Regression: "SIGFPE: erroneous arithmetic operation" while compiling Servo in release mode

[SimonSapin]

Running RUSTFLAGS="-Z time-passes" mach build --release -p style -v, rustc 1.23.0-nightly (5041b3b 2017-11-19) succeeds, rustc 1.23.0-nightly (33374fa 2017-11-20) fails without an error message. Range: 5041b3b...33374fa. #45225 seems to be the only compiler change in that range. CC @eddyb

Full output: https://gist.github.com/anonymous/e0881601130599a2c3853f876b5efc7a

Extract that seems most relevant:

$ RUSTFLAGS="-Z time-passes" mach build --release -p style -v
cargo build -p style --manifest-path /home/simon/servo3/ports/servo/Cargo.toml --release -v
       Fresh unicode-xid v0.0.4
[…]
       Fresh style_traits v0.0.1 (file:///home/simon/servo3/components/style_traits)
   Compiling style v0.0.1 (file:///home/simon/servo3/components/style)
     Running `rustc --crate-name […]`
  time: 0.113; rss: 82MB	parsing
[…]
    time: 0.115; rss: 1659MB	codegen passes [core-fmt-builders.volatile]
    time: 0.042; rss: 1659MB	llvm function passes [style-values-specified-font]
    time: 1.306; rss: 1660MB	llvm module passes [std-collections-hash-map.volatile]
    time: 1.213; rss: 1661MB	llvm module passes [style-properties-declaration_block]
    time: 0.514; rss: 1659MB	llvm module passes [core-result.volatile]
    time: 0.040; rss: 1662MB	llvm function passes [style-animation]
error: Could not compile `style`.

Caused by:
  process didn't exit successfully: `rustc --crate-name style […]` (signal: 8, SIGFPE: erroneous arithmetic operation)

time-passes is used to get some idea of what stage of compilation is happening.

So far I have managed to run rustc with correct arguments within gdb.

My daily "build servo with rust nightly" job did not catch this because this only happens in release mode.

[SimonSapin]

After some gdb wrangling I got a stack trace:

