clang: Assertion fail for simple _Bitint code

[dcb314]

Given this source code:

void f()
{
	static _BitInt(130) p = {}; 
}

then recent clang trunk does this:

$ clang -c bug38.c
clang: /home/dcb38/llvm/trunk/clang/lib/CodeGen/CGDecl.cpp:366: llvm::GlobalVariable* clang::CodeGen::CodeGenFunction::AddInitializerToStaticVarDecl(const clang
::VarDecl&, llvm::GlobalVariable*): Assertion `VarSize == CstSize && "Emitted constant has unexpected size"' failed.

A slightly smaller size of 120 seems to work ok. It looks like the
boundary size is likely to be 128 or so.

If this is a fixed boundary, a tidy "not implemented" error message
is better than an assertion fail and a crash.

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: None (dcb314)

Given this source code:

void f()
{
static _BitInt(130) p = {};
}

then recent clang trunk does this:

$ clang -c bug38.c
clang: /home/dcb38/llvm/trunk/clang/lib/CodeGen/CGDecl.cpp:366: llvm::GlobalVariable* clang::CodeGen::CodeGenFunction::AddInitializerToStaticVarDecl(const clang
::VarDecl&, llvm::GlobalVariable*): Assertion `VarSize == CstSize && "Emitted constant has unexpected size"' failed.

A slightly smaller size of 120 seems to work ok. It looks like the
boundary size is likely to be 128 or so.

If this is a fixed boundary, a tidy "not implemented" error message
is better than an assertion fail and a crash.

[shafik]

Confirmed: https://godbolt.org/z/YWodd8qn9

Does not happen in clang-17

Backtrace:

