Is there an intrinsic similar to _BitScanForward on Linux?

["bootmgr@163.com" <bootmgr@163.com>]

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Greetings,
I'm not sure if it's appropriate to ask a question here, so please understand.


I trying to write a function like strlen to calculate the length of a null-terminated string. Here is a code
snippet.


[[nodiscard]] inline std::size_t strlen(const char *str) noexcept {
  const auto uint_ptr = reinterpret_cast<std::uintptr_t>(str);
  using v16qi [[gnu::vector_size(16)]] = char;
  constexpr std::uintptr_t mask = 0xF;
  auto xmm_word = reinterpret_cast<const v16qi *>(uint_ptr & ~mask);
  v16qi xmm0;
  __builtin_memset(std::addressof(xmm0), 0, sizeof(xmm0));
  unsigned long result;
  if (_BitScanForward(std::addressof(result),
          __builtin_ia32_pmovmskb128(*xmm_word == xmm0) >> (uint_ptr & mask)))
    return result;
  /* ... */
}


And I want to port it to Linux, so I am looking for any functions like _BitScanForward. I trying to use
__builtin_ctz instead _BitScanForward, but it is undefined when x is 0 and it can't
determine whether x is 0 or not. Then I found __bsfd in x86intrin.h, but it's the same as
__builtin_ctz, which can't determine x is 0.


So is there any solution, preferably cross-platform?