#0  0x00007fffeff32159 in llvm::APInt::rotl(unsigned int) const ()
   from /home/simon/tmp/servo-share/rust/2017-11-21/rustc-nightly-x86_64-unknown-linux-gnu/rustc/bin/../lib/../lib/../lib/librustc_llvm-50edb4a75bc0fe7c.so
#1  0x00007fffeff3238b in llvm::APInt::isSplat(unsigned int) const ()
   from /home/simon/tmp/servo-share/rust/2017-11-21/rustc-nightly-x86_64-unknown-linux-gnu/rustc/bin/../lib/../lib/../lib/librustc_llvm-50edb4a75bc0fe7c.so
#2  0x00007fffefc9d1c7 in llvm::isBytewiseValue(llvm::Value*) ()
   from /home/simon/tmp/servo-share/rust/2017-11-21/rustc-nightly-x86_64-unknown-linux-gnu/rustc/bin/../lib/../lib/../lib/librustc_llvm-50edb4a75bc0fe7c.so
#3  0x00007fffef8fe7d7 in llvm::MemCpyOptPass::processStore(llvm::StoreInst*, llvm::ilist_iterator<llvm::ilist_detail::node_options<llvm::Instruction, false, false, void>, false, false>&) ()
   from /home/simon/tmp/servo-share/rust/2017-11-21/rustc-nightly-x86_64-unknown-linux-gnu/rustc/bin/../lib/../lib/../lib/librustc_llvm-50edb4a75bc0fe7c.so
#4  0x00007fffef8fff2b in llvm::MemCpyOptPass::iterateOnFunction(llvm::Function&) ()
   from /home/simon/tmp/servo-share/rust/2017-11-21/rustc-nightly-x86_64-unknown-linux-gnu/rustc/bin/../lib/../lib/../lib/librustc_llvm-50edb4a75bc0fe7c.so
#5  0x00007fffef900239 in llvm::MemCpyOptPass::runImpl(llvm::Function&, llvm::MemoryDependenceResults*, llvm::TargetLibraryInfo*, std::function<llvm::AAResults& ()>, std::function<llvm::AssumptionCache& ()>, std::function<llvm::DominatorTree& ()>) ()
   from /home/simon/tmp/servo-share/rust/2017-11-21/rustc-nightly-x86_64-unknown-linux-gnu/rustc/bin/../lib/../lib/../lib/librustc_llvm-50edb4a75bc0fe7c.so
#6  0x00007fffef9003fc in (anonymous namespace)::MemCpyOptLegacyPass::runOnFunction(llvm::Function&) ()
   from /home/simon/tmp/servo-share/rust/2017-11-21/rustc-nightly-x86_64-unknown-linux-gnu/rustc/bin/../lib/../lib/../lib/librustc_llvm-50edb4a75bc0fe7c.so
#7  0x00007fffefdf5f4a in llvm::FPPassManager::runOnFunction(llvm::Function&) ()
   from /home/simon/tmp/servo-share/rust/2017-11-21/rustc-nightly-x86_64-unknown-linux-gnu/rustc/bin/../lib/../lib/../lib/librustc_llvm-50edb4a75bc0fe7c.so
#8  0x00007fffefb6c5df in (anonymous namespace)::CGPassManager::runOnModule(llvm::Module&) ()
   from /home/simon/tmp/servo-share/rust/2017-11-21/rustc-nightly-x86_64-unknown-linux-gnu/rustc/bin/../lib/../lib/../lib/librustc_llvm-50edb4a75bc0fe7c.so
#9  0x00007fffefdf6950 in llvm::legacy::PassManagerImpl::run(llvm::Module&) ()
   from /home/simon/tmp/servo-share/rust/2017-11-21/rustc-nightly-x86_64-unknown-linux-gnu/rustc/bin/../lib/../lib/../lib/librustc_llvm-50edb4a75bc0fe7c.so
#10 0x00007fffefd8d879 in LLVMRunPassManager ()
   from /home/simon/tmp/servo-share/rust/2017-11-21/rustc-nightly-x86_64-unknown-linux-gnu/rustc/bin/../lib/../lib/../lib/librustc_llvm-50edb4a75bc0fe7c.so
#11 0x00007ffff5b592f5 in rustc::util::common::time ()
   from /home/simon/tmp/servo-share/rust/2017-11-21/rustc-nightly-x86_64-unknown-linux-gnu/rustc/bin/../lib/../lib/librustc_trans-64828bfa1a07d0dd.so
#12 0x00007ffff5b1eaba in std::sys_common::backtrace::__rust_begin_short_backtrace ()
   from /home/simon/tmp/servo-share/rust/2017-11-21/rustc-nightly-x86_64-unknown-linux-gnu/rustc/bin/../lib/../lib/librustc_trans-64828bfa1a07d0dd.so
#13 0x00007ffff5b23408 in std::panicking::try::do_call ()
   from /home/simon/tmp/servo-share/rust/2017-11-21/rustc-nightly-x86_64-unknown-linux-gnu/rustc/bin/../lib/../lib/librustc_trans-64828bfa1a07d0dd.so
#14 0x00007ffff76b3e6f in panic_unwind::__rust_maybe_catch_panic () at /checkout/src/libpanic_unwind/lib.rs:101
#15 0x00007ffff5b29d0b in <F as alloc::boxed::FnBox<A>>::call_box ()
   from /home/simon/tmp/servo-share/rust/2017-11-21/rustc-nightly-x86_64-unknown-linux-gnu/rustc/bin/../lib/../lib/librustc_trans-64828bfa1a07d0dd.so
#16 0x00007ffff76a73cc in alloc::boxed::{{impl}}::call_once<(),()> () at /checkout/src/liballoc/boxed.rs:782
#17 std::sys_common::thread::start_thread () at /checkout/src/libstd/sys_common/thread.rs:24
#18 std::sys::imp::thread::{{impl}}::new::thread_start () at /checkout/src/libstd/sys/unix/thread.rs:90
#19 0x00007ffff15e27fc in start_thread (arg=0x7fff661ff700) at pthread_create.c:465
#20 0x00007ffff7362b0f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

