[Regression] LLVM asserts "conflicting locations for variable" since 1.82

[cuviper]

Code

I tried this code: cargo itself, building release + debug=2
(multiple versions, but at the time of this report I have cargo cf53cc54bb593b5ec3dc2be4b1702f50c36d24d5)

I expected to see this happen: successful compilation

Instead, this happened:

In Fedora 39 builds (with its LLVM 17), rustc hits a libstdc++ assertion via LLVM:

/usr/bin/../lib/gcc/x86_64-redhat-linux/13/../../../../include/c++/13/optional:479: _Tp &std::_Optional_base_impl<llvm::DIExpression::FragmentInfo, std::_Optional_base<llvm::DIExpression::FragmentInfo>>::_M_get() [_Tp = llvm::DIExpression::FragmentInfo, _Dp = std::_Optional_base<llvm::DIExpression::FragmentInfo>]: Assertion 'this->_M_is_engaged()' failed.
rustc exited with signal: 6 (SIGABRT) (core dumped)

While investigating, I found that llc also crashes with the extracted IR:
https://bugzilla.redhat.com/show_bug.cgi?id=2226564#c10

In local builds with rust's LLVM 19 and assertions enabled, and also with CI "alt" builds, LLVM asserts "conflicting locations for variable":

.../cargo$ CARGO_PROFILE_RELEASE_DEBUG=2 cargo +e92993dbb43f0a5d17fe56e2d82f90435d6521c8-alt build --release
[...]
rustc: /checkout/src/llvm-project/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp:303: void llvm::Loc::MMI::addFrameIndexExpr(const DIExpression *, int): Assertion `(FrameIndexExprs.size() == 1 || llvm::all_of(FrameIndexExprs, [](const FrameIndexExpr &FIE) { return FIE.Expr && FIE.Expr->isFragment(); })) && "conflicting locations for variable"' failed.
error: could not compile `cargo` (lib)

Version it worked on

It most recently worked on: Rust 1.81.0

Version with regression

Rust 1.82.0, but at first it only crashed in my Fedora 39 build with LLVM 17. However, using the bundled rust-llvm with assertions enabled does trigger a debuginfo assertion.

Current nightly (e92993d) in alt mode reproduces this as well.

i.e. rustup-toolchain-install-master --alt e92993dbb43f0a5d17fe56e2d82f90435d6521c8

$ rustc +e92993dbb43f0a5d17fe56e2d82f90435d6521c8-alt -Vv
rustc 1.84.0-nightly (e92993dbb 2024-10-18)
binary: rustc
commit-hash: e92993dbb43f0a5d17fe56e2d82f90435d6521c8
commit-date: 2024-10-18
host: x86_64-unknown-linux-gnu
release: 1.84.0-nightly
LLVM version: 19.1.1

