[flang] Flang goes into infinite loop when compiling gfortran test: strictly-structured-block-1.f90

[tarunprabhu]

Steps to reproduce:

Assuming that the LLVM test suite is checked out in ./llvm-test-suite, compile the test as follows

$ flang -fopenmp ./llvm-test-suite/Fortran/gfortran/regression/gomp/strictly-structured-block-1.f90

Actual behavior: the compiler seems to be in an infinite loop. The memory usage as reported by top seems to be insignificant. Compilation did not terminate even after at least 5 minutes on both x86_64 and aarch64.

Expected behavior: Compilation should terminate successfully. If some OpenMP construct used in the test is not supported yet, flang should crash with a TODO message.

[eugeneepshteyn]

I started flang -fc1 -fopenmp strictly-structured-block-1.f90 in gdb, let it run for a while, then CTRL-C. I did this a couple times and both times it broke in parsing. The backtrace contained between 400 and 640 calls, so something strange is going on with the parser in this case. The source file strictly-structured-block-1.f90 doesn't seem to be very complicated.

[eugeneepshteyn]

Partial backtrace:

#0  std::__1::__compressed_pair<unsigned long, std::__1::allocator<std::__1::__list_node<Fortran::parser::Message, void*> > >::second[abi:nn190106]() (this=0x7ffffffdc928)
    at /home/eepshteyn/compilers/clang-19.1.6/bin/../include/c++/v1/__memory/compressed_pair.h:138
#1  0x0000555560e6e8d9 in std::__1::__list_imp<Fortran::parser::Message, std::__1::allocator<Fortran::parser::Message> >::__node_alloc[abi:nn190106]() (this=0x7ffffffdc918) at /home/eepshteyn/compilers/clang-19.1.6/bin/../include/c++/v1/list:508
#2  0x0000555563bfbcf1 in std::__1::list<Fortran::parser::Message, std::__1::allocator<Fortran::parser::Message> >::list (
    this=0x7ffffffda810, __c=...) at /home/eepshteyn/compilers/clang-19.1.6/bin/../include/c++/v1/list:1053
#3  0x0000555563bfba0d in Fortran::parser::Messages::Messages (this=0x7ffffffda810, that=...)
    at /home/eepshteyn/src/flang-upstream/llvm-project/flang/include/flang/Parser/message.h:322
#4  0x0000555563c0766f in Fortran::parser::ParseState::ParseState (this=0x7ffffffda800, that=...)
    at /home/eepshteyn/src/flang-upstream/llvm-project/flang/include/flang/Parser/parse-state.h:48
#5  0x0000555563cf9083 in Fortran::parser::AlternativesParser<Fortran::parser::ApplyConstructor<Fortran::parser::IntrinsicTypeSpec, Fortran::parser::Parser<Fortran::parser::IntegerTypeSpec> >, Fortran::parser::ApplyConstructor<Fortran::parser::IntrinsicTypeSpec, Fortran::parser::ApplyConstructor<Fortran::parser::IntrinsicTypeSpec::Real, Fortran::parser::SequenceParser<Fortran::parser::TokenStringMatch<false, false>, Fortran::parser::MaybeParser<Fortran::parser::Parser<Fortran::parser::KindSelector> > > > >, Fortran::parser::ApplyConstructor<Fortran::parser::IntrinsicTypeSpec, Fortran::parser::SequenceParser<Fortran::parser::TokenStringMatch<false, false>, Fortran::parser::ApplyConstructor<Fortran::parser::IntrinsicTypeSpec::DoublePrecision> > >, Fortran::parser::ApplyConstructor<Fortran::parser::IntrinsicTypeSpec, Fortran::parser::ApplyConstructor<Fortran::parser::IntrinsicTypeSpec::Complex, Fortran::parser::SequenceParser<Fortran::parser::TokenStringMatch<false, false>, Fortran::parser::MaybeParser<Fortran::parser::Parser<Fortran::parser::KindSelector> > > > >, Fortran::parser::ApplyConstructor<Fortran::parser::IntrinsicTypeSpec, Fortran::parser::ApplyConstructor<Fortran::parser::IntrinsicTypeSpec::Character, Fortran::parser::SequenceParser<Fortran::parser::TokenStringMatch<false, false>, Fortran::parser::MaybeParser<Fortran::parser::Parser<Fortran::parser::CharSelector> > > > >, Fortran::parser::ApplyConstructor<Fortran::parser::IntrinsicTypeSpec, Fortran::parser::ApplyConstructor<Fortran::parser::IntrinsicTypeSpec::Logical, Fortran::parser::SequenceParser<Fortran::parser::TokenStringMatch<false, false>, Fortran::parser::MaybeParser<Fortran::parser::Parser<Fortran::parser::KindSelector> > > > >, Fortran::parser::ApplyConstructor<Fortran::parser::IntrinsicTypeSpec, Fortran::parser::Parser<Fortran::parser::UnsignedTypeSpec> >, Fortran::parser::NonstandardParser<(Fortran::common::LanguageFeature)12, Fortran::parser::ApplyConstructor<Fortran::parser::IntrinsicTypeSpec, Fortran::parser::SequenceParser<Fortran::parser::TokenStringMatch<true, false>, Fortran::parser::ApplyConstructor<Fortran::parser::IntrinsicTypeSpec::DoubleComplex> > > >, Fortran::parser::NonstandardParser<(Fortran::common::LanguageFeature)13, Fortran::parser::ApplyConstructor<Fortran::parser::IntrinsicTypeSpec, Fortran::parser::ApplyConstructor<Fortran::parser::IntegerTypeSpec, Fortran::parser::SequenceParser<Fortran::parser::TokenStringMatch<false, false>, Fortran::parser::ApplyConstructor<std::__1::optional<Fortran::parser::KindSelector>, Fortran::parser::PureParser<int> > > > > > >::ParseRest<8> (
    this=0x555569f8fb28 <Fortran::parser::Parser<Fortran::parser::IntrinsicTypeSpec>::Parse(Fortran::parser::ParseState&)::parser+48>, result=..., state=..., backtrack=...) at /home/eepshteyn/src/flang-upstream/llvm-project/flang/lib/Parser/basic-parsers.h:327
...
    this=0x555569f89768 <Fortran::parser::Parser<Fortran::parser::Program>::Parse(Fortran::parser::ParseState&)::parser>, 
    state=...) at /home/eepshteyn/src/flang-upstream/llvm-project/flang/lib/Parser/basic-parsers.h:316
#488 0x0000555563c092a8 in _ZNK7Fortran6parser16ApplyConstructorINS0_7ProgramEJNS0_18AlternativesParserINS0_17NonstandardParserILNS_6common15LanguageFeatureE55ENS0_14SequenceParserINS7_INS0_24SkipStuffBeforeStatementENS0_13NegatedParserINS0_6NextChEEEEENS0_17PureDefaultParserINSt3__14listINS0_11ProgramUnitENSE_9allocatorISG_EEEEEEEEEEJNS0_12FollowParserINS0_10SomeParserINS3_INS3_INS1_ISG_JNS1_INS5_11IndirectionINS0_17CompilerDirectiveELb0EEEJNS0_6ParserISQ_EEEEEEEEJNS1_ISG_JNS1_INSP_INS0_23OpenACCRoutineConstructELb0EEEJNS7_INS7_IS8_NS0_16TokenStringMatchILb1ELb0EEEEENSS_ISW_EEEEEEEEEEEEEJNS7_INS0_18StartNewSubprogramENSN_INSN_INSN_INS3_INS3_INS3_INS3_INS3_INS1_ISG_JNS1_INSP_INS0_6ModuleELb0EEEJNSS_IS17_EEEEEEEEJNS1_ISG_JNS1_INSP_INS0_20SubroutineSubprogramELb0EEEJNSS_IS1C_EEEEEEEEEEEJNS1_ISG_JNS1_INSP_INS0_9SubmoduleELb0EEEJNSS_IS1I_EEEEEEEEEEEJNS1_ISG_JNS1_INSP_INS0_9BlockDataELb0EEEJNSS_IS