[SimonSapin]

disassemble in gdb:

=> 0x00007fffeff32159 <+25>:	div    %r8d

Details

Dump of assembler code for function _ZNK4llvm5APInt4rotlEj:
   0x00007fffeff32140 <+0>:	push   %r14
   0x00007fffeff32142 <+2>:	mov    %edx,%eax
   0x00007fffeff32144 <+4>:	xor    %edx,%edx
   0x00007fffeff32146 <+6>:	push   %r13
   0x00007fffeff32148 <+8>:	push   %r12
   0x00007fffeff3214a <+10>:	push   %rbp
   0x00007fffeff3214b <+11>:	mov    %rsi,%rbp
   0x00007fffeff3214e <+14>:	push   %rbx
   0x00007fffeff3214f <+15>:	mov    %rdi,%rbx
   0x00007fffeff32152 <+18>:	sub    $0x20,%rsp
   0x00007fffeff32156 <+22>:	mov    (%rsi),%r8d
=> 0x00007fffeff32159 <+25>:	div    %r8d
   0x00007fffeff3215c <+28>:	test   %edx,%edx
   0x00007fffeff3215e <+30>:	mov    %edx,%r12d
   0x00007fffeff32161 <+33>:	jne    0x7fffeff32190 <_ZNK4llvm5APInt4rotlEj+80>
   0x00007fffeff32163 <+35>:	cmp    $0x40,%r8d
   0x00007fffeff32167 <+39>:	mov    %r8d,(%rdi)
   0x00007fffeff3216a <+42>:	movq   $0x0,0x8(%rdi)
   0x00007fffeff32172 <+50>:	ja     0x7fffeff32238 <_ZNK4llvm5APInt4rotlEj+248>
   0x00007fffeff32178 <+56>:	mov    0x8(%rsi),%rax
   0x00007fffeff3217c <+60>:	mov    %rax,0x8(%rdi)
   0x00007fffeff32180 <+64>:	add    $0x20,%rsp
   0x00007fffeff32184 <+68>:	mov    %rbx,%rax
   0x00007fffeff32187 <+71>:	pop    %rbx
   0x00007fffeff32188 <+72>:	pop    %rbp
   0x00007fffeff32189 <+73>:	pop    %r12
   0x00007fffeff3218b <+75>:	pop    %r13
   0x00007fffeff3218d <+77>:	pop    %r14
   0x00007fffeff3218f <+79>:	retq   
   0x00007fffeff32190 <+80>:	lea    0x10(%rsp),%r13
   0x00007fffeff32195 <+85>:	mov    %r8d,%edx
   0x00007fffeff32198 <+88>:	sub    %r12d,%edx
   0x00007fffeff3219b <+91>:	mov    %r13,%rdi
   0x00007fffeff3219e <+94>:	callq  0x7fffee7b3910 <_ZNK4llvm5APInt4lshrEj@plt>
   0x00007fffeff321a3 <+99>:	mov    0x0(%rbp),%eax
   0x00007fffeff321a6 <+102>:	cmp    $0x40,%eax
   0x00007fffeff321a9 <+105>:	ja     0x7fffeff322b0 <_ZNK4llvm5APInt4rotlEj+368>
   0x00007fffeff321af <+111>:	cmp    %eax,%r12d
   0x00007fffeff321b2 <+114>:	jae    0x7fffeff32250 <_ZNK4llvm5APInt4rotlEj+272>
   0x00007fffeff321b8 <+120>:	mov    0x8(%rbp),%rdx
   0x00007fffeff321bc <+124>:	mov    %r12d,%ecx
   0x00007fffeff321bf <+127>:	mov    %rsp,%rdi
   0x00007fffeff321c2 <+130>:	mov    %eax,(%rsp)
   0x00007fffeff321c5 <+133>:	mov    %rsp,%r14
   0x00007fffeff321c8 <+136>:	shl    %cl,%rdx
   0x00007fffeff321cb <+139>:	mov    %rdx,0x8(%rsp)
   0x00007fffeff321d0 <+144>:	callq  0x7fffee94cab0 <_ZN4llvm5APInt15clearUnusedBitsEv>
   0x00007fffeff321d5 <+149>:	mov    (%rsp),%eax
   0x00007fffeff321d8 <+152>:	cmp    $0x40,%eax
   0x00007fffeff321db <+155>:	jbe    0x7fffeff32273 <_ZNK4llvm5APInt4rotlEj+307>
   0x00007fffeff321e1 <+161>:	mov    %r13,%rdx
   0x00007fffeff321e4 <+164>:	mov    %r14,%rsi
   0x00007fffeff321e7 <+167>:	mov    %rbx,%rdi
   0x00007fffeff321ea <+170>:	callq  0x7fffee7c1290 <_ZNK4llvm5APInt10OrSlowCaseERKS0_@plt>
   0x00007fffeff321ef <+175>:	cmpl   $0x40,(%rsp)
   0x00007fffeff321f3 <+179>:	jbe    0x7fffeff32204 <_ZNK4llvm5APInt4rotlEj+196>
   0x00007fffeff321f5 <+181>:	mov    0x8(%rsp),%rdi
