[LLVMdev] Merging custom metadata

[Dimitri Tcaciuc]

Hello everyone,

I'm trying to tag certain load instructions with specialized metadata and
make use of it in an AA pass. However, it seems that metadata is getting
lost when instructions are merged together.

Here's a toy problem with one intentionally duplicated load to illustrate
the issue.

   http://susepaste.org/3af3a676

The above paste has two before/after IR pairs (the `before` code gets
passed through `opt -O3 test.ll | llvm-dis`). Regular `tbaa` tags are still
present when loads are merged, however custom ones seem to perish.

Is this expected? If yes, is there any way I can make them persist from
within a custom AA pass at all? I tried overwriting copyValue() in the AA
pass but it doesn't seem to get called at all. The main idea is to
recognize certain types of loads and have other information in the my.tbaa
metadata that regular tbaa doesn't provide. The only other solution I can
think of right now is to encode that extra information in tbaa node
strings, but that seems rather gross.

Thanks,


Dimitri.
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