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: /opt/compiler-explorer/clang-assertions-trunk/bin/clang++ -gdwarf-4 -g -o /app/output.s -mllvm --x86-asm-syntax=intel -S --gcc-toolchain=/opt/compiler-explorer/gcc-snapshot -fcolor-diagnostics -fno-crash-diagnostics -x c -std=c2x <source>
1.	<eof> parser at end of file
2.	<source>:1:6: LLVM IR generation of declaration 'f'
3.	<source>:1:6: Generating code for declaration 'f'
 #0 0x00000000038d2b38 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x38d2b38)
 #1 0x00000000038d081c llvm::sys::CleanupOnSignal(unsigned long) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x38d081c)
 #2 0x0000000003818cf8 CrashRecoverySignalHandler(int) CrashRecoveryContext.cpp:0:0
 #3 0x00007ff06e642520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
 #4 0x00007ff06e6969fc pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x969fc)
 #5 0x00007ff06e642476 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x42476)
 #6 0x00007ff06e6287f3 abort (/lib/x86_64-linux-gnu/libc.so.6+0x287f3)
 #7 0x00007ff06e62871b (/lib/x86_64-linux-gnu/libc.so.6+0x2871b)
 #8 0x00007ff06e639e96 (/lib/x86_64-linux-gnu/libc.so.6+0x39e96)
 #9 0x0000000003fa6ac8 clang::CodeGen::CodeGenFunction::AddInitializerToStaticVarDecl(clang::VarDecl const&, llvm::GlobalVariable*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3fa6ac8)
#10 0x0000000003fb05f9 clang::CodeGen::CodeGenFunction::EmitStaticVarDecl(clang::VarDecl const&, llvm::GlobalValue::LinkageTypes) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3fb05f9)
#11 0x0000000003fb543b clang::CodeGen::CodeGenFunction::EmitVarDecl(clang::VarDecl const&) (.part.0) CGDecl.cpp:0:0
#12 0x0000000003fb56c3 clang::CodeGen::CodeGenFunction::EmitDecl(clang::Decl const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3fb56c3)
#13 0x0000000003be8427 clang::CodeGen::CodeGenFunction::EmitDeclStmt(clang::DeclStmt const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3be8427)
#14 0x0000000003bfcdf9 clang::CodeGen::CodeGenFunction::EmitSimpleStmt(clang::Stmt const*, llvm::ArrayRef<clang::Attr const*>) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3bfcdf9)
#15 0x0000000003bf5775 clang::CodeGen::CodeGenFunction::EmitStmt(clang::Stmt const*, llvm::ArrayRef<clang::Attr const*>) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3bf5775)
#16 0x0000000003bfc79c clang::CodeGen::CodeGenFunction::EmitCompoundStmtWithoutScope(clang::CompoundStmt const&, bool, clang::CodeGen::AggValueSlot) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3bfc79c)
#17 0x0000000003c5d452 clang::CodeGen::CodeGenFunction::EmitFunctionBody(clang::Stmt const*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3c5d452)
#18 0x0000000003c707e4 clang::CodeGen::CodeGenFunction::GenerateCode(clang::GlobalDecl, llvm::Function*, clang::CodeGen::CGFunctionInfo const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3c707e4)
#19 0x0000000003ccead2 clang::CodeGen::CodeGenModule::EmitGlobalFunctionDefinition(clang::GlobalDecl, llvm::GlobalValue*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3ccead2)
#20 0x0000000003cc93e5 clang::CodeGen::CodeGenModule::EmitGlobalDefinition(clang::GlobalDecl, llvm::GlobalValue*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3cc93e5)
#21 0x0000000003cc99e3 clang::CodeGen::CodeGenModule::EmitGlobal(clang::GlobalDecl) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x3cc99e3)
#22 0x0000000003cd39b1 clang::CodeGen::CodeGenModule::EmitTopLevelDecl(clang::Decl*) (.part.0) CodeGenModule.cpp:0:0
#23 0x0000000004168dc6 (anonymous namespace)::CodeGeneratorImpl::HandleTopLevelDecl(clang::DeclGroupRef) ModuleBuilder.cpp:0:0
#24 0x00000000041597c8 clang::BackendConsumer::HandleTopLevelDecl(clang::DeclGroupRef) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x41597c8)
#25 0x00000000061375d4 clang::ParseAST(clang::Sema&, bool, bool) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x61375d4)
#26 0x00000000041669e8 clang::CodeGenAction::ExecuteAction() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x41669e8)
#27 0x00000000043e1099 clang::FrontendAction::Execute() (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x43e1099)
#28 0x000000000435fc0e clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x435fc0e)
#29 0x00000000044c327e clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x44c327e)
#30 0x0000000000c1ea66 cc1_main(llvm::ArrayRef<char const*>, char const*, void*) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc1ea66)
#31 0x0000000000c1677a ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&, llvm::ToolContext const&) driver.cpp:0:0
#32 0x00000000041a7ff9 void llvm::function_ref<void ()>::callback_fn<clang::driver::CC1Command::Execute(llvm::ArrayRef<std::optional<llvm::StringRef>>, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>*, bool*) const::'lambda'()>(long) Job.cpp:0:0
#33 0x00000000038191a4 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x38191a4)
#34 0x00000000041a85ef clang::driver::CC1Command::Execute(llvm::ArrayRef<std::optional<llvm::StringRef>>, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>*, bool*) const (.part.0) Job.cpp:0:0
#35 0x0000000004170275 clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4170275)
#36 0x0000000004170cdd clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&, bool) const (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4170cdd)
#37 0x0000000004178c15 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*>>&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0x4178c15)
#38 0x0000000000c1bf1d clang_main(int, char**, llvm::ToolContext const&) (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc1bf1d)
#39 0x0000000000b12824 main (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xb12824)
#40 0x00007ff06e629d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
#41 0x00007ff06e629e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
#42 0x0000000000c1623e _start (/opt/compiler-explorer/clang-assertions-trunk/bin/clang+++0xc1623e)
clang++: error: clang frontend command failed with exit code 134 (use -v to see invocation)
Compiler returned: 134

[shafik]

CC @AaronBallman

[AaronBallman]

Can confirm that this appears to be new behavior in Clang 18. The CodeGen code around the assertion has not changed in quite some time, so the issue was introduced elsewhere (I've not had the chance to track down where). This would probably be a good candidate for bisection (CC @Endilll who may be able to help narrow down how this got introduced).

[dcb314]

I ran a quick bisect over some recent clangs here and it appears
to me that the bug was introduced sometime between
65066c0, dated 20240201
and 2147a2a, dated 20240202.

I think that's about 110 commits.

[Endilll]

I don't any related changes to /clang/ in this range: https://github.com/llvm/llvm-project/commits/main/clang?before=6c39fa9e9f198498ff7cf9646081437a0fc0882a+620 (from b67ce7e to 2147a2a). Proper bisecting is due.

[dcb314]

Proper bisecting is due.

Is there a "needs-bisection" that could be added ? Someone might pick it up.

Given that it takes me over an hour to build any clang, I am unlikely to be able to help.