   0x00007fffeff321fa <+186>:	test   %rdi,%rdi
   0x00007fffeff321fd <+189>:	je     0x7fffeff32204 <_ZNK4llvm5APInt4rotlEj+196>
   0x00007fffeff321ff <+191>:	callq  0x7fffee7e99f0 <_ZdaPv@plt>
   0x00007fffeff32204 <+196>:	cmpl   $0x40,0x10(%rsp)
   0x00007fffeff32209 <+201>:	jbe    0x7fffeff32180 <_ZNK4llvm5APInt4rotlEj+64>
   0x00007fffeff3220f <+207>:	mov    0x18(%rsp),%rdi
   0x00007fffeff32214 <+212>:	test   %rdi,%rdi
   0x00007fffeff32217 <+215>:	je     0x7fffeff32180 <_ZNK4llvm5APInt4rotlEj+64>
   0x00007fffeff3221d <+221>:	callq  0x7fffee7e99f0 <_ZdaPv@plt>
   0x00007fffeff32222 <+226>:	add    $0x20,%rsp
   0x00007fffeff32226 <+230>:	mov    %rbx,%rax
   0x00007fffeff32229 <+233>:	pop    %rbx
   0x00007fffeff3222a <+234>:	pop    %rbp
   0x00007fffeff3222b <+235>:	pop    %r12
   0x00007fffeff3222d <+237>:	pop    %r13
   0x00007fffeff3222f <+239>:	pop    %r14
   0x00007fffeff32231 <+241>:	retq   
   0x00007fffeff32232 <+242>:	nopw   0x0(%rax,%rax,1)
   0x00007fffeff32238 <+248>:	callq  0x7fffee7d1d20 <_ZN4llvm5APInt12initSlowCaseERKS0_@plt>
   0x00007fffeff3223d <+253>:	add    $0x20,%rsp
   0x00007fffeff32241 <+257>:	mov    %rbx,%rax
   0x00007fffeff32244 <+260>:	pop    %rbx
   0x00007fffeff32245 <+261>:	pop    %rbp
   0x00007fffeff32246 <+262>:	pop    %r12
   0x00007fffeff32248 <+264>:	pop    %r13
   0x00007fffeff3224a <+266>:	pop    %r14
   0x00007fffeff3224c <+268>:	retq   
   0x00007fffeff3224d <+269>:	nopl   (%rax)
   0x00007fffeff32250 <+272>:	mov    %rsp,%rdi
   0x00007fffeff32253 <+275>:	mov    %eax,(%rsp)
   0x00007fffeff32256 <+278>:	movq   $0x0,0x8(%rsp)
   0x00007fffeff3225f <+287>:	callq  0x7fffee94cab0 <_ZN4llvm5APInt15clearUnusedBitsEv>
   0x00007fffeff32264 <+292>:	mov    (%rsp),%eax
   0x00007fffeff32267 <+295>:	mov    %rsp,%r14
   0x00007fffeff3226a <+298>:	cmp    $0x40,%eax
   0x00007fffeff3226d <+301>:	ja     0x7fffeff321e1 <_ZNK4llvm5APInt4rotlEj+161>
   0x00007fffeff32273 <+307>:	mov    0x18(%rsp),%rdx
   0x00007fffeff32278 <+312>:	or     0x8(%rsp),%rdx
   0x00007fffeff3227d <+317>:	mov    %eax,(%rbx)
   0x00007fffeff3227f <+319>:	and    $0x3f,%eax
   0x00007fffeff32282 <+322>:	mov    %rdx,0x8(%rbx)
   0x00007fffeff32286 <+326>:	je     0x7fffeff32204 <_ZNK4llvm5APInt4rotlEj+196>
   0x00007fffeff3228c <+332>:	mov    $0x40,%ecx
   0x00007fffeff32291 <+337>:	sub    %eax,%ecx
   0x00007fffeff32293 <+339>:	mov    $0xffffffffffffffff,%rax
   0x00007fffeff3229a <+346>:	shr    %cl,%rax
   0x00007fffeff3229d <+349>:	and    %rdx,%rax
   0x00007fffeff322a0 <+352>:	mov    %rax,0x8(%rbx)
   0x00007fffeff322a4 <+356>:	jmpq   0x7fffeff32204 <_ZNK4llvm5APInt4rotlEj+196>
   0x00007fffeff322a9 <+361>:	nopl   0x0(%rax)
   0x00007fffeff322b0 <+368>:	mov    %r12d,%edx
   0x00007fffeff322b3 <+371>:	mov    %rbp,%rsi
   0x00007fffeff322b6 <+374>:	mov    %rsp,%rdi
   0x00007fffeff322b9 <+377>:	mov    %rsp,%r14
   0x00007fffeff322bc <+380>:	callq  0x7fffee7b3b90 <_ZNK4llvm5APInt11shlSlowCaseEj@plt>
   0x00007fffeff322c1 <+385>:	jmpq   0x7fffeff321d5 <_ZNK4llvm5APInt4rotlEj+149>
End of assembler dump.

