[llvm-dev] [RFC] Lazy-loading of debug info metadata

[Teresa Johnson via llvm-dev]

On Wed, Mar 23, 2016 at 11:06 PM, Duncan P. N. Exon Smith <
dexonsmith at apple.com> wrote:

>
> > On 2016-Mar-22, at 19:28, Duncan P. N. Exon Smith via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> > 1. If a piece of metadata is referenced from only a single `Function`,
> >    serialize that metadata in the function's metadata block instead of
> >    the global metadata block.
> >
> >    This addresses problems (a) and (b), primarily targeting
> >    `DILocation`s.  It should pick up lots of other stuff, depending on
> >    how much inlining has happened.
> >
> >    (I have a draft of the writer side, still working on the reader.)
>
> Teresa: I had trouble make this work with the delayed metadata parsing
> (the logic around "SeenModuleValuesRecord").  My WIP patch rips that
> out rather unceremoniously.
>
> (Attached the patch for reference, but it needs to be cleaned up, split
> up, etc.; not ready for review (although comments always welcome)).
>
> My understanding from Mehdi is that maybe ThinLTO isn't currently
> relying on the delayed parsing.  I.e., the original scheme was:
> - cherry-pick functions one at a time (never reading metadata), then
> - come back at the end to read the metadata all at once.
> But the scheme has evolved to:
> - calculate the desired functions from each module, then
> - load the module and link all the functions in one go.
>

Right, that is what the FunctionImporter logic has changed to. I was
keeping that support alive on the concern that for very large modules the
overhead of keeping all the source modules materialized while importing
decisions are made would be too high. But since I added a full
reference/call graph to the summary and Mehdi has sent a patch to change
the importer to do summary-based importing, that concern should be moot.

>
> Is that accurate?  If so, I don't see remaining benefit in delaying the
> global metadata parsing, just an extra code path to maintain.  Do you
> agree?
>
>
I think we can take this support out at this point now. Note however that
llvm-link (used for testing) is still doing function at a time importing
and therefore relying on this support. However it shouldn't be hard to
rework that to collect all the imports from a given module for batch
importing.

Let me take a stab at moving llvm-link over to using batch importing
hopefully today or tomorrow, then at least no tests will rely on the
post-pass metadata importing and I can pull that out independently.

Teresa

-- 
Teresa Johnson |  Software Engineer |  tejohnson at google.com |  408-460-2413
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