1O_EEEEEEEEEEEJNS7_INS0_15LookAheadParserINS7_INS0_11MaybeParserINS7_INS0_5SpaceENSN_INS0_13DigitString64ENS0_10SpaceCheckEEEEEEENS3_INSN_INS1_INS0_12FunctionStmtEJNS0_10ManyParserINSS_INS0_10PrefixSpecEEEEENS7_INSY_ILb0ELb0EEENSS_INS0_4NameEEEEENS7_IS27_NSN_INS0_15DefaultedParserINS0_17NonemptySeparatedIS29_S27_EEEES27_EEEENS1V_INSS_INS0_6SuffixEEEEEEEENS7_IS1W_NS0_17WithMessageParserINS1U_INS0_10AnyOfCharsEEEEEEEEEJNS1_IS22_JS26_NS7_IS27_NSN_IS29_S2P_EEEENS4_ILS6_33ENSD_INSF_IS28_NSH_IS28_EEEEEEEENSD_INSE_8optionalIS2H_EEEEEEEEEEEEEENS1_ISG_JNS1_INSP_INS0_18FunctionSubprogramELb0EEEJNSS_IS34_EEEEEEEEEEEEEJNS1_ISG_JNS1_INSP_INS0_11MainProgramELb0EEEJNSS_IS3B_EEEEEEEEEEENS0_14SkipManyParserIS27_EEEES1W_EENS0_14RecoveryParserINS3_INS7_IS2M_NS0_8OkParserEEEJNS0_10FailParserINS0_7SuccessEEEEEENS0_8SkipPastILc10EEEEEEEEEEEEEES8_EEEEEEE8ParseOneERNS0_10ParseStateE (
    this=0x555569f89768 <Fortran::parser::Parser<Fortran::parser::Program>::Parse(Fortran::parser::ParseState&)::parser>, 
    state=...) at /home/eepshteyn/src/flang-upstream/llvm-project/flang/lib/Parser/basic-parsers.h:748
#489 0x0000555563bf7f88 in _ZNK7Fortran6parser16ApplyConstructorINS0_7ProgramEJNS0_18AlternativesParserINS0_17NonstandardParserILNS_6common15LanguageFeatureE55ENS0_14SequenceParserINS7_INS0_24SkipStuffBeforeStatementENS0_13NegatedParserINS0_6NextChEEEEENS0_17PureDefaultParserINSt3__14listINS0_11ProgramUnitENSE_9allocatorISG_EEEEEEEEEEJNS0_12FollowParserINS0_10SomeParserINS3_INS3_INS1_ISG_JNS1_INS5_11IndirectionINS0_17CompilerDirectiveELb0EEEJNS0_6ParserISQ_EEEEEEEEJNS1_ISG_JNS1_INSP_INS0_23OpenACCRoutineConstructELb0EEEJNS7_INS7_IS8_NS0_16TokenStringMatchILb1ELb0EEEEENSS_ISW_EEEEEEEEEEEEEJNS7_INS0_18StartNewSubprogramENSN_INSN_INSN_INS3_INS3_INS3_INS3_INS3_INS1_ISG_JNS1_INSP_INS0_6ModuleELb0EEEJNSS_IS17_EEEEEEEEJNS1_ISG_JNS1_INSP_INS0_20SubroutineSubprogramELb0EEEJNSS_IS1C_EEEEEEEEEEEJNS1_ISG_JNS1_INSP_INS0_9SubmoduleELb0EEEJNSS_IS1I_EEEEEEEEEEEJNS1_ISG_JNS1_INSP_INS0_9BlockDataELb0EEEJNSS_IS1O_EEEEEEEEEEEJNS7_INS0_15LookAheadParserINS7_INS0_11MaybeParserINS7_INS0_5SpaceENSN_INS0_13DigitString64ENS0_10SpaceCheckEEEEEEENS3_INSN_INS1_INS0_12FunctionStmtEJNS0_10ManyParserINSS_INS0_10PrefixSpecEEEEENS7_INSY_ILb0ELb0EEENSS_INS0_4NameEEEEENS7_IS27_NSN_INS0_15DefaultedParserINS0_17NonemptySeparatedIS29_S27_EEEES27_EEEENS1V_INSS_INS0_6SuffixEEEEEEEENS7_IS1W_NS0_17WithMessageParserINS1U_INS0_10AnyOfCharsEEEEEEEEEJNS1_IS22_JS26_NS7_IS27_NSN_IS29_S2P_EEEENS4_ILS6_33ENSD_INSF_IS28_NSH_IS28_EEEEEEEENSD_INSE_8optionalIS2H_EEEEEEEEEEEEEENS1_ISG_JNS1_INSP_INS0_18FunctionSubprogramELb0EEEJNSS_IS34_EEEEEEEEEEEEEJNS1_ISG_JNS1_INSP_INS0_11MainProgramELb0EEEJNSS_IS3B_EEEEEEEEEEENS0_14SkipManyParserIS27_EEEES1W_EENS0_14RecoveryParserINS3_INS7_IS2M_NS0_8OkParserEEEJNS0_10FailParserINS0_7SuccessEEEEEENS0_8SkipPastILc10EEEEEEEEEEEEEES8_EEEEEEE5ParseERNS0_10ParseStateE (
    this=0x555569f89768 <Fortran::parser::Parser<Fortran::parser::Program>::Parse(Fortran::parser::ParseState&)::parser>, 
    state=...) at /home/eepshteyn/src/flang-upstream/llvm-project/flang/lib/Parser/basic-parsers.h:729
