[libcxx-commits] [PATCH] D96946: [libcxx][RFC] Unspecified behavior randomization in libcxx

[Arthur O'Dwyer via Phabricator via libcxx-commits]

Quuxplusone added inline comments.


================
Comment at: libcxx/include/algorithm:3113
+
+template <class _Dummy>
+class _LIBCPP_TYPE_VIS __random_unspecified_behavior_gen_
----------------
danlark wrote:
> ldionne wrote:
> > Why are we making this a template?
> Because there is no algorithm.cpp and I decided not to create one because of such feature. It is needed to instantiate `static const void* const __seed_static;` and inline variables are only a C++17 feature, before that it was a common trick to put the static variable definition in header
> 
> See
> 
> ```
> template <class _Void>
> const void* const __random_unspecified_behavior_gen_<_Void>::__seed_static =
>     &__seed_static;
> ```
Seems like it would be way simpler to just do the //other// C++ singleton trick, which is "make the thing a function-local static variable":
```
    _LIBCPP_ALWAYS_INLINE static result_type __seed() {
#ifdef _LIBCPP_RANDOMIZE_UNSPECIFIED_STABILITY_SEED
      return _LIBCPP_RANDOMIZE_UNSPECIFIED_STABILITY_SEED;
#else
      static char __x;
      return reinterpret_cast<uintptr_t>(&__x);
#endif
    }
```
I see no reason for any of the helper machinery here; it looks like we're just trying to generate a "random seed" from an address, so let's just do that. No global variables, no templates (not even any unnecessary casts!), nice and simple.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D96946/new/

https://reviews.llvm.org/D96946