Build failure against libstdc++ 14.2 with `-std=c++20`

[nickdesaulniers]

Building against libstdc++14.2 fails:

external/tcmalloc+/tcmalloc/parameters.cc:253:17: error: variable does not have a constant initializer
  253 |     Parameters::per_cpu_caches_dynamic_slab_shrink_threshold_(0.4);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
external/tcmalloc+/tcmalloc/parameters.cc:252:1: note: required by 'constinit' specifier here
  252 | ABSL_CONST_INIT std::atomic<double>
      | ^~~~~~~~~~~~~~~
external/abseil-cpp+/absl/base/attributes.h:755:25: note: expanded from macro 'ABSL_CONST_INIT'
  755 | #define ABSL_CONST_INIT constinit
      |                         ^~~~~~~~~
[redacted]/14.2.0/include/c++/bits/atomic_base.h:1286:9: note: non-constexpr function '__clear_padding<double>' cannot be used in a constant expression
 1286 |       { __atomic_impl::__clear_padding(_M_fp); }
      |         ^
[redacted]/14.2.0/../../../gcc/x86_64-conda-linux-gnu/14.2.0/include/c++/atomic:1644:38: note: in call to '__atomic_float(4.000000e-01)'
 1644 |       atomic(double __fp) noexcept : __atomic_float<double>(__fp)
      |                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
external/tcmalloc+/tcmalloc/parameters.cc:253:17: note: in call to 'atomic(4.000000e-01)'
  253 |     Parameters::per_cpu_caches_dynamic_slab_shrink_threshold_(0.4);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[redacted]/14.2.0/include/c++/bits/atomic_base.h:972:7: note: declared here
  972 |       __clear_padding(_Tp& __val) noexcept
      |       ^

FWICT, I think libstdc++ had a bug in their 14.2 release where their std::atomic maybe wasn't constinit. This appears fixed in their 14.3 release (hence me not searching their issue tracker for a reference).

https://godbolt.org/z/b1bxeGzcY

I don't know if I'd call this a bug in abseil's ABSL_CONST_INIT; it's moreso a (now fixed) bug in libstdc++.

Maybe the answer is just "move to newer libstdc++ (there is 14.3, and 15.1) or libc++." But figured I'd file this in case compatibility with 14.2 is important.

One way to work around this, if libstdc++ exposes version info via preprocessor define, is to wrap ABSL_CONST_INIT when using it on std::atomic, I think, or simply omit ABSL_CONST_INIT. 🤷

Also, we're building with -std=gnu++20, hence ABSL_CONST_INIT expanding to constinit.

[ckennelly]

Just to understand the scope here, is this just a problem with the floating point atomics, or are there other build errors regarding constinit on GCC 14.2?

Stepping back, though: The bug here isn't with constinit. TCMalloc might access some of our state very early, before the initializers for that particular file have run (example: an initializer in one file allocates), and we need to both

1. Know what state it might be in
2. Ensure that there isn't an initializer that might fire that overwrites state after we've started using it and corrupts the heap

If I drop constinit, GCC 14.2 compiles, but it introduces an initializer (https://godbolt.org/z/xcMzaWvvj). This particular atomic is fairly benign: We're not going to mutate it while allocating, nor would doing the initialization a little late be a problem either.

The bigger issue is that constinit is loadbearing for correctness elsewhere in TCMalloc. constinit is only advisory in a sense that it doesn't make constinit-able things actually constinit'd, but it's a really valuable double-check that the compiler is doing the right thing where it matters by balking when it doesn't happen (https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1143r2.html).

It looks like the fix for 14.3 was gcc-mirror/gcc@ae91b5d btw

[nickdesaulniers]

are there other build errors regarding constinit on GCC 14.2?

Not that I'm aware of; let me try upgrading my env to libstdc++ 14.3 then report back. (We are using clang, just clang with libstdc++).