<charconv>: for integer to hex conversion instead of looping you can convert digits in parallel using bitwise operations

#include <cstddef>
#include <cstring>
#ifdef _MSVC
#include <intrin.h>
#else
#include <x86intrin.h>
#endif

uint32_t log2(uint32_t i) {
#ifdef _MSVC  
  return 32u - __lzcnt(i);
#else
  return 32u - __builtin_clz(i);
#endif
}

void writeHexNumber(char *s, uint32_t number)
{
  // inspired by https://johnnylee-sde.github.io/Fast-unsigned-integer-to-hex-string/
  // convert to 64 bit: 0x00000000'1234abcd
  uint64_t digits = number;
  // send low part to high 32 bits, high part to low 16 bit: 0x0000abcd'00001234
  digits = (digits << 32) | (digits >> 16);
  // spread bytes to higher bytes: 0x00cd00ab'00340012
  digits = ((digits & 0x0000ff00'0000ff00) >> 8) | ((digits & 0x000000ff'000000ff) << 16);
  // move high hex digits to separate bytes: 0x0d0c0b0a'04030201
  digits = ((digits & 0x00f000f0'00f000f0) >> 4) | ((digits & 0x000f000f'000f000f) << 8);
  // make mask for byte values larger than '9' by overflowing into neighbor bits
  const uint64_t mask = ((digits + 0x06060606'06060606) >> 4) & 0x01010101'01010101;
  // convert to ASCII by adding value of '0' which is '48' of '0x30'
  digits |= 0x30303030'30303030;
  // fix up large value by adding value of ('A' - ('9' + 1)) which is '7'
  digits += mask * 0x7;
  // we got 'digits' containing ASCII codes for each byte
  // we have to memcpy data to formally avoid UB
  //memcpy(s, &digits, 8);
  // skip leading zeros
  const auto size = (log2(number | 1) + 3) / 4;
  memcpy(s, reinterpret_cast<const std::byte*>(&digits) + (8 - size), size);
}