 22 bytes,
+// this causes both libstdc++ and MSSTL to allocate.
+// 2) strings_internal::STLStringResizeUninitialized() only has an
+// implementation that avoids initialization when using libc++. This isn't as
+// relevant as (1), and the cost should be benchmarked if (1) ever changes on
+// libstc++ or MSSTL.
+#ifdef _LIBCPP_VERSION
+#define ABSL_INTERNAL_STRCAT_ENABLE_FAST_CASE true
+#else
+#define ABSL_INTERNAL_STRCAT_ENABLE_FAST_CASE false
+#endif
+
+template <typename T, typename = std::enable_if_t<
+                          ABSL_INTERNAL_STRCAT_ENABLE_FAST_CASE &&
+                          std::is_arithmetic<T>{} && !std::is_same<T, char>{}>>
 using EnableIfFastCase = T;
 
+#undef ABSL_INTERNAL_STRCAT_ENABLE_FAST_CASE
+
 }  // namespace strings_internal
 
 ABSL_MUST_USE_RESULT inline std::string StrCat() { return std::string(); }
@@ -558,67 +600,6 @@ inline void StrAppend(absl::Nonnull<std::string*> dest, const AlphaNum& a,
              static_cast<const AlphaNum&>(args).Piece()...});
 }
 
-template <class String, class T>
-std::enable_if_t<
-    std::is_integral<absl::strings_internal::EnableIfFastCase<T>>::value, void>
-StrAppend(absl::Nonnull<String*> result, T i) {
-  return absl::strings_internal::SingleArgStrAppend(*result, i);
-}
-
-// This overload is only selected if all the parameters are numbers that can be
-// handled quickly.
-// Later we can look into how we can extend this to more general argument
-// mixtures without bloating codegen too much, or copying unnecessarily.
-template <typename String, typename... T>
-std::enable_if_t<
-    (sizeof...(T) > 1),
-    std::common_type_t<std::conditional_t<
-        true, void, absl::strings_internal::EnableIfFastCase<T>>...>>
-StrAppend(absl::Nonnull<String*> str, T... args) {
-  // Do not add unnecessary variables, logic, or even "free" lambdas here.
-  // They can add overhead for the compiler and/or at run time.
-  // Furthermore, assume this function will be inlined.
-  // This function is carefully tailored to be able to be largely optimized away
-  // so that it becomes near-equivalent to the caller handling each argument
-  // individually while minimizing register pressure, so that the compiler
-  // can inline it with minimal overhead.
-
-  // First, calculate the total length, so we can perform just a single resize.
-  // Save all the lengths for later.
-  size_t total_length = 0;
-  const ptrdiff_t lengths[] = {
-      absl::numbers_internal::GetNumDigitsOrNegativeIfNegative(args)...};
-  for (const ptrdiff_t possibly_negative_length : lengths) {
-    // Lengths are negative for negative numbers. Keep them for later use, but
-    // take their absolute values for calculating total lengths;
-    total_length += possibly_negative_length < 0
-                        ? static_cast<size_t>(-possibly_negative_length)
-                        : static_cast<size_t>(possibly_negative_length);
-  }
-
-  // Now reserve space for all the arguments.
-  const size_t old_size = str->size();
-  absl::strings_internal::STLStringAppendUninitializedAmortized(str,
-                                                                total_length);
-
-  // Finally, output each argument one-by-one, from left to right.
-  size_t i = 0;  // The current argument we're processing
-  ptrdiff_t n;   // The length of the current argument
-  typename String::pointer pos = &(*str)[old_size];
-  using SomeTrivialEmptyType = std::false_type;
-  const SomeTrivialEmptyType dummy;
-  // Ugly code due to the lack of C++17 fold expressions
-  const SomeTrivialEmptyType dummies[] = {
-      (/* Comma expressions are poor man's C++17 fold expression for C++14 */
-       (void)(n = lengths[i]),
-       (void)(n < 0 ? (void)(*pos++ = '-'), (n = ~n) : 0),
-       (void)absl::numbers_internal::FastIntToBufferBackward(
-           absl::numbers_internal::UnsignedAbsoluteValue(std::move(args)),
-           pos += n, static_cast<uint32_t>(n)),
-       (void)++i, dummy)...};
-  (void)dummies;  // Remove & migrate to fold expressions in C++17
-}
-
 // Helper function for the future StrCat default floating-point format, %.6g
 // This is fast.
 inline strings_internal::AlphaNumBuffer<
diff --git a/absl/strings/str_cat_test.cc b/absl/strings/str_cat_test.cc
index b30a86fe..66eddf0d 100644
--- a/absl/strings/str_cat_test.cc
+++ b/absl/strings/str_cat_test.cc
@@ -39,24 +39,6 @@
 
 namespace {
 
-template <typename Integer>
-void VerifyInteger(Integer value) {
-  const std::string expected = std::to_string(value);
-
-  EXPECT_EQ(absl::StrCat(value), expected);
-
-  const char* short_prefix = "x";
-  const char* long_prefix = "2;k.msabxiuow2[09i;o3k21-93-9=29]";
-
-  std::string short_str = short_prefix;
-  absl::StrAppend(&short_str, value);
-  EXPECT_EQ(short_str, short_prefix + expected);
-
-  std::string long_str = long_prefix;
-  absl::StrAppend(&long_str, value);
-  EXPECT_EQ(long_str, long_prefix + expected);
-}
-
 // Test absl::StrCat of ints and longs of various sizes and signdedness.
 TEST(StrCat, Ints) {
   const short s = -1;  // NOLINT(runtime/int)
@@ -86,34 +68,6 @@ TEST(StrCat, Ints) {
   EXPECT_EQ(answer, "-9-12");
   answer = absl::StrCat(uintptr, 0);
   EXPECT_EQ(answer, "130");
-
-  for (const uint32_t base : {2u, 10u}) {
-    for (const int extra_shift : {0, 12}) {
-      for (uint64_t i = 0; i < (1 << 8); ++i) {
-        uint64_t j = i;
-        while (true) {
-          uint64_t v = j ^ (extra_shift != 0 ? (j << extra_shift) * base : 0);
-          VerifyInteger(static_cast<bool>(v));
-          VerifyInteger(static_cast<wchar_t>(v));
-          VerifyInteger(static_cast<signed char>(v));
-          VerifyInteger(static_cast<unsigned char>(v));
-          VerifyInteger(static_cast<short>(v));               // NOLINT
-          VerifyInteger(static_cast<unsigned short>(v));      // NOLINT
-          VerifyInteger(static_cast<int>(v));                 // NOLINT
-          VerifyInteger(static_cast<unsigned int>(v));        // NOLINT
-          VerifyInteger(static_cast<long>(v));                // NOLINT
-          VerifyInteger(static_cast<unsigned long>(v));       // NOLINT
-          VerifyInteger(static_cast<long long>(v));           // NOLINT
-          VerifyInteger(static_cast<unsigned long long>(v));  // NOLINT
-          const uint64_t next = j == 0 ? 1 : j * base;
-          if (next <= j) {
-            break;
-          }
-          j = next;
-        }
-      }
-    }
-  }
 }
 
 TEST(StrCat, Enums) {