[PATCH] IR: Move the slot tracker out of AsmWriter into a separate public module.

[Alex L]

2015-06-17 13:52 GMT-07:00 Duncan P. N. Exon Smith <dexonsmith at apple.com>:

>
> > On 2015 Jun 17, at 13:20, Alex Lorenz <arphaman at gmail.com> wrote:
> >
> > Hi dexonsmith, bob.wilson, bogner,
> >
> > This patch moves the SlotTracker class out of AsmWriter.cpp into a
> separate module that's publicly accessible.
> >
> > This patch would be useful for MIR Serialization, in particular it would
> enable the MIR parser to parse metadata machine operands. The metadata
> machine operands are serialized using the familiar '!' <slot> notation, and
> the MIR parser has to be able to map from slot numbers to the actual
> metadata nodes. The SlotTracker class would allow the MIRParser to create
> this mapping.
>
> I can see that this would be useful for *writing* .mir files, but I
> don't think you can safely use this for *reading* .mir files.
>
> Metadata slots can be assigned arbitrarily in an LLVM IR file, such as:
>
>     !named = !{!36, !72}
>     !72 = !{!"string"}}
>     !36 = !{!72, !{!{}}}
>
> If you were to parse the module and then run the slot tracker, you'd get:
>
>     !named = !{!1, !2}
>     !1 = !{!2, !3}
>     !2 = !{!"string"}
>     !3 = !{!4}
>     !4 = !{}
>
> or something close to that.
>
> You couldn't safely take an already-parsed Module, run the slot-tracker
> on it, and then parse machine functions that referenced metadata.  But
> it sounds like that's what you're suggesting?
>

This makes sense, yeah this patch wouldn't really work then.


>
> Instead, I think you need to:
>
>  1. Yes, surface the slot tracker (exactly this patch), but for completely
>     different reasons:  so that you can write out correct metadata numbers
>     for metadata references within machine functions, to match the metadata
>     that you wrote out for the LLVM IR.
>  2. Use (1) so that the same slots are used when writing LLVM IR portion of
>     MIR as the machine functions.
>

I don't need to surface the slot tracker then, as I can print out the
correct metadata slot numbers by printing the metadata nodes as operands.
They create the slot tracker and initialize it for the whole module, so the
correct slot numbers are printed.


>  3. Find a way to surface `LLParser::NumberedMetadata` in a similar way
>     (could be some way of copying them out?) when parsing the LLVM IR part
>     of MIR.
>  4. Use that slot mapping when you're reading machine functions.
>

Yeah, I should surface the slot mapping from the LLParser then.


>
> Also, I'm not sure this just affects metadata.  What about llvm::Type?
> llvm::GlobalValue?  (Maybe their numbered slots are more strictly organized
> so they're okay, but I'm not sure.)
>

I'm not sure yet, but I don't think they're affected in the same manner.
The only other things that I can think of are some of the LLVM IR
references that I serialize - like the allocas, and memory operand values,
as they can be numbered, and their numbering can be different when parsing
and printing. I will have to surface the mappings for those things as well.


>
> Does this make sense?
>
> >
> > REPOSITORY
> >  rL LLVM
> >
> > http://reviews.llvm.org/D10517
> >
> > Files:
> >  include/llvm/IR/SlotTracker.h
> >  lib/IR/AsmWriter.cpp
> >  lib/IR/CMakeLists.txt
> >  lib/IR/SlotTracker.cpp
> >
> > EMAIL PREFERENCES
> >  http://reviews.llvm.org/settings/panel/emailpreferences/
> > <D10517.27862.patch>
>
>
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