#490 0x0000555563bf7047 in Fortran::parser::Parser<Fortran::parser::Program>::Parse (state=...)
    at /home/eepshteyn/src/flang-upstream/llvm-project/flang/lib/Parser/program-parsers.cpp:86
#491 0x0000555563bd0079 in Fortran::parser::Parsing::Parse (this=0x55556a186a60, out=...)
    at /home/eepshteyn/src/flang-upstream/llvm-project/flang/lib/Parser/parsing.cpp:287
#492 0x000055555f17fd73 in Fortran::frontend::FrontendAction::runParse (this=0x55556a1ba080, emitMessages=false)
    at /home/eepshteyn/src/flang-upstream/llvm-project/flang/lib/Frontend/FrontendAction.cpp:165
#493 0x000055555f18dea2 in Fortran::frontend::PrescanAndSemaAction::beginSourceFileAction (this=0x55556a1ba080)
    at /home/eepshteyn/src/flang-upstream/llvm-project/flang/lib/Frontend/FrontendActions.cpp:140
#494 0x000055555f17fa55 in Fortran::frontend::FrontendAction::beginSourceFile (this=0x55556a1ba080, ci=..., realInput=...)
    at /home/eepshteyn/src/flang-upstream/llvm-project/flang/lib/Frontend/FrontendAction.cpp:109
#495 0x000055555f147dca in Fortran::frontend::CompilerInstance::executeAction (this=0x55556a1b0220, act=...)
    at /home/eepshteyn/src/flang-upstream/llvm-project/flang/lib/Frontend/CompilerInstance.cpp:191
#496 0x000055555f18692c in Fortran::frontend::executeCompilerInvocation (flang=0x55556a1b0220)
    at /home/eepshteyn/src/flang-upstream/llvm-project/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp:222
#497 0x000055555e7a5c4a in fc1_main (argv=..., argv0=0x7fffffffdeb7 "/home/eepshteyn/compilers/flang-upstream/bin/flang")
    at /home/eepshteyn/src/flang-upstream/llvm-project/flang/tools/flang-driver/fc1_main.cpp:91
#498 0x000055555e7a0441 in executeFC1Tool (argV=...)
    at /home/eepshteyn/src/flang-upstream/llvm-project/flang/tools/flang-driver/driver.cpp:66
#499 0x000055555e79fcd6 in main (argc=4, argv=0x7fffffffdae8)
    at /home/eepshteyn/src/flang-upstream/llvm-project/flang/tools/flang-driver/driver.cpp:110
(gdb) 

[eugeneepshteyn]

Next step: reduce strictly-structured-block-1.f90 to try and find the construct that causes the parser to choke.

[klausler]

Kiran, please reassign to whomever takes care of OpenMP parsing.

[abidh]

I tried it today and it does not hang anymore but gives this error instead.

strictly-structured-block-1.f90:65:9: error: The structured block in a WORKSHARE construct may consist of only SCALAR or ARRAY assignments, FORALL or WHERE statements, FORALL, WHERE, ATOMIC, CRITICAL or PARALLEL constructs
    !$omp workshare

[llvmbot]

@llvm/issue-subscribers-flang-frontend

Author: Tarun Prabhu (tarunprabhu)

Steps to reproduce:

Assuming that the LLVM test suite is checked out in ./llvm-test-suite, compile the test as follows

$ flang -fopenmp ./llvm-test-suite/Fortran/gfortran/regression/gomp/strictly-structured-block-1.f90

Actual behavior: the compiler seems to be in an infinite loop. The memory usage as reported by top seems to be insignificant. Compilation did not terminate even after at least 5 minutes on both x86_64 and aarch64.

Expected behavior: Compilation should terminate successfully. If some OpenMP construct used in the test is not supported yet, flang should crash with a TODO message.