[llvm-bugs] [Bug 45681] New: Request for LangRef clarification: allocation size of global variable declaration

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=45681

            Bug ID: 45681
           Summary: Request for LangRef clarification: allocation size of
                    global variable declaration
           Product: Documentation
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: General docs
          Assignee: unassignedbugs at nondot.org
          Reporter: mschiffer at universe-factory.net
                CC: llvm-bugs at lists.llvm.org

Working on Rust, we have come across the question whether LLVM makes any
assumptions about the allocation size of declared global variables based on
their type when the definition is unknown (or, turning it around, if it is
guaranteed that LLVM makes no such assumptions).

It seems fine to access data (for example using getelementptr inbounds) past
the "end" of such a global variable at least on some cases - I've noticed that
clang compiles a `extern char foo[];` to `@foo = external global [0 x i8],
align 1` (and a comment in llvm/test/Analysis/BasicAA/global-size.ll backs
this).

What is not entirely clear to us is whether such accesses are *always* valid
(given that a definition of appropriate length comes in at link time), in
particular:

- when the length of the declared type is not 0
- when the declared type is not an array, but a struct, and the last element of
the struct is no array

If all of this is indeed valid, it would be great if such a statement could be
added to the LangRef.

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