Thanks. The source code of ghc-heap has led me to what I was looking for. That whole library hinges upon unpackClosure# (stg_unpackClosurezh), whose implementation opens with the telling:

    info  = %GET_STD_INFO(UNTAG(closure));

In CmmParse.y, we find the implementation of GET_STD_INFO, and just a few lines below is the INFO_TYPE macro. The combination of these (along with a cmm implementation of UNTAG, which I am still unsure of exactly how to accomplish) should do the trick.

On Wed, Feb 6, 2019 at 2:53 PM Matthew Pickering <matthewtpickering@gmail.com> wrote:

Unless I am misunderstanding you want something which works like ghc-heap.

If so, perhaps these three files will be instructive?

https://gitlab.haskell.org/ghc/ghc/blob/master/libraries/ghc-heap/GHC/Exts/Heap/InfoTable.hsc
https://gitlab.haskell.org/ghc/ghc/blob/master/libraries/ghc-heap/GHC/Exts/Heap/InfoTableProf.hsc
https://gitlab.haskell.org/ghc/ghc/blob/master/libraries/ghc-heap/GHC/Exts/Heap/Closures.hs#L25

Cheers,

Matt

On Wed, Feb 6, 2019 at 7:29 PM Andrew Martin <andrew.thaddeus@gmail.com> wrote:
>
> For an optimization I’m implementing, I’m trying to write an inline primop that gives me lets me inspect the closure type of an arbitrary heap-allocated value. I’m close to figuring this out, and the FCode machinery is starting to make sense, but one thing eludes me. Starting from a StgHeader, I get the info table pointer. This is easy since it’s the first field in the struct. But next I have to deference the info table pointer and get the closure type. There are two fields at the beginning of an info table that may or may not be present. I don’t know how to figure out if they are present or not. I suspect that there is something I can get out of DynFlags that tells me this, but I can’t find anything else in the stg-to-cmm pass that reads from an info table.
>
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-Andrew Thaddeus Martin