Figure out the interaction of GVN and function/vtable pointers · Issue #123670 · rust-lang/rust (original) (raw)

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Description

GVN has special treatment for function/vtable pointers. I haven't fully understood why.

Function pointers have unstable comparison. We set the unnamed_addr flag in LLVM. I am not entirely sure how far that attribute goes, but I think it means that == involving such functions is basically non-deterministic? @nikic @bjorn3 maybe you can help here -- can the result of comparing function pointers depend on whether or not optimizations are applied? Or how exactly unnamed_addr affect the generated code?

@cjgillot could you give an example for the code you are concerned about, where GVN would go wrong if it did not special-case function pointers? I think you gave an example a while ago but I can't find it right now.