deeply-nested chain hangs with Item = u32

[cuviper]

I tried this code:

pub fn foo() -> Box<Iterator<Item = u32>> {
    use std::iter::empty;

    Box::new(empty()
        .chain(empty())
        .chain(empty())
        .chain(empty())
        .chain(empty())
        .chain(empty())
        .chain(empty())
        .chain(empty())
        .chain(empty())
        .chain(empty())
        .chain(empty()) // 10th .chain(empty())
        .chain(empty())
        .chain(empty())
        .chain(empty())
        .chain(empty())
        .chain(empty())
        .chain(empty()) // 16th .chain(empty())
    )
}

This is adapted from the deeply-nested test, just changing Item = () to Item = u32.

Building with rustc +nightly --crate-type lib -Copt-level=2 src/lib.rs seems to hang indefinitely.

I expected to see this happen: completion in a few seconds at most.

Instead, this happened: I'm still waiting...

Meta

rustc --version --verbose:

rustc 1.44.0-nightly (537ccdf3a 2020-04-02)
binary: rustc
commit-hash: 537ccdf3ac44c8c7a8d36cbdbe6fb224afabb7ae
commit-date: 2020-04-02
host: x86_64-unknown-linux-gnu
release: 1.44.0-nightly
LLVM version: 9.0

Also happens on 1.42.0 and 1.43.0-beta.3.

Attaching rust-gdb, I see two busy LLVM threads:

Backtrace

Thread 6 (Thread 0x7f70bd5ff700 (LWP 207946)):
#0  0x00007f70c72133db in llvm::DominatorTreeBase<llvm::BasicBlock, false>::dominates(llvm::BasicBlock const*, llvm::BasicBlock const*) const () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#1  0x00007f70c7e2298a in llvm::GVN::findLeader(llvm::BasicBlock const*, unsigned int) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#2  0x00007f70c7e23279 in llvm::GVN::processInstruction(llvm::Instruction*) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#3  0x00007f70c7e24977 in llvm::GVN::processBlock(llvm::BasicBlock*) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#4  0x00007f70c7e1aff0 in llvm::GVN::runImpl(llvm::Function&, llvm::AssumptionCache&, llvm::DominatorTree&, llvm::TargetLibraryInfo const&, llvm::AAResults&, llvm::MemoryDependenceResults*, llvm::LoopInfo*, llvm::OptimizationRemarkEmitter*) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#5  0x00007f70c7e27098 in llvm::gvn::GVNLegacyPass::runOnFunction(llvm::Function&) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#6  0x00007f70c727842f in llvm::FPPassManager::runOnFunction(llvm::Function&) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#7  0x00007f70c8233a78 in (anonymous namespace)::CGPassManager::runOnModule(llvm::Module&) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#8  0x00007f70c7279030 in llvm::legacy::PassManagerImpl::run(llvm::Module&) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#9  0x00007f70c71e210a in LLVMRunPassManager () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#10 0x00007f70caa79b43 in rustc_codegen_llvm::back::write::optimize () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/librustc_driver-ebfe476c9299964b.so
#11 0x00007f70caa3ef17 in rustc_codegen_ssa::back::write::execute_work_item () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/librustc_driver-ebfe476c9299964b.so
#12 0x00007f70cab2680e in std::sys_common::backtrace::__rust_begin_short_backtrace () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/librustc_driver-ebfe476c9299964b.so
#13 0x00007f70caa08eb5 in core::ops::function::FnOnce::call_once{{vtable-shim}} () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/librustc_driver-ebfe476c9299964b.so
#14 0x00007f70c9fe92ff in <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once () at /rustc/537ccdf3ac44c8c7a8d36cbdbe6fb224afabb7ae/src/liballoc/boxed.rs:1008
#15 0x00007f70ca01c813 in <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once () at /rustc/537ccdf3ac44c8c7a8d36cbdbe6fb224afabb7ae/src/liballoc/boxed.rs:1008
#16 std::sys_common::thread::start_thread () at src/libstd/sys_common/thread.rs:13
#17 std::sys::unix::thread::Thread::new::thread_start () at src/libstd/sys/unix/thread.rs:80
#18 0x00007f70c9f534e2 in start_thread () from /lib64/libpthread.so.0
#19 0x00007f70c9e706d3 in clone () from /lib64/libc.so.6

