GeezerVison is showing github.com — Link-time-optimization with clang is broken · Issue #96761 · python/cpython
Link-time-optimization with clang is broken #96761DescriptionI tried to enable link time optimizations for clang.
export CC="clang"
export LD="clang"
nice "${here}/configure" \
--enable-optimizations \
--with-lto=thin
nice make -j8
Eventually building reaches this step:
clang -fprofile-instr-generate -o _bootstrap_python Modules/getbuildinfo.o Parser/token.o Parser/pegen.o Parser/pegen_errors.o Parser/action_helpers.o Parser/parser.o Parser/string_parser.o Parser/peg_api.o Parser/myreadline.o Parser/tokenizer.o Objects/abstract.o Objects/boolobject.o Objects/bytes_methods.o Objects/bytearrayobject.o Objects/bytesobject.o Objects/call.o Objects/capsule.o Objects/cellobject.o Objects/classobject.o Objects/codeobject.o Objects/complexobject.o Objects/descrobject.o Objects/enumobject.o Objects/exceptions.o Objects/genericaliasobject.o Objects/genobject.o Objects/fileobject.o Objects/floatobject.o Objects/frameobject.o Objects/funcobject.o Objects/interpreteridobject.o Objects/iterobject.o Objects/listobject.o Objects/longobject.o Objects/dictobject.o Objects/odictobject.o Objects/memoryobject.o Objects/methodobject.o Objects/moduleobject.o Objects/namespaceobject.o Objects/object.o Objects/obmalloc.o Objects/picklebufobject.o Objects/rangeobject.o Objects/setobject.o Objects/sliceobject.o Objects/structseq.o Objects/tupleobject.o Objects/typeobject.o Objects/unicodeobject.o Objects/unicodectype.o Objects/unionobject.o Objects/weakrefobject.o Objects/perf_trampoline.o Objects/asm_trampoline.o Python/_warnings.o Python/Python-ast.o Python/Python-tokenize.o Python/asdl.o Python/ast.o Python/ast_opt.o Python/ast_unparse.o Python/bltinmodule.o Python/ceval.o Python/codecs.o Python/compile.o Python/context.o Python/dynamic_annotations.o Python/errors.o Python/frame.o Python/frozenmain.o Python/future.o Python/getargs.o Python/getcompiler.o Python/getcopyright.o Python/getplatform.o Python/getversion.o Python/ceval_gil.o Python/hamt.o Python/hashtable.o Python/import.o Python/importdl.o Python/initconfig.o Python/marshal.o Python/modsupport.o Python/mysnprintf.o Python/mystrtoul.o Python/pathconfig.o Python/preconfig.o Python/pyarena.o Python/pyctype.o Python/pyfpe.o Python/pyhash.o Python/pylifecycle.o Python/pymath.o Python/pystate.o Python/pythonrun.o Python/pytime.o Python/bootstrap_hash.o Python/specialize.o Python/structmember.o Python/symtable.o Python/sysmodule.o Python/thread.o Python/traceback.o Python/getopt.o Python/pystrcmp.o Python/pystrtod.o Python/pystrhex.o Python/dtoa.o Python/formatter_unicode.o Python/fileutils.o Python/suggestions.o Python/dynload_shlib.o Modules/config.o Modules/main.o Modules/gcmodule.o Modules/atexitmodule.o Modules/faulthandler.o Modules/posixmodule.o Modules/signalmodule.o Modules/_tracemalloc.o Modules/_codecsmodule.o Modules/_collectionsmodule.o Modules/errnomodule.o Modules/_io/_iomodule.o Modules/_io/iobase.o Modules/_io/fileio.o Modules/_io/bytesio.o Modules/_io/bufferedio.o Modules/_io/textio.o Modules/_io/stringio.o Modules/itertoolsmodule.o Modules/_sre/sre.o Modules/_threadmodule.o Modules/timemodule.o Modules/_weakref.o Modules/_abc.o Modules/_functoolsmodule.o Modules/_localemodule.o Modules/_operator.o Modules/_stat.o Modules/symtablemodule.o Modules/pwdmodule.o Programs/_bootstrap_python.o Modules/getpath.o -ldl -lm
And fails:
Modules/getbuildinfo.o: file not recognized: file format not recognized
clang-14: error: linker command failed with exit code 1 (use -v to see invocation)
Your environment
I'm on Archlinux x86-64.
