Clang does not optimize out _LIBCPP_ASSERT_NON_OVERLAPPING_RANGES from libc++

[davidben]

libc++ contains assertions of the following form:

llvm-project/libcxx/include/__string/char_traits.h

Lines 239 to 246 in 933ad5c

	static inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
	char_type* copy(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT {
	if (!__libcpp_is_constant_evaluated())
	_LIBCPP_ASSERT_NON_OVERLAPPING_RANGES(
	__s2 < __s1 || __s2 >= __s1 + __n, "char_traits::copy overlapped range");
	std::copy_n(__s2, __n, __s1);
	return __s1;
	}

These are enabled with _LIBCPP_ENABLE_SAFE_MODE and are, among other places, used when constructing std::strings from string literals:

llvm-project/libcxx/include/string

Line 940 in 933ad5c

__init(__s, traits_type::length(__s));

llvm-project/libcxx/include/string

Line 2168 in 933ad5c

traits_type::copy(std::__to_address(__p), __s, __sz);

Looking at the disassembly of some code in Chromium, this does not seem to get optimized out, even when all we're doing is constructing a std::string from a string literal. The compiler should know that string literals cannot alias with much of anything.

I put together this minimal program in godbolt and it seems, even with -O3, Clang (and GCC) do not optimize these checks out. That suggests Clang (and GCC) just don't know how to optimize this pattern? Separately, I'm not sure if the check is correct. See the comment about the "fixed" versions.
https://godbolt.org/z/oef93T9xb

[ldionne]

@var-const Can you take a look?

Also @fhahn for optimizations opportunities.

[davidben]

I was asked whether __is_pointer_in_range changes things, and the answer is it doesn't. That just moved some code around, from what I can tell. Here's a reproducer in the context of std::string: https://godbolt.org/z/7bK9P46jv

I believe the snippets that look like this are the non-overlapping range checks:

        cmp     rax, rdx
        seta    sil
        cmp     rcx, rdx
        setbe   dl
        or      dl, sil
        je      .LBB0_1