================
@@ -9242,3 +9242,15 @@ Declares that a function potentially allocates heap
memory, and prevents any pot
of ``nonallocating`` by the compiler.
}];
}
+
+def ValueTypeDocs : Documentation {
+ let Category = DocCatDecl;
+ let Content = [{
+The ``value_type`` attribute can be used to mark user-defined types as 'value
+types'. When objects of value types are passed value to functions, the objects
+are always considered to be formally copied into a new object. This means the
+argument itself must be the only value referencing the passed object. This
----------------
rjmccall wrote:
If the argument satisfies the formal conditions for being passed in registers,
the implementation is allowed to introduce a copy despite NRVO, which might be
enough to make any escaped/self pointers formally invalid even if we don't
actually copy. But yeah, NRVO might mean we're out of luck for an arbitrary
type to which that doesn't apply.
We could probably talk to the C++ committee about this if we can show that we
could do useful optimizations except for some contrived and dangerous code
pattern like escaping the address of a copyable class value prior to returning
it via NRVO.
https://github.com/llvm/llvm-project/pull/95004
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