[SimonSapin]

With a "alt" compiler downloaded from https://s3.amazonaws.com/rust-lang-ci/rustc-builds-alt/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea/rust-nightly-x86_64-unknown-linux-gnu.tar.gz, the output is:

$ cargo +alt-2017-12-04 build --release -p style --manifest-path ports/servo/Cargo.toml -v
[…]
rustc: /checkout/src/llvm/include/llvm/Support/Casting.h:236: typename llvm::cast_retty<X, Y*>::ret_type llvm::cast(Y*) [with X = llvm::IntegerType; Y = llvm::Type; typename llvm::cast_retty<X, Y*>::ret_type = llvm::IntegerType*]: Assertion `isa<X>(Val) && "cast<Ty>() argument of incompatible type!"' failed.
error: Could not compile `style`.

Caused by:
  process didn't exit successfully: `rustc --crate-name style […]` (signal: 6, SIGABRT: process abort signal)

[SimonSapin]

And gdb stack track for that:

#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51
#1  0x00007ffff726ff5d in __GI_abort () at abort.c:90
#2  0x00007ffff7265f17 in __assert_fail_base (fmt=<optimized out>, 
    assertion=assertion@entry=0x7fffef3ea0e0 "isa<X>(Val) && \"cast<Ty>() argument of incompatible type!\"", 
    file=file@entry=0x7fffef547a20 "/checkout/src/llvm/include/llvm/Support/Casting.h", line=line@entry=236, 
    function=function@entry=0x7ffff0539ba0 <_ZZN4llvm4castINS_11IntegerTypeENS_4TypeEEENS_10cast_rettyIT_PT0_E8ret_typeES6_E19__PRETTY_FUNCTION__> "typename llvm::cast_retty<X, Y*>::ret_type llvm::cast(Y*) [with X = llvm::IntegerType; Y = llvm::Type; typename llvm::cast_retty<X, Y*>::ret_type = llvm::IntegerType*]") at assert.c:92
#3  0x00007ffff7265fc2 in __GI___assert_fail (
    assertion=0x7fffef3ea0e0 "isa<X>(Val) && \"cast<Ty>() argument of incompatible type!\"", 
    file=0x7fffef547a20 "/checkout/src/llvm/include/llvm/Support/Casting.h", line=236, 
    function=0x7ffff0539ba0 <_ZZN4llvm4castINS_11IntegerTypeENS_4TypeEEENS_10cast_rettyIT_PT0_E8ret_typeES6_E19__PRETTY_FUNCTION__> "typename llvm::cast_retty<X, Y*>::ret_type llvm::cast(Y*) [with X = llvm::IntegerType; Y = llvm::Type; typename llvm::cast_retty<X, Y*>::ret_type = llvm::IntegerType*]") at assert.c:101
#4  0x00007fffef0f5eca in LLVMConstIntOfArbitraryPrecision ()
   from /home/simon/tmp/servo-share/rust/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea-alt/bin/../lib/../lib/../lib/librustc_llvm-fe79b73fb19ea62e.so
#5  0x00007ffff5a400c7 in rustc_trans::mir::place::PlaceRef::trans_set_discr ()
   from /home/simon/tmp/servo-share/rust/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea-alt/bin/../lib/../lib/librustc_trans-84cc35c4fa80a856.so
#6  0x00007ffff5a51546 in rustc_trans::mir::rvalue::<impl rustc_trans::mir::MirContext<'a, 'tcx>>::trans_rvalue ()
   from /home/simon/tmp/servo-share/rust/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea-alt/bin/../lib/../lib/librustc_trans-84cc35c4fa80a856.so

[…]

