[libc-commits] [libc] d34214a - [libc] Add and use 'cpp::launder' to guard placement new (#146123)

[via libc-commits]

Author: Joseph Huber
Date: 2025-06-27T14:34:33-05:00
New Revision: d34214a85e3b9b94c2ccc6d63f3592be7eca0c24

URL: https://github.com/llvm/llvm-project/commit/d34214a85e3b9b94c2ccc6d63f3592be7eca0c24
DIFF: https://github.com/llvm/llvm-project/commit/d34214a85e3b9b94c2ccc6d63f3592be7eca0c24.diff

LOG: [libc] Add and use 'cpp::launder' to guard placement new (#146123)

Summary:
In the GPU allocator we reinterpret cast from a void pointer. We know
that an actual object was constructed there according to the C++ object
model, but to make it fully standards compliant we need to 'launder' it
to forward that information to the compiler. Add this function and call
it as appropriate.

Added: 
    

Modified: 
    libc/src/__support/CPP/new.h
    libc/src/__support/GPU/allocator.cpp

Removed: 
    


################################################################################
diff  --git a/libc/src/__support/CPP/new.h b/libc/src/__support/CPP/new.h
index 8694d9c475507..fe36de29468a8 100644
--- a/libc/src/__support/CPP/new.h
+++ b/libc/src/__support/CPP/new.h
@@ -29,6 +29,14 @@ enum class align_val_t : size_t {};
 
 namespace LIBC_NAMESPACE_DECL {
 
+namespace cpp {
+template <class T> [[nodiscard]] constexpr T *launder(T *p) {
+  static_assert(__has_builtin(__builtin_launder),
+                "cpp::launder requires __builtin_launder");
+  return __builtin_launder(p);
+}
+} // namespace cpp
+
 class AllocChecker {
   bool success = false;
 

diff  --git a/libc/src/__support/GPU/allocator.cpp b/libc/src/__support/GPU/allocator.cpp
index 66ab155e5c299..5ea27a9c44b66 100644
--- a/libc/src/__support/GPU/allocator.cpp
+++ b/libc/src/__support/GPU/allocator.cpp
@@ -544,8 +544,8 @@ void deallocate(void *ptr) {
     return impl::rpc_free(ptr);
 
   // The original slab pointer is the 2MiB boundary using the given pointer.
-  Slab *slab = reinterpret_cast<Slab *>(
-      (reinterpret_cast<uintptr_t>(ptr) & ~SLAB_ALIGNMENT));
+  Slab *slab = cpp::launder(reinterpret_cast<Slab *>(
+      (reinterpret_cast<uintptr_t>(ptr) & ~SLAB_ALIGNMENT)));
   slab->deallocate(ptr);
   release_slab(slab);
 }