$ clang --version
clang version 14.0.6
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
I tested against main. Reacted by Rin Arakaki and Claudia Pellegrino Activitycorona10 commented on Sep 12, 2022 -no-lto flag was first introduced at #29859 by @tiran
Would you like to take a look at this issue please? matthiasgoergens commented on Sep 12, 2022 ContributorAuthor More actions @corona10 Thanks! That seems to be mostly about speeding up the GCC build. LLVM is already pretty fast with thin-lto. corona10 commented on Sep 13, 2022 I will try to reproduce the issue in my Linux environment with clang14 before reviewing the PR.
cc @tiran corona10 commented on Sep 16, 2022 Okay, I am able to reproduce the issue even with the lld. Reacted by Matthias Görgens matthiasgoergens commented on Sep 17, 2022 ContributorAuthor More actions Thanks for confirming that it's not just some idiosyncratic weirdness of my environment. corona10 commented on Sep 18, 2022 For @matthiasgoergens, @tiran @vstinner cc @pablogsal as release manager.
My conclusion
- At this moment
Modules/*.o are actually LLVM bitcode not object files.
- so the LTO processor needs to transform the LLVM bitcode into the object file for the traditional linker but looks like without
-flto it doesn't transform to object files for the traditional linker.
- This issue not only happens for ThinLTO but also in full LTO because both processes have the phase for transforming the LLVM bitcode into the object file.
- see http://blog.llvm.org/2016/06/thinlto-scalable-and-incremental-lto.html
- IMHO we can add the transformation phase for
-fno-lto but it could be complicated, I prefer using gh-96761: Fix lto-build for clang #96762 as a workaround approach and we need to backport to Python 3.11 too.
- But I am still curious about the fact why apple clang does not raise this issue.
Reference
file format
corona10@python-dev:~/cpython$ file Modules/getbuildinfo.o
Modules/getbuildinfo.o: LLVM IR bitcode clang full LTO process
clang ThinLTO process
Reacted by Matthias Görgens and Claudia Pellegrino Reacted by Matthias Görgens and Rin Arakaki pablogsal commented on Sep 18, 2022 Why is this only affecting 3.11 and not previous version? corona10 commented on Sep 18, 2022
Why is this only affecting 3.11 and not previous version?
because #29859 was first introduced in Python 3.11 for _bootstrap_python. corona10 commented on Sep 18, 2022 When building the final binary for CPython, we actually pass the -flto flags that the process will transform the LLVM bitcode into object files, so this issue only affects while building the _bootstrap_python.
I didn't check if the issue will affect to old versions (< 3.10) corona10 commented on Sep 18, 2022 Manual transformation example:
corona10@python-dev:~/cpython$ llc-14 -filetype=obj Modules/getbuildinfo.o -o Modules/getbuildinfo.o
corona10@python-dev:~/cpython$ file Modules/getbuildinfo.o
Modules/getbuildinfo.o: ELF 64-bit LSB relocatable, x86-64, version 1 (GNU/Linux), with debug_info, not stripped
pablogsal commented on Sep 18, 2022 Thanks for the quick answer and great analysis @corona10. I think the fix for this should go to 3.11.1 as this likely involve configure changes unless someone fundamentally disagrees. matthiasgoergens commented on Sep 18, 2022 ContributorAuthor More actions As far as I can tell, building _bootstrap_python with different flags than the full Python binary was only a hack to speed up the build, wasn't it?
Thin LTO is pretty fast, and compatible with the other LTO settings (apart perhaps from no-LTO), so we can build the bootstrap with that one, if any LTO is requested for the main build? corona10 commented on Sep 19, 2022
Thin LTO is pretty fast, and compatible with the other LTO settings (apart perhaps from no-LTO), so we can build the bootstrap with that one, if any LTO is requested for the main build?
Please follow my last review: #96762 (comment)
Since we should support all C11 compilers including old clangs which doesn't support ThinLTO, the configuration should use ThinLTO only if the compiler supports it. We should not break the build for old compilers too.
see: https://clang.llvm.org/c_status.html and https://peps.python.org/pep-0007/#c-dialect matthiasgoergens commented on Sep 19, 2022 ContributorAuthor More actions @corona10 Yes, definitely. I need to add checks. added 2 commits that reference this issue on Sep 20, 2022pythongh-96761: Fix build process of the clang compiler for _bootstra… gh-96761: Fix build process of the clang compiler for _bootstrap_pyth… added 2 commits that reference this issue on Sep 23, 2022pythongh-96761: Fix build process of the clang compiler for _bootstra… gh-96761: Fix build process of the clang compiler for _bootstrap_pyth… corona10 commented on Sep 23, 2022 All patches are merged.
Feel free to reopen if we need other solutions.
cc @tiran @pablogsal
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