Details

#7  0x00007ffff5a49a69 in rustc_trans::mir::trans_mir ()
   from /home/simon/tmp/servo-share/rust/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea-alt/bin/../lib/../lib/librustc_trans-84cc35c4fa80a856.so
#8  0x00007ffff5a66c36 in rustc_trans::base::trans_instance ()
   from /home/simon/tmp/servo-share/rust/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea-alt/bin/../lib/../lib/librustc_trans-84cc35c4fa80a856.so
#9  0x00007ffff5a97b75 in rustc_trans::trans_item::TransItemExt::define ()
   from /home/simon/tmp/servo-share/rust/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea-alt/bin/../lib/../lib/librustc_trans-84cc35c4fa80a856.so
#10 0x00007ffff5a6afc4 in rustc_trans::base::compile_codegen_unit ()
   from /home/simon/tmp/servo-share/rust/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea-alt/bin/../lib/../lib/librustc_trans-84cc35c4fa80a856.so
#11 0x00007ffff369af92 in rustc::ty::maps::<impl rustc::ty::maps::queries::compile_codegen_unit<'tcx>>::compute_result
    ()
   from /home/simon/tmp/servo-share/rust/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea-alt/bin/../lib/../lib/librustc-d6aca8ae434d20ce.so
#12 0x00007ffff3885ea1 in rustc::dep_graph::graph::DepGraph::with_task_impl ()
   from /home/simon/tmp/servo-share/rust/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea-alt/bin/../lib/../lib/librustc-d6aca8ae434d20ce.so
#13 0x00007ffff34be2d2 in rustc_errors::Handler::track_diagnostics ()
   from /home/simon/tmp/servo-share/rust/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea-alt/bin/../lib/../lib/librustc-d6aca8ae434d20ce.so
#14 0x00007ffff3a28240 in rustc::ty::maps::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::cycle_check ()
   from /home/simon/tmp/servo-share/rust/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea-alt/bin/../lib/../lib/librustc-d6aca8ae434d20ce.so
#15 0x00007ffff369b03a in rustc::ty::maps::<impl rustc::ty::maps::queries::compile_codegen_unit<'tcx>>::force ()
   from /home/simon/tmp/servo-share/rust/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea-alt/bin/../lib/../lib/librustc-d6aca8ae434d20ce.so
#16 0x00007ffff369ba0f in rustc::ty::maps::<impl rustc::ty::maps::queries::compile_codegen_unit<'tcx>>::try_get ()
   from /home/simon/tmp/servo-share/rust/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea-alt/bin/../lib/../lib/librustc-d6aca8ae434d20ce.so
#17 0x00007ffff37b04bd in rustc::ty::maps::TyCtxtAt::compile_codegen_unit ()
   from /home/simon/tmp/servo-share/rust/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea-alt/bin/../lib/../lib/librustc-d6aca8ae434d20ce.so
#18 0x00007ffff3a4704a in rustc::ty::maps::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::compile_codegen_unit ()
   from /home/simon/tmp/servo-share/rust/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea-alt/bin/../lib/../lib/librustc-d6aca8ae434d20ce.so
#19 0x00007ffff5a68f6e in rustc_trans::base::trans_crate ()
   from /home/simon/tmp/servo-share/rust/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea-alt/bin/../lib/../lib/librustc_trans-84cc35c4fa80a856.so