Backtrace

(gdb) backtrace

#0  __pthread_kill_implementation (threadid=<optimized out>, signo=signo@entry=6, no_tid=no_tid@entry=0) at pthread_kill.c:44
#1  0x00007f70eb682793 in __pthread_kill_internal (threadid=<optimized out>, signo=6) at pthread_kill.c:78
#2  0x00007f70eb629d0e in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
#3  0x00007f70eb611942 in __GI_abort () at abort.c:79
#4  0x00007f70eb61185e in __assert_fail_base (fmt=0x7f70eb7c5cb0 "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n",
    assertion=assertion@entry=0x7f70db9545d4 <.L.str.70> "(FrameIndexExprs.size() == 1 || llvm::all_of(FrameIndexExprs, [](const FrameIndexExpr &FIE) { return FIE.Expr && FIE.Expr->isFragment(); })) && \"conflicting locations for variable\"",
    file=file@entry=0x7f70db8eb1f8 <str.67.llvm> "/checkout/src/llvm-project/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp", line=line@entry=303,
    function=function@entry=0x7f70db233d86 <.L__PRETTY_FUNCTION__._ZN4llvm3Loc3MMI17addFrameIndexExprEPKNS_12DIExpressionEi> "void llvm::Loc::MMI::addFrameIndexExpr(const DIExpression *, int)") at assert.c:94
#5  0x00007f70eb621e47 in __assert_fail (
    assertion=0x7f70db9545d4 <.L.str.70> "(FrameIndexExprs.size() == 1 || llvm::all_of(FrameIndexExprs, [](const FrameIndexExpr &FIE) { return FIE.Expr && FIE.Expr->isFragment(); })) && \"conflicting locations for variable\"",
    file=0x7f70db8eb1f8 <str.67.llvm> "/checkout/src/llvm-project/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp", line=303,
    function=0x7f70db233d86 <.L__PRETTY_FUNCTION__._ZN4llvm3Loc3MMI17addFrameIndexExprEPKNS_12DIExpressionEi> "void llvm::Loc::MMI::addFrameIndexExpr(const DIExpression *, int)") at assert.c:103
#6  0x00007f70e3710e95 in llvm::Loc::MMI::addFrameIndexExpr(llvm::DIExpression const*, int) [clone .cold] () from /home/jistone/.rustup/toolchains/e92993dbb43f0a5d17fe56e2d82f90435d6521c8-alt/lib/libLLVM.so.19.1-rust-1.84.0-nightly
#7  0x00007f70e22f2086 in llvm::DwarfDebug::collectVariableInfoFromMFTable(llvm::DwarfCompileUnit&, llvm::DenseSet<std::pair<llvm::DINode const*, llvm::DILocation const*>, llvm::DenseMapInfo<std::pair<llvm::DINode const*, llvm::DILocation const*>, void> >&) () from /home/jistone/.rustup/toolchains/e92993dbb43f0a5d17fe56e2d82f90435d6521c8-alt/lib/libLLVM.so.19.1-rust-1.84.0-nightly
#8  0x00007f70e25914ab in llvm::DwarfDebug::collectEntityInfo(llvm::DwarfCompileUnit&, llvm::DISubprogram const*, llvm::DenseSet<std::pair<llvm::DINode const*, llvm::DILocation const*>, llvm::DenseMapInfo<std::pair<llvm::DINode const*, llvm::DILocation const*>, void> >&) () from /home/jistone/.rustup/toolchains/e92993dbb43f0a5d17fe56e2d82f90435d6521c8-alt/lib/libLLVM.so.19.1-rust-1.84.0-nightly
#9  0x00007f70e2592fe8 in llvm::DwarfDebug::endFunctionImpl(llvm::MachineFunction const*) () from /home/jistone/.rustup/toolchains/e92993dbb43f0a5d17fe56e2d82f90435d6521c8-alt/lib/libLLVM.so.19.1-rust-1.84.0-nightly
#10 0x00007f70e267f0a4 in llvm::DebugHandlerBase::endFunction(llvm::MachineFunction const*) () from /home/jistone/.rustup/toolchains/e92993dbb43f0a5d17fe56e2d82f90435d6521c8-alt/lib/libLLVM.so.19.1-rust-1.84.0-nightly
#11 0x00007f70e2bb6059 in llvm::AsmPrinter::emitFunctionBody() () from /home/jistone/.rustup/toolchains/e92993dbb43f0a5d17fe56e2d82f90435d6521c8-alt/lib/libLLVM.so.19.1-rust-1.84.0-nightly
#12 0x00007f70e223d646 in llvm::X86AsmPrinter::runOnMachineFunction(llvm::MachineFunction&) () from /home/jistone/.rustup/toolchains/e92993dbb43f0a5d17fe56e2d82f90435d6521c8-alt/lib/libLLVM.so.19.1-rust-1.84.0-nightly
