Clang assertion failure in clang::CodeGen::LValue::getPointer when accessing a bitfield in an asm block: Assertion `isSimple()' failed

[tahonermann]

Clang (16/trunk) fails the following test case when built with assertions enabled.

$ cat t.c
struct {
  unsigned bf:1;
} s;
void f()
{
   __asm { mov s.bf,eax }
}

$ clang --version
clang version 16.0.0 (https://github.com/tahonermann/llvm-project.git 6073e0a2f7018fb01283f48e946a45aff5512eae)
Target: x86_64-unknown-linux-gnu
...

$ clang -c -fasm-blocks t.c
clang: /iusers/thonerma/llvm-project/clang/lib/CodeGen/CGValue.h:338: llvm::Value* clang::CodeGen::LValue::getPointer(clang::CodeGen::CodeGenFunction&) const: Assertion `isSimple()' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0.      Program arguments: /iusers/thonerma/build-llvm-project-main-Debug-On/bin/clang -c -fasm-blocks t.c
1.      <eof> parser at end of file
2.      t.c:4:6: LLVM IR generation of declaration 'f'
3.      t.c:4:6: Generating code for declaration 'f'
 #0 0x0000000005aa8fbd llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) /iusers/thonerma/llvm-project/llvm/lib/Support/Unix/Signals.inc:569:22
 #1 0x0000000005aa9074 PrintStackTraceSignalHandler(void*) /iusers/thonerma/llvm-project/llvm/lib/Support/Unix/Signals.inc:636:1
 #2 0x0000000005aa6fe2 llvm::sys::RunSignalHandlers() /iusers/thonerma/llvm-project/llvm/lib/Support/Signals.cpp:103:20
 #3 0x0000000005aa8908 llvm::sys::CleanupOnSignal(unsigned long) /iusers/thonerma/llvm-project/llvm/lib/Support/Unix/Signals.inc:361:31
 #4 0x00000000059d2605 (anonymous namespace)::CrashRecoveryContextImpl::HandleCrash(int, unsigned long) /iusers/thonerma/llvm-project/llvm/lib/Support/CrashRecoveryContext.cpp:77:5
 #5 0x00000000059d2aeb CrashRecoverySignalHandler(int) /iusers/thonerma/llvm-project/llvm/lib/Support/CrashRecoveryContext.cpp:398:1
 #6 0x00007f24d6c64b20 __restore_rt sigaction.c:0:0
 #7 0x00007f24d574537f raise (/lib64/libc.so.6+0x3737f)
 #8 0x00007f24d572fdb5 abort (/lib64/libc.so.6+0x21db5)
 #9 0x00007f24d572fc89 _nl_load_domain.cold.0 loadmsgcat.c:0:0
#10 0x00007f24d573da76 .annobin___GI___assert_fail.end assert.c:0:0
#11 0x00000000060e6e97 clang::CodeGen::LValue::getPointer(clang::CodeGen::CodeGenFunction&) const /iusers/thonerma/llvm-project/clang/lib/CodeGen/CGValue.h:339:12
#12 0x00000000060e6edf clang::CodeGen::LValue::getAddress(clang::CodeGen::CodeGenFunction&) const /iusers/thonerma/llvm-project/clang/lib/CodeGen/CGValue.h:342:12
#13 0x00000000060e13de clang::CodeGen::CodeGenFunction::EmitAsmStmt(clang::AsmStmt const&) /iusers/thonerma/llvm-project/clang/lib/CodeGen/CGStmt.cpp:2457:47
#14 0x00000000060d7c1f clang::CodeGen::CodeGenFunction::EmitStmt(clang::Stmt const*, llvm::ArrayRef<clang::Attr const*>) /iusers/thonerma/llvm-project/clang/lib/CodeGen/CGStmt.cpp:157:75
#15 0x00000000060d8d41 clang::CodeGen::CodeGenFunction::EmitCompoundStmtWithoutScope(clang::CompoundStmt const&, bool, clang::CodeGen::AggValueSlot) /iusers/thonerma/llvm-project/clang/lib/CodeGen/CGStmt.cpp:496:3
#16 0x000000000618e328 clang::CodeGen::CodeGenFunction::EmitFunctionBody(clang::Stmt const*) /iusers/thonerma/llvm-project/clang/lib/CodeGen/CodeGenFunction.cpp:1239:36
#17 0x000000000618f1c1 clang::CodeGen::CodeGenFunction::GenerateCode(clang::GlobalDecl, llvm::Function*, clang::CodeGen::CGFunctionInfo const&) /iusers/thonerma/llvm-project/clang/lib/CodeGen/CodeGenFunction.cpp:1447:21
#18 0x00000000061bdce1 clang::CodeGen::CodeGenModule::EmitGlobalFunctionDefinition(clang::GlobalDecl, llvm::GlobalValue*) /iusers/thonerma/llvm-project/clang/lib/CodeGen/CodeGenModule.cpp:5322:3
#19 0x00000000061b6885 clang::CodeGen::CodeGenModule::EmitGlobalDefinition(clang::GlobalDecl, llvm::GlobalValue*) /iusers/thonerma/llvm-project/clang/lib/CodeGen/CodeGenModule.cpp:3597:47
#20 0x00000000061b5828 clang::CodeGen::CodeGenModule::EmitGlobal(clang::GlobalDecl) /iusers/thonerma/llvm-project/clang/lib/CodeGen/CodeGenModule.cpp:3338:5
#21 0x00000000061c1dde clang::CodeGen::CodeGenModule::EmitTopLevelDecl(clang::Decl*) /iusers/thonerma/llvm-project/clang/lib/CodeGen/CodeGenModule.cpp:6181:37
#22 0x0000000007356136 (anonymous namespace)::CodeGeneratorImpl::HandleTopLevelDecl(clang::DeclGroupRef) /iusers/thonerma/llvm-project/clang/lib/CodeGen/ModuleBuilder.cpp:188:7
#23 0x000000000734e264 clang::BackendConsumer::HandleTopLevelDecl(clang::DeclGroupRef) /iusers/thonerma/llvm-project/clang/lib/CodeGen/CodeGenAction.cpp:234:11
#24 0x0000000008e8f302 clang::ParseAST(clang::Sema&, bool, bool) /iusers/thonerma/llvm-project/clang/lib/Parse/ParseAST.cpp:166:20
#25 0x0000000006a1a751 clang::ASTFrontendAction::ExecuteAction() /iusers/thonerma/llvm-project/clang/lib/Frontend/FrontendAction.cpp:1152:11
#26 0x000000000734b9ef clang::CodeGenAction::ExecuteAction() /iusers/thonerma/llvm-project/clang/lib/CodeGen/CodeGenAction.cpp:1145:5
#27 0x0000000006a1a07d clang::FrontendAction::Execute() /iusers/thonerma/llvm-project/clang/lib/Frontend/FrontendAction.cpp:1049:38
#28 0x00000000069580ba clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) /iusers/thonerma/llvm-project/clang/lib/Frontend/CompilerInstance.cpp:1043:42
#29 0x0000000006b9f80c clang::ExecuteCompilerInvocation(clang::CompilerInstance*) /iusers/thonerma/llvm-project/clang/lib/FrontendTool/ExecuteCompilerInvocation.cpp:266:38
#30 0x0000000002710caf cc1_main(llvm::ArrayRef<char const*>, char const*, void*) /iusers/thonerma/llvm-project/clang/tools/driver/cc1_main.cpp:250:40
#31 0x00000000027028fb ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&) /iusers/thonerma/llvm-project/clang/tools/driver/driver.cpp:317:20
#32 0x000000000682995d 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'()::operator()() const /iusers/thonerma/llvm-project/clang/lib/Driver/Job.cpp:428:32
#33 0x0000000006829ec4 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) /iusers/thonerma/llvm-project/llvm/include/llvm/ADT/STLFunctionalExtras.h:46:40
#34 0x00000000047bd23c llvm::function_ref<void ()>::operator()() const /iusers/thonerma/llvm-project/llvm/include/llvm/ADT/STLFunctionalExtras.h:68:62
#35 0x00000000059d2ca5 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) /iusers/thonerma/llvm-project/llvm/lib/Support/CrashRecoveryContext.cpp:434:10
#36 0x0000000006829b4a clang::driver::CC1Command::Execute(llvm::ArrayRef<llvm::Optional<llvm::StringRef>>, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>*, bool*) const /iusers/thonerma/llvm-project/clang/lib/Driver/Job.cpp:428:7
#37 0x00000000067d1a6c clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const /iusers/thonerma/llvm-project/clang/lib/Driver/Compilation.cpp:200:22
#38 0x00000000067d1da5 clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&, bool) const /iusers/thonerma/llvm-project/clang/lib/Driver/Compilation.cpp:254:62
#39 0x00000000067e2612 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&) /iusers/thonerma/llvm-project/clang/lib/Driver/Driver.cpp:1783:28
#40 0x0000000002703d1a clang_main(int, char**) /iusers/thonerma/llvm-project/clang/tools/driver/driver.cpp:513:39
#41 0x000000000272d22d main /iusers/thonerma/build-llvm-project-main-Debug-On/tools/clang/tools/driver/clang-driver.cpp:11:63
#42 0x00007f24d5731493 __libc_start_main (/lib64/libc.so.6+0x23493)
#43 0x000000000270120e _start (/iusers/thonerma/build-llvm-project-main-Debug-On/bin/clang+0x270120e)
clang-15: error: clang frontend command failed with exit code 134 (use -v to see invocation)
clang version 16.0.0 (https://github.com/tahonermann/llvm-project.git 6073e0a2f7018fb01283f48e946a45aff5512eae)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /iusers/thonerma/build-llvm-project-main-Debug-On/bin