#20 0x00007ffff5af88c7 in <rustc_trans::LlvmTransCrate as rustc_trans_utils::trans_crate::TransCrate>::trans_crate ()
   from /home/simon/tmp/servo-share/rust/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea-alt/bin/../lib/../lib/librustc_trans-84cc35c4fa80a856.so
#21 0x00007ffff7a356d8 in rustc::util::common::time ()
   from /home/simon/tmp/servo-share/rust/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea-alt/bin/../lib/librustc_driver-33a33ebd533ddabe.so
#22 0x00007ffff7a61fae in rustc_driver::driver::phase_4_translate_to_llvm ()
   from /home/simon/tmp/servo-share/rust/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea-alt/bin/../lib/librustc_driver-33a33ebd533ddabe.so
#23 0x00007ffff7aa5213 in rustc_driver::driver::compile_input::{{closure}} ()
   from /home/simon/tmp/servo-share/rust/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea-alt/bin/../lib/librustc_driver-33a33ebd533ddabe.so
#24 0x00007ffff7a9aaa2 in <std::thread::local::LocalKey<T>>::with ()
   from /home/simon/tmp/servo-share/rust/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea-alt/bin/../lib/librustc_driver-33a33ebd533ddabe.so
#25 0x00007ffff7a9e407 in <std::thread::local::LocalKey<T>>::with ()
   from /home/simon/tmp/servo-share/rust/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea-alt/bin/../lib/librustc_driver-33a33ebd533ddabe.so
#26 0x00007ffff7ae98a8 in rustc::ty::context::TyCtxt::create_and_enter ()
   from /home/simon/tmp/servo-share/rust/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea-alt/bin/../lib/librustc_driver-33a33ebd533ddabe.so
#27 0x00007ffff7a5d195 in rustc_driver::driver::compile_input ()
   from /home/simon/tmp/servo-share/rust/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea-alt/bin/../lib/librustc_driver-33a33ebd533ddabe.so
#28 0x00007ffff7af6ca4 in rustc_driver::run_compiler ()
   from /home/simon/tmp/servo-share/rust/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea-alt/bin/../lib/librustc_driver-33a33ebd533ddabe.so
#29 0x00007ffff7a3bed2 in std::sys_common::backtrace::__rust_begin_short_backtrace ()
   from /home/simon/tmp/servo-share/rust/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea-alt/bin/../lib/librustc_driver-33a33ebd533ddabe.so
#30 0x00007ffff76a94bf in panic_unwind::__rust_maybe_catch_panic () at /checkout/src/libpanic_unwind/lib.rs:101
#31 0x00007ffff7ab8823 in <F as alloc::boxed::FnBox<A>>::call_box ()
   from /home/simon/tmp/servo-share/rust/1956d5535ad77ddf46e4b29ba089a8b4a73cfaea-alt/bin/../lib/librustc_driver-33a33ebd533ddabe.so
#32 0x00007ffff766567c in alloc::boxed::{{impl}}::call_once<(),()> () at /checkout/src/liballoc/boxed.rs:827
#33 std::sys_common::thread::start_thread () at /checkout/src/libstd/sys_common/thread.rs:24
#34 std::sys::unix::thread::{{impl}}::new::thread_start () at /checkout/src/libstd/sys/unix/thread.rs:90
#35 0x00007ffff166c7fc in start_thread (arg=0x7fffec1ff700) at pthread_create.c:465
#36 0x00007ffff734bb0f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

[SimonSapin]

This LLVM assertion fails in both debug and release mode, where as the division by zero in a "non-alt" compiler only happens in release mode. (Compilation succeeds in debug non-alt.)