Thread 5 (Thread 0x7f70be1ff700 (LWP 207945)):
#0  0x00007f70c7213354 in llvm::DominatorTreeBase<llvm::BasicBlock, false>::dominates(llvm::BasicBlock const*, llvm::BasicBlock const*) const () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#1  0x00007f70c7e2298a in llvm::GVN::findLeader(llvm::BasicBlock const*, unsigned int) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#2  0x00007f70c7e23279 in llvm::GVN::processInstruction(llvm::Instruction*) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#3  0x00007f70c7e24977 in llvm::GVN::processBlock(llvm::BasicBlock*) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#4  0x00007f70c7e1aff0 in llvm::GVN::runImpl(llvm::Function&, llvm::AssumptionCache&, llvm::DominatorTree&, llvm::TargetLibraryInfo const&, llvm::AAResults&, llvm::MemoryDependenceResults*, llvm::LoopInfo*, llvm::OptimizationRemarkEmitter*) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#5  0x00007f70c7e27098 in llvm::gvn::GVNLegacyPass::runOnFunction(llvm::Function&) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#6  0x00007f70c727842f in llvm::FPPassManager::runOnFunction(llvm::Function&) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#7  0x00007f70c8233a78 in (anonymous namespace)::CGPassManager::runOnModule(llvm::Module&) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#8  0x00007f70c7279030 in llvm::legacy::PassManagerImpl::run(llvm::Module&) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#9  0x00007f70c71e210a in LLVMRunPassManager () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#10 0x00007f70caa79b43 in rustc_codegen_llvm::back::write::optimize () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/librustc_driver-ebfe476c9299964b.so
#11 0x00007f70caa3ef17 in rustc_codegen_ssa::back::write::execute_work_item () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/librustc_driver-ebfe476c9299964b.so
#12 0x00007f70cab2680e in std::sys_common::backtrace::__rust_begin_short_backtrace () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/librustc_driver-ebfe476c9299964b.so
#13 0x00007f70caa08eb5 in core::ops::function::FnOnce::call_once{{vtable-shim}} () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/librustc_driver-ebfe476c9299964b.so
#14 0x00007f70c9fe92ff in <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once () at /rustc/537ccdf3ac44c8c7a8d36cbdbe6fb224afabb7ae/src/liballoc/boxed.rs:1008
#15 0x00007f70ca01c813 in <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once () at /rustc/537ccdf3ac44c8c7a8d36cbdbe6fb224afabb7ae/src/liballoc/boxed.rs:1008
#16 std::sys_common::thread::start_thread () at src/libstd/sys_common/thread.rs:13
#17 std::sys::unix::thread::Thread::new::thread_start () at src/libstd/sys/unix/thread.rs:80
#18 0x00007f70c9f534e2 in start_thread () from /lib64/libpthread.so.0
#19 0x00007f70c9e706d3 in clone () from /lib64/libc.so.6

[cuviper]

I found this in the process of evaluating changes to Chain: #70332 (comment)

Importantly, this test code does not hang with those changes. So if the compiler is fixed to handle this better, it should probably come with a self-contained regression test including the "old" Chain (assuming my changes do land).

[cuviper]

Instead, this happened: I'm still waiting...

The good news is that it did finish when I wasn't looking -- then I ran it again to actually time it:

real    39m25.171s
user    50m13.046s
sys     0m5.276s

[wesleywiser]

I ran into a very similar issue while trying to enable the MIR inline pass by default. (More info in this Zulip thread)

If I remember correctly, using -Znew-llvm-pass-manager seemed to resolve the issue.

[cuviper]

Hmm, with new PM it completed in "only" 8 minutes here, and then 13 minutes the second time, with a snapshot of the stack like this:

