[llvm-dev] Find uses of Metadata / DITypes

[Henrik Olsson via llvm-dev]

Ah yes, of course! For our thesis we're trying to reconstruct Value names
from the IR to C syntax, to help with clarity in optimisation remarks. To
do this for something like a GetElementPointer we first have to find the
name of the pointer operand, and then we try to name the offsets. Naming
array offsets is relatively straightforward, but for structs we need the
DICompositeType which contains the struct field names. So we make the
recursive call to name the pointer operand also return the operand's
DIType, and from this we get the base type of the pointer. However we're
struggling with handling bitcasts properly at the moment. When the pointer
operand of a GEP is a bitcast, say from a struct pointer type to a pointer
to the struct's first field we can figure that out by diffing the Value
types and then traversing the DIType accordingly. When the cast is from
smaller to wider type, on the other hand, we cannot just traverse the
DIType structure to the wider type as the link is only in one direction. We
recognise that there may be several potential wider types matching the
Value type, but this is a best effort matching.

We have discussed traversing all the DITypes in the module to find uses of
the smaller type, and we have also thought about making changes to clang to
emit debug variable intrinsics for bitcasts so we can find the correct
DIType from the bitcast instruction itself. We're not sure how feasible
these are in the grander scheme of things. Would emitting debug variable
intrinsics for all bitcasts have a semantics mismatch in later passes when
DWARF data is generated? Is it feasible to traverse all of the DITypes
without damaging performance too much? Would saving the uses in the reverse
direction like for Values have too large of a memory footprint? Obviously
we'll make the changes we need to make things work for our thesis, but if
possible we would like to not make changes that are invasive enough that
it's unmergeable back into mainline LLVM.

/Henrik

On Wed, 25 Mar 2020, 18:10 Adrian Prantl, <aprantl at apple.com> wrote:

> It might help if you could explain what you are *actually* trying to do,
> since there are often other solutions for the higher-level problem that fit
> better into the design and architecture.
>
> -- adrian
>
> > On Mar 25, 2020, at 4:01 AM, Henrik Olsson via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> > I'm looking for a way to efficiently traverse the Metadata structure in
> reverse, or otherwise find DITypes that refer to another DIType. As an
> example, say I have a DICompositeType describing a struct. Now I want to
> find other DITypes containing this type, e.g. a DIDerivedType describing
> its pointer type, or another struct containing this struct as one of its
> fields. Is this possible in LLVM today? Would it be unreasonable to save
> Metadata to Metadata uses, like what is done for Value to Value uses?
> >
> > Any help would be greatly appreciated,
> > Henrik
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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