[eddyb]

@SimonSapin Any chance you can get a hold of a compiler with debug assertions enabled to get the last few lines of compilation with RUST_LOG=rustc_trans?

[SimonSapin]

Sure.

INFO:rustc_trans::base: trans_instance(values::specified::font::FontFeatureSettings::system_font)
INFO:rustc_trans::base: trans_instance(values::specified::font::FontFeatureSettings::get_system)
INFO:rustc_trans::base: trans_instance(<values::specified::font::FontFeatureSettings as values::computed::ToComputedValue>::to_computed_value)
INFO:rustc_trans::base: trans_instance(<values::specified::font::FontFeatureSettings as values::computed::ToComputedValue>::from_computed_value)
INFO:rustc_trans::base: trans_instance(<values::specified::font::FontFeatureSettings as parser::Parse>::parse)
INFO:rustc_trans::base: trans_instance(<values::specified::font::FontSynthesis as parser::Parse>::parse)
INFO:rustc_trans::base: trans_instance(values::specified::font::FontLanguageOverride::system_font)
rustc: /checkout/src/llvm/include/llvm/Support/Casting.h:236: typename llvm::cast_retty<X, Y*>::ret_type llvm::cast(Y*) [with X = llvm::IntegerType; Y = llvm::Type; typename llvm::cast_retty<X, Y*>::ret_type = llvm::IntegerType*]: Assertion `isa<X>(Val) && "cast<Ty>() argument of incompatible type!"' failed.
error: Could not compile `style`.

Type definition for the last one:

#[derive(Clone, Debug, Eq, MallocSizeOf, PartialEq, ToCss)]
/// Allows authors to explicitly specify the language system of the font,
/// overriding the language system implied by the content language
pub enum FontLanguageOverride {
    /// When rendering with OpenType fonts,
    /// the content language of the element is
    /// used to infer the OpenType language system
    Normal,
    /// Single three-letter case-sensitive OpenType language system tag,
    /// specifies the OpenType language system to be used instead of
    /// the language system implied by the language of the element
    Override(Box<str>),
    /// Use system font
    System(SystemFont)
}

impl FontLanguageOverride {
    /// Get `font-language-override` with `system font`
    pub fn system_font(f: SystemFont) -> Self {
        FontLanguageOverride::System(f)
    }
}

        <%
            system_fonts = """caption icon menu message-box small-caption status-bar
                              -moz-window -moz-document -moz-workspace -moz-desktop
                              -moz-info -moz-dialog -moz-button -moz-pull-down-menu
                              -moz-list -moz-field""".split()
        %>

        #[derive(Clone, Copy, Debug, Eq, Hash, MallocSizeOf, PartialEq, ToCss)]
        pub enum SystemFont {
            % for font in system_fonts:
                ${to_camel_case(font)},
            % endfor
        }

The second enum is generated by a Mako template, it looks like it’s "C-like"

[eddyb]

Okay, so the culprit appears to be a missing check for uninhabited enums, between these lines:

rust/src/librustc_trans/mir/place.rs

Lines 361 to 362 in abe85ab

	pub fn trans_set_discr(&self, bcx: &Builder<'a, 'tcx>, variant_index: usize) {
	match self.layout.variants {

The check should look like this:

    if self.layout.for_variant(bcx.ccx, variant_index).abi == layout::Abi::Uninhabited {
        return;
    }

This also lets us simplify an existing check:

rust/src/librustc_trans/mir/place.rs

Lines 364 to 369 in abe85ab

	if index != variant_index {
	// If the layout of an enum is `Single`, all
	// other variants are necessarily uninhabited.
	assert_eq!(self.layout.for_variant(bcx.ccx, variant_index).abi,
	layout::Abi::Uninhabited);
	}

To just:

                assert_eq!(index, variant_index);