Backtrace

Thread 5 (Thread 0x7f79b23ff700 (LWP 218091)):
#0  0x00007f79bb8c4354 in llvm::DominatorTreeBase<llvm::BasicBlock, false>::dominates(llvm::BasicBlock const*, llvm::BasicBlock const*) const () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#1  0x00007f79bc4d398a in llvm::GVN::findLeader(llvm::BasicBlock const*, unsigned int) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#2  0x00007f79bc4d4279 in llvm::GVN::processInstruction(llvm::Instruction*) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#3  0x00007f79bc4d5977 in llvm::GVN::processBlock(llvm::BasicBlock*) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#4  0x00007f79bc4cbff0 in llvm::GVN::runImpl(llvm::Function&, llvm::AssumptionCache&, llvm::DominatorTree&, llvm::TargetLibraryInfo const&, llvm::AAResults&, llvm::MemoryDependenceResults*, llvm::LoopInfo*, llvm::OptimizationRemarkEmitter*) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#5  0x00007f79bc4cbbe6 in llvm::GVN::run(llvm::Function&, llvm::AnalysisManager<llvm::Function>&) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#6  0x00007f79bdd1552d in llvm::detail::PassModel<llvm::Function, llvm::GVN, llvm::PreservedAnalyses, llvm::AnalysisManager<llvm::Function>>::run(llvm::Function&, llvm::AnalysisManager<llvm::Function>&) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#7  0x00007f79bb95abb3 in llvm::PassManager<llvm::Function, llvm::AnalysisManager<llvm::Function>>::run(llvm::Function&, llvm::AnalysisManager<llvm::Function>&) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#8  0x00007f79bdd1817b in llvm::detail::PassModel<llvm::LazyCallGraph::SCC, llvm::CGSCCToFunctionPassAdaptor<llvm::PassManager<llvm::Function, llvm::AnalysisManager<llvm::Function>> >, llvm::PreservedAnalyses, llvm::AnalysisManager<llvm::LazyCallGraph::SCC, llvm::LazyCallGraph&>, llvm::LazyCallGraph&, llvm::CGSCCUpdateResult&>::run(llvm::LazyCallGraph::SCC&, llvm::AnalysisManager<llvm::LazyCallGraph::SCC, llvm::LazyCallGraph&>&, llvm::LazyCallGraph&, llvm::CGSCCUpdateResult&) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#9  0x00007f79bc8d85a4 in llvm::PassManager<llvm::LazyCallGraph::SCC, llvm::AnalysisManager<llvm::LazyCallGraph::SCC, llvm::LazyCallGraph&>, llvm::LazyCallGraph&, llvm::CGSCCUpdateResult&>::run(llvm::LazyCallGraph::SCC&, llvm::AnalysisManager<llvm::LazyCallGraph::SCC, llvm::LazyCallGraph&>&, llvm::LazyCallGraph&, llvm::CGSCCUpdateResult&) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#10 0x00007f79bdd22000 in llvm::DevirtSCCRepeatedPass<llvm::PassManager<llvm::LazyCallGraph::SCC, llvm::AnalysisManager<llvm::LazyCallGraph::SCC, llvm::LazyCallGraph&>, llvm::LazyCallGraph&, llvm::CGSCCUpdateResult&> >::run(llvm::LazyCallGraph::SCC&, llvm::AnalysisManager<llvm::LazyCallGraph::SCC, llvm::LazyCallGraph&>&, llvm::LazyCallGraph&, llvm::CGSCCUpdateResult&) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#11 0x00007f79bdd20f83 in llvm::detail::PassModel<llvm::Module, llvm::ModuleToPostOrderCGSCCPassAdaptor<llvm::DevirtSCCRepeatedPass<llvm::PassManager<llvm::LazyCallGraph::SCC, llvm::AnalysisManager<llvm::LazyCallGraph::SCC, llvm::LazyCallGraph&>, llvm::LazyCallGraph&, llvm::CGSCCUpdateResult&> > >, llvm::PreservedAnalyses, llvm::AnalysisManager<llvm::Module>>::run(llvm::Module&, llvm::AnalysisManager<llvm::Module>&) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#12 0x00007f79bb958f6a in llvm::PassManager<llvm::Module, llvm::AnalysisManager<llvm::Module>>::run(llvm::Module&, llvm::AnalysisManager<llvm::Module>&) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#13 0x00007f79bdd2820d in llvm::detail::PassModel<llvm::Module, llvm::PassManager<llvm::Module, llvm::AnalysisManager<llvm::Module>>, llvm::PreservedAnalyses, llvm::AnalysisManager<llvm::Module>>::run(llvm::Module&, llvm::AnalysisManager<llvm::Module>&) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#14 0x00007f79bb958f6a in llvm::PassManager<llvm::Module, llvm::AnalysisManager<llvm::Module>>::run(llvm::Module&, llvm::AnalysisManager<llvm::Module>&) () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/../lib/libLLVM-9-rust-1.44.0-nightly.so
#15 0x00007f79bf1fc2d1 in LLVMRustOptimizeWithNewPassManager () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/librustc_driver-ebfe476c9299964b.so
#16 0x00007f79bf129c93 in rustc_codegen_llvm::back::write::optimize_with_new_llvm_pass_manager () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/librustc_driver-ebfe476c9299964b.so
#17 0x00007f79bf12a616 in rustc_codegen_llvm::back::write::optimize () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/librustc_driver-ebfe476c9299964b.so
#18 0x00007f79bf0eff17 in rustc_codegen_ssa::back::write::execute_work_item () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/librustc_driver-ebfe476c9299964b.so
#19 0x00007f79bf1d780e in std::sys_common::backtrace::__rust_begin_short_backtrace () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/librustc_driver-ebfe476c9299964b.so
#20 0x00007f79bf0b9eb5 in core::ops::function::FnOnce::call_once{{vtable-shim}} () from /home/jistone/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/../lib/librustc_driver-ebfe476c9299964b.so
#21 0x00007f79be69a2ff in <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once () at /rustc/537ccdf3ac44c8c7a8d36cbdbe6fb224afabb7ae/src/liballoc/boxed.rs:1008
#22 0x00007f79be6cd813 in <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once () at /rustc/537ccdf3ac44c8c7a8d36cbdbe6fb224afabb7ae/src/liballoc/boxed.rs:1008
#23 std::sys_common::thread::start_thread () at src/libstd/sys_common/thread.rs:13
#24 std::sys::unix::thread::Thread::new::thread_start () at src/libstd/sys/unix/thread.rs:80
#25 0x00007f79be6044e2 in start_thread () from /lib64/libpthread.so.0
#26 0x00007f79be5216d3 in clone () from /lib64/libc.so.6

