[libc++] Avoid relying on non-portable behaviour in std::align

[arichardson]

Summary

Round-tripping pointers via size_t is not portable, the C/C++ standards
only require this to be valid when using (u)intptr_t. While touching
these lines also make use of the Clang 10+ __builtin_align_up to avoid
the need for reinterpret_cast.

Originally committed to the CHERI fork of LLVM as
https://github.com/CTSRD-CHERI/llvm-project/commit/dd01245185ab9e71b70b418bee8f11ea0199e1a3,
but I forgot to upstream the change. I rediscovered this issue due to a
compiler warning when building libc++ on a Arm Morello system.

[ldionne]

LGTM but please re-upload to get a CI run.

[philnik]

Is there any benefit of the builtin other than removing reinterpret_cast? If not I'd say just drop it, since we have to keep the reinterpret_casts anyways.

[arichardson]

Drop __builtin_align_up

[arichardson]

In D134363#3806644, @philnik wrote:

Is there any benefit of the builtin other than removing reinterpret_cast? If not I'd say just drop it, since we have to keep the reinterpret_casts anyways.

Not really - in theory it could improve codegen, but it just emits the same thing as the hand-written code anyway. I think the main reason I originally used it is that CHERI Clang emitted a (false-positive) warning on the arithmetic expression back in 2019 that has since been fixed. I've dropped the builtin from this version.

[arichardson]

rebase to try and get a working CI run

[ldionne]

LGTM once CI is green. You should try rebasing now, the CI is back online.

[arichardson]

rebase for CI

libcxx/src/memory.cpp

 
 void*
 align(size_t alignment, size_t size, void*& ptr, size_t& space)
 {
     void* r = nullptr;
     if (size <= space)
     {
         char* p1 = static_cast<char*>(ptr);
-        char* p2 = reinterpret_cast<char*>(reinterpret_cast<size_t>(p1 + (alignment - 1)) & -alignment);
+        char* p2 = reinterpret_cast<char*>(reinterpret_cast<uintptr_t>(p1 + (alignment - 1)) & -alignment);
         size_t d = static_cast<size_t>(p2 - p1);
         if (d <= space - size)
         {
             r = p2;
             ptr = r;
             space -= d;
         }
     }
     return r;
 }
 
 _LIBCPP_END_NAMESPACE_STD