InstrProf backward compatibility

Project Infrastructure LLVM Dev List Archives
1

Hi all,

I am working on adding PGO to LDC (LLVM D Compiler). The current implementation

  1. uses LLVM’s InstrProf pass to generate an instrumented executable

  2. links to compiler-rt/lib/profile for the runtime functionality to write a raw profile data file

  3. uses llvm-profdata to merge profile data and convert from profraw to profdata format

  4. uses llvm::IndexedInstrProfReader to read-in the profdata file for the 2nd PGO compile pass

This all works well with LLVM 3.7. While testing this with LLVM trunk, I run into a version compatibility problem between different versions of LLVM: I made a local copy of the compiler-rt/lib/profile release_37 version (with a few fixes for MSVC) and so steps 1,2, and 4 use LLVM trunk, but step 2 uses an older version of compiler-rt/lib/profile. Apparently, trunk’s llvm-profdata cannot read raw profile data files generated by compiler-rt 3.7. Is this intended? Is llvm-profdata’s output also incompatible between different versions of LLVM (InstrProfWriter/Reader are not meant to be compatible across LLVM versions) ?

LDC is currently compatible with LLVM 3.5 up to trunk. To support PGO for multiple LLVM versions (>= 3.7), I think that we will need to have a copy of compiler-rt/lib/profile for each version of LLVM >= 3.7. We also need to have a copy of llvm-profdata for each version of LLVM >= 3.7. We already do this with the FileCheck utility and will not be a big problem.

Does that make sense? What do you advise?

Thanks very much for your comments,

Johan

2

Hi Johan,

Hi all,
  I am working on adding PGO to LDC (LLVM D Compiler). The current implementation
1) uses LLVM's InstrProf pass to generate an instrumented executable
2) links to compiler-rt/lib/profile for the runtime functionality to write a raw profile data file
3) uses llvm-profdata to merge profile data and convert from profraw to profdata format
4) uses llvm::IndexedInstrProfReader to read-in the profdata file for the 2nd PGO compile pass

This all works well with LLVM 3.7. While testing this with LLVM trunk, I run into a version compatibility problem between different versions of LLVM: I made a local copy of the compiler-rt/lib/profile release_37 version (with a few fixes for MSVC) and so steps 1,2, and 4 use LLVM trunk, but step 2 uses an older version of compiler-rt/lib/profile. Apparently, trunk's llvm-profdata cannot read raw profile data files generated by compiler-rt 3.7. Is this intended?

Yes, as far as I'm aware, the raw profile format is meant to buy llvm flexibility. There are backwards compatibility guarantees for the indexed (*.profdata) format. I'm not sure if these guarantees have been written down yet -- if they haven't I'll put a document together.

The profraw/profdata split is analogous to the textual IR / bitcode split.

Is llvm-profdata's output also incompatible between different versions of LLVM (InstrProfWriter/Reader are not meant to be compatible across LLVM versions) ?

The indexed profdata reader for version X+1 of the format should be able to read versions <= X. There are no forward-compatibility promises: the reader for version X may not be able to read version X+k files.

LDC is currently compatible with LLVM 3.5 up to trunk. To support PGO for multiple LLVM versions (>= 3.7), I think that we will need to have a copy of compiler-rt/lib/profile for each version of LLVM >= 3.7. We also need to have a copy of llvm-profdata for each version of LLVM >= 3.7. We already do this with the FileCheck utility and will not be a big problem.

You should only ever need the latest compiler-rt you can use (i.e, one compatible with your version of llvm). That should work if you only rely on backwards compatibility for *.profdata files.

Does this work for your purposes? Is there anything we can do to make life easier for downstream projects?

best,
vedant

3

Hi all,
  I am working on adding PGO to LDC (LLVM D Compiler). The current
implementation
1) uses LLVM's InstrProf pass to generate an instrumented executable
2) links to compiler-rt/lib/profile for the runtime functionality to write a
raw profile data file
3) uses llvm-profdata to merge profile data and convert from profraw to
profdata format
4) uses llvm::IndexedInstrProfReader to read-in the profdata file for the
2nd PGO compile pass

This all works well with LLVM 3.7. While testing this with LLVM trunk, I run
into a version compatibility problem between different versions of LLVM: I
made a local copy of the compiler-rt/lib/profile release_37 version (with a
few fixes for MSVC) and so steps 1,2, and 4 use LLVM trunk, but step 2 uses
an older version of compiler-rt/lib/profile. Apparently, trunk's

Runtime and InstrProf pass have to be in sync. You can not use 3.7
version of runtime with trunk version of 1).

Perhaps you want to explain more about your use cases? If you want to
use 3.7 compiler/runtime to produce profile to be used by trunk
compiler -- that is doable.

1) instrument the code with 3.7 compiler and link with 3.7 runtime
2) run and dump the profile
3) use 3.7 llvm-profdata tool to do a merge and generate indexed format profile
4) use that profile with the trunk compiler

However, the other direction does not work -- you can not expect older
version of compiler to consume indexed format profile of the new
compiler.

llvm-profdata cannot read raw profile data files generated by compiler-rt
3.7. Is this intended?

yes. New version of raw profile reader does not understand old version
of raw profile data. You need to use older version of llvm-profdata to
produce old version of indexed format, and then use new version of
llvm-profdata to convert the indexed format into new version. Here is
flow:

1) llvm-profdata_3.7 merge -o my_profdata_old.profdata my_profdata_old.profraw

At this point, my_profdata_old.profdata *can* be consumed by trunk
version of compiler already, but if you want to convert it to new
version of indexed format, do this:

2) llvm-profdata_trunk merge -o my_profdata_new.profdata
my_profdata_old.profdata

Now my_profdata_new.profdata is in shiny new version (indexed).

Is llvm-profdata's output also incompatible between

different versions of LLVM (InstrProfWriter/Reader are not meant to be
compatible across LLVM versions) ?

The merge output format (indexed format) is backward compatible as
explained by Vedant.

LDC is currently compatible with LLVM 3.5 up to trunk. To support PGO for
multiple LLVM versions (>= 3.7), I think that we will need to have a copy of
compiler-rt/lib/profile for each version of LLVM >= 3.7. We also need to
have a copy of llvm-profdata for each version of LLVM >= 3.7. We already do
this with the FileCheck utility and will not be a big problem.

yes -- see above. Remember compiler instrumentation and supporting
runtime must be in sync.

thanks,

David

4

Thanks for the clear answers.

At the moment, my only request would be to also make sure that profile lib works with MSVC (3.7 needed small patching, trunk is fine now).

Thanks!

Johan