diff --git a/libcxx/benchmarks/to_chars.bench.cpp b/libcxx/benchmarks/to_chars.bench.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/benchmarks/to_chars.bench.cpp
@@ -0,0 +1,58 @@
+//===----------------------------------------------------------------------===//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include <array>
+#include <charconv>
+#include <random>
+
+#include "benchmark/benchmark.h"
+#include "test_macros.h"
+
+static const std::array<unsigned, 1000> input = [] {
+  std::mt19937 generator;
+  std::uniform_int_distribution<unsigned> distribution(0, std::numeric_limits<unsigned>::max());
+  std::array<unsigned, 1000> result;
+  std::generate_n(result.begin(), result.size(), [&] { return distribution(generator); });
+  return result;
+}();
+
+static void BM_to_chars_good(benchmark::State& state) {
+  char buffer[128];
+  int base = state.range(0);
+  while (state.KeepRunningBatch(input.size()))
+    for (auto value : input)
+      benchmark::DoNotOptimize(std::to_chars(buffer, &buffer[128], value, base));
+}
+BENCHMARK(BM_to_chars_good)->DenseRange(2, 36, 1);
+
+static void BM_to_chars_bad(benchmark::State& state) {
+  char buffer[128];
+  int base = state.range(0);
+  struct sample {
+    unsigned size;
+    unsigned value;
+  };
+  std::array<sample, 1000> data;
+  // Assume the failure occurs, on average, halfway during the conversion.
+  std::transform(input.begin(), input.end(), data.begin(), [&](unsigned value) {
+    std::to_chars_result result = std::to_chars(buffer, &buffer[128], value, base);
+    return sample{unsigned((result.ptr - buffer) / 2), value};
+  });
+
+  while (state.KeepRunningBatch(data.size()))
+    for (auto element : data)
+      benchmark::DoNotOptimize(std::to_chars(buffer, &buffer[element.size], element.value, base));
+}
+BENCHMARK(BM_to_chars_bad)->DenseRange(2, 36, 1);
+
+int main(int argc, char** argv) {
+  benchmark::Initialize(&argc, argv);
+  if (benchmark::ReportUnrecognizedArguments(argc, argv))
+    return 1;
+
+  benchmark::RunSpecifiedBenchmarks();
+}
diff --git a/libcxx/include/charconv b/libcxx/include/charconv
--- a/libcxx/include/charconv
+++ b/libcxx/include/charconv
@@ -78,6 +78,7 @@
 #include <__errc>
 #include <cmath> // for log2f
 #include <cstdint>
+#include <cstdlib> // for _LIBCPP_UNREACHABLE
 #include <cstring>
 #include <limits>
 #include <type_traits>
@@ -400,33 +401,54 @@
     return __to_chars_integral(__first, __last, __x, __base, false_type());
 }
 
+template <typename _Tp>
+_LIBCPP_AVAILABILITY_TO_CHARS _LIBCPP_INLINE_VISIBILITY int __to_chars_integral_width(_Tp __value, unsigned __base) {
+  _LIBCPP_ASSERT(__value >= 0, "The function requires a non-negative value.");
+
+  unsigned __pow_2 = __base * __base;
+  unsigned __pow_3 = __pow_2 * __base;
+  unsigned __pow_4 = __pow_3 * __base;
+
+  int __r = 0;
+  while (true) {
+    if (__value < __base)
+      return __r + 1;
+    if (__value < __pow_2)
+      return __r + 2;
+    if (__value < __pow_3)
+      return __r + 3;
+    if (__value < __pow_4)
+      return __r + 4;
+
+    __value /= __pow_4;
+    __r += 4;
+  }
+
+  _LIBCPP_UNREACHABLE();
+}
+
 template <typename _Tp>
 _LIBCPP_AVAILABILITY_TO_CHARS
 inline _LIBCPP_INLINE_VISIBILITY to_chars_result
 __to_chars_integral(char* __first, char* __last, _Tp __value, int __base,
                     false_type)
 {
-    if (__base == 10)
-        return __to_chars_itoa(__first, __last, __value, false_type());
-
-    auto __p = __last;
-    while (__p != __first)
-    {
-        auto __c = __value % __base;
-        __value /= __base;
-        *--__p = "0123456789abcdefghijklmnopqrstuvwxyz"[__c];
-        if (__value == 0)
-            break;
-    }
-
-    auto __len = __last - __p;
-    if (__value != 0 || !__len)
-        return {__last, errc::value_too_large};
-    else
-    {
-        _VSTD::memmove(__first, __p, __len);
-        return {__first + __len, {}};
-    }
+  if (__base == 10)
+    return __to_chars_itoa(__first, __last, __value, false_type());
+
+  ptrdiff_t __diff = __last - __first;
+  int __width = __to_chars_integral_width(__value, __base);
+  if (__width > __diff)
+    return {__last, errc::value_too_large};
+
+  __last = __first + __width;
+  char* __p = __last;
+  do {
+    unsigned __c = __value % __base;
+    __value /= __base;
+    *--__p = "0123456789abcdefghijklmnopqrstuvwxyz"[__c];
+  } while (__value != 0);
+  return {__last, errc(0)};
 }
 
 template <typename _Tp, typename enable_if<is_integral<_Tp>::value, int>::type = 0>
diff --git a/libcxx/test/support/charconv_test_helpers.h b/libcxx/test/support/charconv_test_helpers.h
--- a/libcxx/test/support/charconv_test_helpers.h
+++ b/libcxx/test/support/charconv_test_helpers.h
@@ -9,6 +9,7 @@
 #ifndef SUPPORT_CHARCONV_TEST_HELPERS_H
 #define SUPPORT_CHARCONV_TEST_HELPERS_H
 
+#include <algorithm>
 #include <charconv>
 #include <cassert>
 #include <limits>
@@ -105,8 +106,12 @@
         using std::to_chars;
         std::to_chars_result r;
 
+        // Poison the buffer for testing whether a successful std::to_chars
+        // doesn't modify data beyond r.ptr.
+        std::fill(buf, buf + sizeof(buf), '~');
         r = to_chars(buf, buf + sizeof(buf), v, args...);
         assert(r.ec == std::errc{});
+        assert(std::all_of(r.ptr, buf + sizeof(buf), [](char c) { return c == '~'; }));
         *r.ptr = '\0';
 
         auto a = fromchars(buf, r.ptr, args...);