clang crashes when bitfields are initialized out-of-order

[ahatanak]

$ cat test.cpp

struct {
  unsigned : 2;
  unsigned a : 6;
  unsigned : 1;
  unsigned b : 6;
  unsigned : 2;
  unsigned c : 6;
  unsigned d : 1;
  unsigned e : 2;
} data = {
    .e = 1,
    .c = 1,
    .a = 1,
};

$ clang++ -c test.cpp
PLEASE submit a bug report to https://bugs.llvm.org/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0.	Program arguments: /opt/compiler-explorer/clang-11.0.1/bin/clang++ -gdwarf-4 -g -o /app/output.s -mllvm --x86-asm-syntax=intel -S --gcc-toolchain=/opt/compiler-explorer/gcc-10.2.0 -fcolor-diagnostics -fno-crash-diagnostics -Wall -Wdoc-deprecated-not-sync <source> 
1.	<source>:14:2: current parser token ';'
 #0 0x000055ce73eacc5a llvm::sys::PrintStackTrace(llvm::raw_ostream&) (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x2cacc5a)
 #1 0x000055ce73eaaa04 llvm::sys::RunSignalHandlers() (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x2caaa04)
 #2 0x000055ce73eaac75 llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x2caac75)
 #3 0x000055ce73e1f4a0 CrashRecoverySignalHandler(int) (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x2c1f4a0)
 #4 0x00007fb1a9a93420 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x14420)
 #5 0x000055ce7660331f clang::Stmt::getBeginLoc() const (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x540331f)
 #6 0x000055ce75eaec6b (anonymous namespace)::InitListChecker::CheckDesignatedInitializer(clang::InitializedEntity const&, clang::InitListExpr*, clang::DesignatedInitExpr*, unsigned int, clang::QualType&, clang::DeclContext::specific_decl_iterator<clang::FieldDecl>*, llvm::APSInt*, unsigned int&, clang::InitListExpr*, unsigned int&, bool, bool) (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x4caec6b)
 #7 0x000055ce75eab6b9 (anonymous namespace)::InitListChecker::CheckStructUnionTypes(clang::InitializedEntity const&, clang::InitListExpr*, clang::QualType, llvm::iterator_range<clang::CXXBaseSpecifier*>, clang::DeclContext::specific_decl_iterator<clang::FieldDecl>, bool, unsigned int&, clang::InitListExpr*, unsigned int&, bool) (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x4cab6b9)
 #8 0x000055ce75eaa233 (anonymous namespace)::InitListChecker::CheckListElementTypes(clang::InitializedEntity const&, clang::InitListExpr*, clang::QualType&, bool, unsigned int&, clang::InitListExpr*, unsigned int&, bool) (.constprop.893) (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x4caa233)
 #9 0x000055ce75ea138f (anonymous namespace)::InitListChecker::InitListChecker(clang::Sema&, clang::InitializedEntity const&, clang::InitListExpr*, clang::QualType&, bool, bool, bool) (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x4ca138f)
#10 0x000055ce75ea5b01 clang::InitializationSequence::Perform(clang::Sema&, clang::InitializedEntity const&, clang::InitializationKind const&, llvm::MutableArrayRef<clang::Expr*>, clang::QualType*) (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x4ca5b01)
#11 0x000055ce75c174a4 clang::Sema::AddInitializerToDecl(clang::Decl*, clang::Expr*, bool) (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x4a174a4)
#12 0x000055ce759ff207 clang::Parser::ParseDeclarationAfterDeclaratorAndAttributes(clang::Declarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::ForRangeInit*) (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x47ff207)
#13 0x000055ce75a058b5 clang::Parser::ParseDeclGroup(clang::ParsingDeclSpec&, clang::DeclaratorContext, clang::SourceLocation*, clang::Parser::ForRangeInit*) (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x48058b5)
#14 0x000055ce759e7fa9 clang::Parser::ParseDeclOrFunctionDefInternal(clang::Parser::ParsedAttributesWithRange&, clang::ParsingDeclSpec&, clang::AccessSpecifier) (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x47e7fa9)
#15 0x000055ce759e8ca1 clang::Parser::ParseDeclarationOrFunctionDefinition(clang::Parser::ParsedAttributesWithRange&, clang::ParsingDeclSpec*, clang::AccessSpecifier) (.part.228) (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x47e8ca1)
#16 0x000055ce759eeb79 clang::Parser::ParseExternalDeclaration(clang::Parser::ParsedAttributesWithRange&, clang::ParsingDeclSpec*) (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x47eeb79)
#17 0x000055ce759f0289 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&, bool) (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x47f0289)
#18 0x000055ce759f08e7 clang::Parser::ParseFirstTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&) (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x47f08e7)
#19 0x000055ce759e3e93 clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x47e3e93)
#20 0x000055ce74698809 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x3498809)
#21 0x000055ce7465232b clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x345232b)
#22 0x000055ce7475ee9b clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x355ee9b)
#23 0x000055ce720867ac cc1_main(llvm::ArrayRef<char const*>, char const*, void*) (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0xe867ac)
#24 0x000055ce720832ad ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&) (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0xe832ad)
#25 0x000055ce7452a055 void llvm::function_ref<void ()>::callback_fn<clang::driver::CC1Command::Execute(llvm::ArrayRef<llvm::Optional<llvm::StringRef> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*, bool*) const::'lambda'()>(long) (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x332a055)
#26 0x000055ce73e1f583 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x2c1f583)
#27 0x000055ce7452ab30 clang::driver::CC1Command::Execute(llvm::ArrayRef<llvm::Optional<llvm::StringRef> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*, bool*) const (.part.148) (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x332ab30)
#28 0x000055ce74504d05 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&) const (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x3304d05)
#29 0x000055ce7450574f clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*> >&) const (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x330574f)
#30 0x000055ce7450e2e5 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*> >&) (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0x330e2e5)
#31 0x000055ce71fc3421 main (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0xdc3421)
#32 0x00007fb1a9541083 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x24083)
#33 0x000055ce72082e2a _start (/opt/compiler-explorer/clang-11.0.1/bin/clang+++0xe82e2a)
clang-11: error: clang frontend command failed due to signal (use -v to see invocation)
Compiler returned: 254

[ahatanak]

C++ doesn't allow out-of-order initialization, but the compiler shouldn't crash.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[shafik]

So it looks like in InitListChecker::CheckDesignatedInitializer(...) the StructuredList index does not match up with PrevField->getFieldIndex().

I think the correct fix is to use unsigned OldIndex = NumBases + StructuredIndex - 1;.

I did some testing with this and it looks like it does the right thing. I need to do some more testing.

[shafik]

I have a fix but I ran into an ancillary issue w/ the diagnostic and how it was designed, it is not ideal but this bug feels bad enough that maybe I will put a fix in for the main issue and tackle the new issue I found as a second step.

[shafik]

PR: https://reviews.llvm.org/D154675