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

Xinliang David Li via llvm-dev llvm-dev at lists.llvm.org
Mon Jul 3 12:07:32 PDT 2017

With the recent change of profile dumping (merge mode), the IO issue with
name write should no longer be a problem (it is written only once).

For coverage mapping data, another possible solution is to introduce a
post-link tool that strips and compresses the coverage mapping data from
the final binary and copies it to a different file. This step can be
manually done by the user or by the compiler driver when coverage mapping
is on. The name data can be copied too, but it requires slight
llvm-profdata work flow change under a flag.


On Fri, Jun 30, 2017 at 5:54 PM, <vsk at apple.com> wrote:

> 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
>     - 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.
> 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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