[PATCH] D11722: [ThinLTO] Bitcode reading/writing support for ThinLTO function summary/index

[Duncan P. N. Exon Smith via llvm-commits]

> On 2015-Aug-13, at 10:22, Teresa Johnson <tejohnson at google.com> wrote:
> 
> 
> 
> On Thu, Aug 13, 2015 at 9:42 AM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
> 
> > On 2015-Aug-12, at 21:23, Teresa Johnson <tejohnson at google.com> wrote:
> >
> > tejohnson updated the summary for this revision.
> > tejohnson updated this revision to Diff 32027.
> > tejohnson added a comment.
> >
> > Removed native object wrapper support from this patch.
> >
> >
> > http://reviews.llvm.org/D11722
> >
> > Files:
> >  include/llvm/Bitcode/LLVMBitCodes.h
> >  include/llvm/Bitcode/ReaderWriter.h
> >  lib/Bitcode/Reader/BitcodeReader.cpp
> >  lib/Bitcode/Writer/BitcodeWriter.cpp
> >  tools/llvm-bcanalyzer/llvm-bcanalyzer.cpp
> >
> > <D11722.32027.patch>
> 
> (Sorry I'm so late getting to this patch.)
> 
> Hi Duncan,
> Thanks for the comments.
>  
> 
> I'm concerned about the direction.  IIUC, the purpose of this is faster
> lazy-loading.  Why do this in a side-channel?  Why not optimize the
> existing lazy-loader?  I imagine the LTO model implemented by the gold
> plugin would benefit from this, for example.  If we need an extra index
> to make lazy-loading "fast enough", it seems like the index should be
> available (as an opt-in for time/space tradeoff) for other consumers as
> well.  Alternatively, maybe there's something we can change about how
> the existing lazy-loader works (or the layout of bitcode) to be more
> amenable to the partial loading ThinLTO needs.
> 
> Only one part of the index (the function's bitcode offset) is used to do the fast loading of the function from a given module. Much of the other information in the ThinLTO sections is to identify/locate the module to import from (via the module strtab index which is populated when creating the combined (global) index), and information used in making importing decisions from another module (like the function's size, hotness when there is profile, etc, which we don't have since we haven't yet parsed that other module).
> 
> The other issue with using the existing lazy loading support is that we may import from a number of other modules in some interleaved fashion, so we may open/import a function/close a module multiple times. My  understanding is that the lazy loading is more for the case of loading a few different functions in sequence.
> 
> So for example, if we have:
> 
> A.cc:
> 
> a() {
>    b1();
>    ... 
>    c1();
>    ...
>    b2();
>    ...
>    c2();
> }
> 
> B.cc:
> 
> b1() { d1(); }
> b2() { ... }
> 
> C.cc:
> 
> c1() { d2(); }
> c2() { ... }
> 
> D.cc:
> 
> d1() { ... }
> d2() { ... }
> 
> 
> When compiling A.cc through the parallel backend's importing pass, we may have the following sequence of events:
> 
> Consider importing b1  -> Decide to import b1  (exposes call to d1)
> Consider importing d1  -> Decide to import d1
> Consider importing c1  -> Decide to import c1  (exposes call to d2)
> Consider importing d2  -> Decide to import d2
> Consider importing b2  -> Decide *not* to import b2
> Consider importing c2  -> Decide to import c2
> 
> For each of the inter-module calls considered in some priority based order, we would look up the callee in the combined index, which provides some information such as the callee's size, hotness, etc. A decision on whether to import is made based on that information (without doing any parsing of the callee's module). If we decide to import, then the combined index entry for the callee provides the callee module path and the bitcode index, which is used to open the module and import just that function, then the callee's module is closed. Some of the lazy loading support could potentially be used in place of the bitcode index, although I am not convinced it is suited for that purpose. In any case, that is just a small part of what is contained in the function summary/index as described earlier.
> 
> On the flip side, the bitcode index could be used by the lazy loader, in place of building the index on the fly. I'm not sure how much benefit there is to lazy loading, which eventually needs to read the entire module once a function is materialized.

It seems generally useful to be able to cherry-pick functions out of a
large bitcode file without scanning the whole thing.  It's really cool
that you can teardown and buildup the BitcodeReader, swapping different
modules in and out.

I don't like this being tied to ThinLTO, since I think it's useful for
other consumers as well.  To start, the old/normal lazy loader should
use the index when it's available.  I'm sure other people will think of
other crazy uses.

One possible use: could llvm-ar use this index (when available) instead
of walking through all the functions?

> Not included in this patch is support for lazy-loading of metadata,
> something you suggested you had support for in your prototype (and I
> assume ThinLTO relies on it).  I think it's particularly important that
> whatever you do there isn't ThinLTO-specific.
> 
> 
> Right, that is needed when we do the importing. This initial patch is just to write/read the function summary/index. I had briefly outlined the way I was hoping to stage patches earlier today (http://lists.llvm.org/pipermail/llvm-dev/2015-August/089222.html). I was planning to do the importing as a separate set of patches once I have the infrastructure for generating/reading the function indexes as that is needed first to drive the importing. Let me know if the staging of work I describe there makes sense.

(What's difficult for me with the staging is how many threads are going
on in parallel.  This stuff is all related, and some of the patches
don't make sense outside the context of the others, and if we need
changes in one, that'll influence the design of the others.  I don't
really see why the review isn't all in one place.  Maybe I just need to
stop worrying and learn to love my inbox, though... and I guess if I'd
responded more promptly to the RFCs you wouldn't have unleashed all the
threads.)

> To facilitate the process of reading/materializing a single function each time a module is imported from, as described above, I we be parsing the module level metadata once as a post-pass. Otherwise supporting the interleaved importing from multiple modules is probably very difficult.

I seem to remember that you only pull in the necessary metadata?  I'm
wondering how you do this, particularly for debug info, which has all
sorts of circular references.  In particular, !llvm.dbg.cu seems like it
could pull in all the subprograms (and their variables) even if you've
just cherry-picked a couple of functions.  How do you get around that?

(I've been planning some debug info schema changes to make this sort of
thing easier, but were you able to do this without them?  How?
(Frankly, it'd be helpful to look at the complete prototype (warts and
all) if it's available somewhere.))

> This is again different than the existing lazy loading, which is parsing/materializing all necessary functions and metadata out of each module in a single pass.

Right, but during "normal" lazy loading, function bodies can be deleted
before they're loaded, so that they're never loaded at all.  It would be
awesome to delay loading metadata until we know what is actually
needed/used, so it's not all leaked onto the LLVMContext.

----

Stepping back a little, I have felt (and, sadly, have probably been)
confrontational about the ThinLTO work.  It's important to say: this is
a really cool approach, and I want it in tree (somehow).

But at a high level, it feels like you've examined how LLVM does all
these things, determined (correctly!) that it's not good enough for the
ThinLTO use case, and created a new approach for ThinLTO.

I'd rather see patches that fix the current LLVM approach.  Even when
there's a tradeoff (for example, the index takes extra disk space, so
that should be opt-in (via a -cc1 flag like
-emit-llvm-function-index?)), the new awesome shouldn't be ThinLTO-only.

> Let me know if this seems reasonable given the above example and description.
> 
> Thanks,
> Teresa
> 
> 
> -- 
> Teresa Johnson |	 Software Engineer |	 tejohnson at google.com |	 408-460-2413