#13 0x00007f70e2d7ac94 in llvm::FPPassManager::runOnFunction(llvm::Function&) [clone .warm] () from /home/jistone/.rustup/toolchains/e92993dbb43f0a5d17fe56e2d82f90435d6521c8-alt/lib/libLLVM.so.19.1-rust-1.84.0-nightly
#14 0x00007f70e22d6376 in llvm::FPPassManager::runOnModule(llvm::Module&) () from /home/jistone/.rustup/toolchains/e92993dbb43f0a5d17fe56e2d82f90435d6521c8-alt/lib/libLLVM.so.19.1-rust-1.84.0-nightly
#15 0x00007f70e299f210 in llvm::legacy::PassManagerImpl::run(llvm::Module&) () from /home/jistone/.rustup/toolchains/e92993dbb43f0a5d17fe56e2d82f90435d6521c8-alt/lib/libLLVM.so.19.1-rust-1.84.0-nightly
#16 0x00007f70e9f2aab2 in LLVMRustWriteOutputFile () from /home/jistone/.rustup/toolchains/e92993dbb43f0a5d17fe56e2d82f90435d6521c8-alt/lib/librustc_driver-a4eb0db55e024726.so
#17 0x00007f70e9f2a767 in rustc_codegen_llvm::back::write::write_output_file () from /home/jistone/.rustup/toolchains/e92993dbb43f0a5d17fe56e2d82f90435d6521c8-alt/lib/librustc_driver-a4eb0db55e024726.so
#18 0x00007f70e9ef1d5c in rustc_codegen_llvm::back::write::codegen () from /home/jistone/.rustup/toolchains/e92993dbb43f0a5d17fe56e2d82f90435d6521c8-alt/lib/librustc_driver-a4eb0db55e024726.so
#19 0x00007f70e9ef19e2 in rustc_codegen_ssa::back::write::finish_intra_module_work::<rustc_codegen_llvm::LlvmCodegenBackend> () from /home/jistone/.rustup/toolchains/e92993dbb43f0a5d17fe56e2d82f90435d6521c8-alt/lib/librustc_driver-a4eb0db55e024726.so
#20 0x00007f70ea003a36 in std::sys::backtrace::__rust_begin_short_backtrace::<<rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::ExtraBackendMethods>::spawn_named_thread<rustc_codegen_ssa::back::write::spawn_work<rustc_codegen_llvm::LlvmCodegenBackend>::{closure#0}, ()>::{closure#0}, ()> () from /home/jistone/.rustup/toolchains/e92993dbb43f0a5d17fe56e2d82f90435d6521c8-alt/lib/librustc_driver-a4eb0db55e024726.so
#21 0x00007f70ea000b28 in <<std::thread::Builder>::spawn_unchecked_<<rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::ExtraBackendMethods>::spawn_named_thread<rustc_codegen_ssa::back::write::spawn_work<rustc_codegen_llvm::LlvmCodegenBackend>::{closure#0}, ()>::{closure#0}, ()>::{closure#1} as core::ops::function::FnOnce<()>>::call_once::{shim:vtable#0} ()
   from /home/jistone/.rustup/toolchains/e92993dbb43f0a5d17fe56e2d82f90435d6521c8-alt/lib/librustc_driver-a4eb0db55e024726.so
#22 0x00007f70ea000eeb in std::sys::pal::unix::thread::Thread::new::thread_start () from /home/jistone/.rustup/toolchains/e92993dbb43f0a5d17fe56e2d82f90435d6521c8-alt/lib/librustc_driver-a4eb0db55e024726.so
#23 0x00007f70eb680797 in start_thread (arg=<optimized out>) at pthread_create.c:447
#24 0x00007f70eb70478c in __GI___clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:78

I bisected this to #128861 (cc @khuey)

• The assertion fails with 3139ff09e9d07f7700f8d15ed25a231e29c43627-alt (3139ff0)
• Compilation succeeds with 026e9ed3f0c5b3ee6233fd23b5e497cb94cf6434-alt (its parent 026e9ed)

@rustbot modify labels: +regression-from-stable-to-stable -regression-untriaged

[cuviper]

FWIW, I also shared some LLVM bitcode in this thread:
https://rust-lang.zulipchat.com/#narrow/channel/122651-general/topic/Help.20minimizing.20LLVM.20assertion.20reproducer.3F/near/477758192

[khuey]

I can reproduce this. I haven't gotten to the bottom of it yet but I think it's an interaction between MIR inlining and an LLVM optimization (not clear to me yet whether it's SROA, inlining, or both). The final bitcode is obviously garbage. Seeing how the LLVM IR evolves through the passes would be helpful, if you can easily isolate that.

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-critical