[cuviper]

Since #70896 did land, removing the reproducer, here's a self-contained version:

#![crate_type = "lib"]
#![allow(unused)]

pub fn foo() -> Box<dyn Iterator<Item = u32>> {
    use std::iter::empty;

    Box::new(
        empty()
            .my_chain(empty())
            .my_chain(empty())
            .my_chain(empty())
            .my_chain(empty())
            .my_chain(empty())
            .my_chain(empty())
            .my_chain(empty())
            .my_chain(empty())
            .my_chain(empty())
            .my_chain(empty()) // 10th
            .my_chain(empty())
            .my_chain(empty())
            .my_chain(empty())
            .my_chain(empty())
            .my_chain(empty())
            .my_chain(empty()), // 16th
    )
}

// The `Chain` implementation changed in #70896, which stopped reproducing the optimization hang,
// but here's a reduced copy of the old `Chain` for posterity.
struct Chain<A, B> {
    a: A,
    b: B,
    state: ChainState,
}

enum ChainState {
    Both,
    Front,
    Back,
}

impl<A, B> Iterator for Chain<A, B>
where
    A: Iterator,
    B: Iterator<Item = A::Item>,
{
    type Item = A::Item;

    #[inline]
    fn next(&mut self) -> Option<A::Item> {
        match self.state {
            ChainState::Both => match self.a.next() {
                elt @ Some(..) => elt,
                None => {
                    self.state = ChainState::Back;
                    self.b.next()
                }
            },
            ChainState::Front => self.a.next(),
            ChainState::Back => self.b.next(),
        }
    }
}

trait IteratorExt: Iterator {
    fn my_chain<U>(self, other: U) -> Chain<Self, U::IntoIter>
    where
        Self: Sized,
        U: IntoIterator<Item = Self::Item>,
    {
        Chain {
            a: self,
            b: other.into_iter(),
            state: ChainState::Both,
        }
    }
}

impl<I: Iterator> IteratorExt for I {}

This compiles quickly (~150ms) in debug mode, but takes a very long time to optimize. I didn't wait to fully time it yet, but it's more than a minute, which is already unacceptable. (edit: 35m34s)

With -Znew-llvm-pass-manager it only takes 200ms, which is much better than I saw before, but I also didn't copy all of the old Chain methods here.

[tmiasko]

The performance bug can be triggered in original example by slightly increasing the LLVM inlining threshold (currently the issue starts at -Cinline-threshold=336 or above). This also came up in the past attempts to enable MIR inlining by default, e.g., in #68213 and #77307. The problem disappears with a single code generation unit.

The partitioning -Zprint-mono-items=lazy reveals a proximate cause. Implementation of iterator trait for chain is placed in one unit, implementation for empty in another. This makes a difference between large but completely trivial functions once empty is inlined, and large and complex when it isn't .