[flang] compilation hangs in CheckScopeConstraints

[k-arrows]

The following test program causes a hang, and it is documented in llvm-test-suite. (See https://github.com/llvm/llvm-test-suite/blob/5942ca45a1036ace15b70e117d636ecf89224957/Fortran/gfortran/regression/DisabledFiles.cmake#L1974)
https://github.com/llvm/llvm-test-suite/blob/main/Fortran/gfortran/regression/goto_5.f90

I will write here what I confirmed about this hang using the perf command.

perf record & perf report:

$ timeout 10 perf record -g flang-new -c goto_5.f90
[ perf record: Woken up 10 times to write data ]
[ perf record: Captured and wrote 2.393 MB perf.data (39133 samples) ]
$ perf report | c++filt

Here is the output of the perf report command:

    99.88%    99.88%  flang-new  flang-new             [.] Fortran::semantics::CheckScopeConstraints(std::vector<Fortran::semantics::SourceStatementInfoTuplePOD, std::allocator<Fortran::semantics::SourceStatementInfoTuplePOD> > const&, std::map<unsigned long, Fortran::semantics::LabeledStatementInfoTuplePOD, std::less<unsigned long>, std::allocator<std::pair<unsigned long const, Fortran::semantics::LabeledStatementInfoTuplePOD> > > const&, std::vector<Fortran::semantics::ScopeInfo, std::allocator<Fortran::semantics::ScopeInfo> > const&, Fortran::semantics::SemanticsContext&)
            |
            ---Fortran::semantics::CheckScopeConstraints(std::vector<Fortran::semantics::SourceStatementInfoTuplePOD, std::allocator<Fortran::semantics::SourceStatementInfoTuplePOD> > const&, std::map<unsigned long, Fortran::semantics::LabeledStatementInfoTuplePOD, std::less<unsigned long>, std::allocator<std::pair<unsigned long const, Fortran::semantics::LabeledStatementInfoTuplePOD> > > const&, std::vector<Fortran::semantics::ScopeInfo, std::allocator<Fortran::semantics::ScopeInfo> > const&, Fortran::semantics::SemanticsContext&)

From this output, we can see that almost all the time of compilation is spent with the symbol Fortran::semantics::CheckScopeConstraints. This symbol seems to come from a function called CheckScopeConstraints in the namespace Fortran::semantics.

llvm-project/flang/lib/Semantics/resolve-labels.cpp

Line 989 in d923f4c

void CheckScopeConstraints(const SourceStmtList &stmts,

[llvmbot]

@llvm/issue-subscribers-flang-frontend

[llvmbot]

@llvm/issue-subscribers-bug

[klausler]

Fix is coming shortly.

[klausler]

https://reviews.llvm.org/D159040

[klausler]

Fix has been merged into llvm-project/main.