[llvmbot]

@llvm/issue-subscribers-clang-codegen

[int6]

Seems crash here:

llvm-project/clang/lib/CodeGen/CGStmt.cpp

Line 2457 in e07ead8

Address DestAddr = Dest.getAddress(*this);

, because, Dest has a bitfield , maybe we should check Dest's bitfield here and use Dest.getBitFieldAddress()

[tahonermann]

Thank you for that suggestion, @objwyh. I'm wondering if we shouldn't emit a diagnostic for access of a bitfield though. Consider the code gen that is done for the example below (see https://godbolt.org/z/KTcK8Kh5o).

struct {
  unsigned bf1:1;
  unsigned bf2:1;
  unsigned bf3:1;
} s = { true, false, true };
void f(unsigned u)
{
  __asm {
    mov eax,u
    mov s.bf2,eax
  }
}

The generated x86_64 assembly for f() is:

        mov     dword ptr [rsp - 4], edi
        mov     eax, dword ptr [rsp - 4]
        mov     dword ptr [rip + s], eax
        ret

Note that the operations end up clobbering surrounding bitfields. That seems unlikely to be the programmer's intent. Implementing the programmer's presumed intent would require rewriting the assembly to do the necessary bit twiddling and that seems unpalatable as well. I think we should emit an error for access of a bitfield in inline assembly.

It is interesting that icc (as exhibited in the godbolt.org link above) behaves the same way.

[tahonermann]

Following discussion with several other Clang contributors, I posted a code review for a change that diagnoses use of bit-fields as instruction operands in Microsoft style asm blocks as an error. For anyone interested, please review and post your comments. https://reviews.llvm.org/D135500.