[libcxx-dev] Problem with __builtin_strlen among compilers

[Zoe Carver via libcxx-dev]

"__builtin_strlen" works with GCC 7.x and up and clang 6.x and up. Here is
a Godbolt: https://godbolt.org/z/G9NpAX

On Mon, Sep 9, 2019 at 1:50 PM Yan Yichen via libcxx-dev <
libcxx-dev at lists.llvm.org> wrote:

> I got a little bit confused trying to get a repro of __builtin_strlen in
> libc++ with gcc.
>
> At least the behavior of __builtin_strlen is different in clang and gcc.
>
>
>
> This passed
>
>
>
> g++ -o /dev/null -std=c++17 --compile test2.cc
>
> while this failed
>
>
>
> clang++ -o /dev/null -std=c++17 --compile test2.cc
>
> with error messages:
>
>
>
> test2.cc:6:19: error: static_assert expression is not an integral constant
> expression
>
>     static_assert(!test(s));
>
>                   ^~~~~~~~
>
> test2.cc:3:12: note: read of non-constexpr variable 's' is not allowed in
> a constant expression
>
>     return __builtin_strlen(s);
>
>            ^
>
> test2.cc:6:20: note: in call to 'test(&s[0])'
>
>     static_assert(!test(s));
>
>                    ^
>
> 1 error generated.
>
>
>
> For one same file:
>
>
>
>
>
> const char s[] = "";
>
> constexpr int test(const char* s) {
>
>     return __builtin_strlen(s);
>
> }
>
> constexpr void __main() {
>
>     static_assert(!test(s));
>
> }
>
>
>
>
>
>
>
> Hmmm...  I think __builtin_strlen is constexpr on gcc.
>
>
>
> Looking at a more minimal case:
>
>
>
> *int* main() { *static_assert*(!__builtin_strlen("")); }
>
>
>
> It compiles fine under:
>
>
>
> gcc 9.2, libstdc++
>
> /usr/local/gcc/bin/g++-9 -std=c++17 a.cpp
>
>
>
> clang 8, libc++
>
> /usr/local/clang/bin/clang++ -std=c++17 a.cpp
>
>
>
> gcc 9.2, libc++
>
> /usr/local/gcc/bin/g++-9 -nostdinc++ -I /usr/local/clang/include/c++/v1
> -nodefaultlibs -lc++ -lc++abi -lm -lc -lgcc -std=c++17 a.cpp
>
>
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