[LangRef] Global variable declarations imply minimum size

[nikic]

Summary

Adjust the wording added in D78952 to say that global variable declarations (and interposable definitions) do imply a minimum size (and alignment) on the global. They just don't imply a maximum size.

We rely on these semantics in at least two places:

• Global dereferenceability: https://github.com/llvm/llvm-project/blob/2153544865a9733b06579823814c981f735e4201/llvm/lib/IR/Value.cpp#L907
• Global inbounds GEP: https://github.com/llvm/llvm-project/blob/2153544865a9733b06579823814c981f735e4201/llvm/lib/IR/ConstantFold.cpp#L2283

[efriedma]

This seems straightforward enough, and should be compatible with existing practice as far as I know. (If you have a global of unknown size, you can always set the type to a zero-length array.) LGTM.

llvm/docs/LangRef.rst

 case, the extra alignment could be observable: for example, code could
 assume that the globals are densely packed in their section and try to
 iterate over them as an array, alignment padding would break this
 iteration. For TLS variables, the module flag ``MaxTLSAlign``, if present,
 limits the alignment to the given value. Optimizers are not allowed to
 impose a stronger alignment on these variables. The maximum alignment
 is ``1 << 32``.
 
-For global variables declarations, as well as definitions that may be
+For global variable declarations, as well as definitions that may be
 replaced at link time (``linkonce``, ``weak``, ``extern_weak`` and ``common``
-linkage types), LLVM makes no assumptions about the allocation size of the
-variables, except that they may not overlap. The alignment of a global variable
-declaration or replaceable definition must not be greater than the alignment of
-the definition it resolves to.
+linkage types), the allocation size and alignment of the definition it resolves
+to must be greater than or equal to that of the declaration or replaceable
+definition, otherwise the behavior is undefined.
 
 Globals can also have a :ref:`DLL storage class <dllstorageclass>`,
 an optional :ref:`runtime preemption specifier <runtime_preemption_model>`,
 an optional :ref:`global attributes <glattrs>` and
 an optional list of attached :ref:`metadata <metadata>`.
 
 Variables and aliases can have a
 :ref:`Thread Local Storage Model <tls_model>`.