[llvm-dev] [RFC] Placing profile name data, and coverage data, outside of object files

Sean Silva via llvm-dev llvm-dev at lists.llvm.org
Fri Jun 30 22:04:07 PDT 2017

On Fri, Jun 30, 2017 at 5:54 PM, via llvm-dev <llvm-dev at lists.llvm.org>

> Problem
> -------
> Instrumentation for PGO and frontend-based coverage places a large amount
> of
> data in object files, even though the majority of this data is not needed
> at
> run-time. All the data is needlessly duplicated while generating archives,
> and
> again while linking. PGO name data is written out into raw profiles by
> instrumented programs, slowing down the training and code coverage
> workflows.
> Here are some numbers from a coverage + RA build of ToT clang:
>   * Size of the build directory: 4.3 GB
>   * Wall time needed to run "clang -help" with an SSD: 0.5 seconds
>   * Size of the clang binary: 725.24 MB
>   * Space wasted on duplicate name/coverage data (*.o + *.a): 923.49 MB
>     - Size contributed by __llvm_covmap sections: 1.02 GB
>       \_ Just within clang: 340.48 MB

We live with this duplication for debug info. In some sense, if the
overhead is small compared to debug info, should we even bother (i.e., we
assume that users accommodate debug builds, so that is a reasonable bound
on the tolerable build directory size). (I don't know the numbers; this
seems pretty large so maybe it is significant compared to debug info; just
saying that looking at absolute numbers is misleading here; numbers
compared to debug info are a closer measure to the user's perceptions)

In fact, one overall architectural observation I have is that the most
complicated part of all this is simply establishing the workflow to plumb
together data emitted per-TU to a tool that needs that information to do
some post-processing step on the results of running the binary. That sounds
a lot like the role of debug info. In fact, having a debugger open a core
file is precisely equivalent to what llvm-profdata needs to do in this
regard AFAICT.

So it would be best if possible to piggyback on all the effort that has
gone into plumbing that data to make debug info work. For example, I know
that on Darwin there's a fair amount of system-level integration to make
split dwarf "just work" while keeping debug info out of final binaries.

If there is a not-too-hacky way to piggyback on debug info, that's likely
to be a really slick solution. For example, debug info could in principle
(if it doesn't already) contain information about the name of each counter
in the counter array, so in principle it would be a complete enough
description to identify each counter.

I'm not very familiar with DWARF, but I'm imagining something like
reserving an LLVM vendor-specific DWARF opcode/attribute/whatever and then
stick a blob of data in there. Presumably we have code somewhere in LLDB
that is "here's a binary, find debug info for it", and in principle we
could factor out that code and lift it into an LLVM library
(libFindDebugInfo) that llvm-profdata could use.

>     - Size contributed by __llvm_prf_names sections: 327.46 MB
>       \_ Just within clang: 106.76 MB
>     => Space wasted within the clang binary: 447.24 MB
> Running an instrumented clang binary triggers a 143MB raw profile write
> which
> is slow even with an SSD. This problem is particularly bad for
> frontend-based
> coverage because it generates a lot of extra name data: however, the
> situation
> can also be improved for PGO instrumentation.
> Proposal
> --------
> Place PGO name data and coverage data outside of object files. This would
> eliminate data duplication in *.a/*.o files, shrink binaries, shrink raw
> profiles, and speed up instrumented programs.
> In more detail:
> 1. The frontends get a new `-fprofile-metadata-dir=<path>` option. This
> lets
> users specify where llvm will store profile metadata. If the metadata
> starts to
> take up too much space, there's just one directory to clean.
> 2. The frontends continue emitting PGO name data and coverage data in the
> same
> llvm::Module. So does LLVM's IR-based PGO implementation. No change here.
> 3. If the InstrProf lowering pass sees that a metadata directory is
> available,
> it constructs a new module, copies the name/coverage data into it, hashes
> the
> module, and attempts to write that module to:
>   <metadata-dir>/<module-hash>.bc   (the metadata module)
> If this write operation fails, it scraps the new module: it keeps all the
> metadata in the original module, and there are no changes from the current
> process. I.e with this proposal we preserve backwards compatibility.

Based at my experience with Clang's implicit modules, I'm *extremely* wary
of anything that might cause the compiler to emit a file that the build
system cannot guess the name of. In fact, having the compiler emit a file
that is not explicitly listed on the command line is basically just as bad
in practice (in terms of feasibility of informing the build system about

As a simple example, ninja simply cannot represent a dependency of this
type, so if you delete a <metadata-dir>/<module-hash>.bc it won't know
things need to be rebuilt (and it won't know how to clean it, etc.).

So I would really strongly recommend against doing this.

Again, these problems of system integration (in particular build system
integration) are nasty, and if you can bypass this and piggyback on debug
info then everything will "just work" because the folks that care about
making sure that debugging "just works" already did the work for you.
It might be more work in the short term to do the debug info approach (if
it is feasible at all), but I can tell you based on the experience with
implicit modules (and I'm sure you have some experience of your own) that
there's just going to be a neverending tail of hitches and ways that things
don't work (or work poorly) due to not having the build system / overall
system integration right, so it will be worth it in the long run.

-- Sean Silva

> 4. Once the metadata module is written, the name/coverage data are entirely
> stripped out of the original module. They are replaced by a path to the
> metadata module:
>   @__llvm_profiling_metadata = "<metadata-dir>/<module-hash>.bc",
>                                section "__llvm_prf_link"
> This allows incremental builds to work properly, which is an important use
> case
> for code coverage users. When an object is rebuilt, it gets a fresh link
> to a
> fresh profiling metadata file. Although stale files can accumulate in the
> metadata directory, the stale files cannot ever be used.
> In an IDE like Xcode, since there's just one target binary per scheme, it's
> possible to clean the metadata directory by removing the modules which
> aren't
> referenced by the target binary.
> 5. The raw profile format is updated so that links to metadata files are
> written
> out in each profile. This makes it possible for all existing llvm-profdata
> and
> llvm-cov commands to work, seamlessly.
> The indexed profile format will *not* be updated: i.e, it will contain a
> full
> symbol table, and no links. This simplifies the coverage mapping reader,
> because
> a full symbol table is guaranteed to exist before any function records are
> parsed. It also reduces the amount of coding, and makes it easier to
> preserve
> backwards compatibility :).
> 6. The raw profile reader will learn how to read links, open up the
> metadata
> modules it finds links to, and collect name data from those modules.
> 7. The coverage reader will learn how to read the __llvm_prf_link section,
> open
> up metadata modules, and lazily read coverage mapping data.
> Alternate Solutions
> -------------------
> 1. Instead of copying name data into an external metadata module, just
> copy the
> coverage mapping data.
> I've actually prototyped this. This might be a good way to split up
> patches,
> although I don't see why we wouldn't want to tackle the name data problem
> eventually.
> 2. Instead of emitting links to external metadata modules, modify llvm-cov
> and
> llvm-profdata so that they require a path to the metadata directory.
> The issue with this is that it's way too easy to read stale metadata. It's
> also
> less user-friendly, which hurts adoption.
> 3. Use something other than llvm bitcode for the metadata module format.
> Since we're mostly writing large binary blobs (compressed name data or
> pre-encoded source range mapping info), using bitcode shouldn't be too
> slow, and
> we're not likely to get better compression with a different format.
> Bitcode is also convenient, and is nice for backwards compatibility.
> ------------------------------------------------------------
> --------------------
> If you've made it this far, thanks for taking a look! I'd appreciate any
> feedback.
> vedant
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