[LLVMdev] tbaa.struct metadata and variable sized structs

Dan Gohman gohman at apple.com
Wed Sep 26 11:49:54 PDT 2012

On Sep 23, 2012, at 8:20 AM, Duncan Sands <baldrick at free.fr> wrote:

> Hi Dan, I was thinking about having dragonegg produce tbaa.struct metadata and I
> realized that there is a problem with variable sized structs.  What I have in
> mind is for example a struct with a few ordinary fields and a variable length
> array at the end.  In Ada these can be copied; dragonegg turns such copies into
> calls to llvm.memcpy with a non-constant size parameter.  It would be nice to
> use tbaa.struct metadata with such structs, not to describe the variable sized
> part but to describe the tbaa properties and gaps in the fixed size normal part.
> Unfortunately with the current tbaa.struct setup anything that isn't specified
> as a field is considered to be a gap.  Since there is no way to describe the
> variable sized part as a field, it will be considered to be a gap, which would
> be fatal.  (Note that the optimizers, eg inlining, may discover the actual size
> of the variable sized struct, resulting in an llvm.memcpy of fixed size with an
> apparent gap at the end, so such structs may not appear variable sized at the
> LLVM level).
> A possible solution is to say that gaps should be made explicit in tbaa.struct
> metadata.  That way for a variable sized struct the ordinary fields and the gaps
> between them would be explicitly output in the metadata, and "the rest" would be
> treated conservatively as something that has to be copied with unknown aliasing
> properties.
> What do you think?

I think it's beyond the scope of what I'm supposed to be working on for this
feature. But you're surely welcome to propose patches for this yourself.


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