[Nightly] LLVM Assertion: SrcTy must be larger than DestTy for Trunc

[steffengy]

Last good version: rustc 1.19.0-nightly (6a5fc9eec 2017-05-02) (previous nightly)
Version: rustc 1.19.0-nightly (2d4ed8e0c 2017-05-03)

Unfortunately I can't provide a small testcase for reproduction,
but the following works reliably:

Reproduction

EDIT: For readability the reduced example from below:

trait Tc {}
impl Tc for bool {}

fn main() {
    let _: &[&Tc] = &[&true];
}

OLD:

# You'll need openssl for the build process on linux, e.g. for ubuntu:
sudo apt-get install libssl-dev
git clone https://github.com/steffengy/tiberius -b ssl
cd tiberius && cargo test

Output:

rustc: /checkout/src/llvm/lib/IR/Constants.cpp:1568: static llvm::Constant*
 llvm::ConstantExpr::getTrunc(llvm::Constant*, llvm::Type*, bool): 
Assertion `C->getType()->getScalarSizeInBits() > Ty->getScalarSizeInBits()&& 
SrcTy must be larger than DestTy for Trunc!"' failed.

[kennytm]

The error comes from the test_bit_0 and test_bit_1 in src/types/mod.rs.

[steffengy]

That was fast.

Just to add a bit of context:

test_bit_0 and test_bit_1 are tests that ensure that different types work
when passed as query parameters.

The difference between test_bit_0 and test_i8 is only in
what the following two traits return for a specific value
(test_bit_0 tests for bool [more specific for false], test_i8 for i8, ...):

pub trait ToColumnData {
    fn to_column_data(&self) -> ColumnData;
}
pub trait ToSql : ToColumnData {
    fn to_sql(&self) -> &'static str;
}

ColumnData is an enum of several possible values (e.g. ColumnData::I8(i8))

Interestingly enough it's not enough for reproduction to just define
ToSql for bool and call to_sql and/or to_column_data for
a (casted) value boolval as &ToSql.

(And FromColumnData is used for the .get assert)

Therefore I'm not quite sure about what's actually causing this.

[kennytm]

Reduced test case:

trait Tc {}
impl Tc for bool {}

fn main() {
    let _: &[&Tc] = &[&true];
}

(Tc is the ToSql or ToColumnData trait in the real code).

[kennytm]

LLDB Stack trace

(lldb) r
Process 34235 launched: '~/.rustup/toolchains/nightly-x86_64-apple-darwin/bin/rustc' (x86_64)
Assertion failed: (C->getType()->getScalarSizeInBits() > Ty->getScalarSizeInBits()&& "SrcTy must be larger than DestTy for Trunc!"), function getTrunc, file /Users/travis/build/rust-lang/rust/src/llvm/lib/IR/Constants.cpp, line 1568.
Process 34235 stopped
* thread #2, name = 'rustc', stop reason = signal SIGABRT
    frame #0: 0x00007fffc99b9d42 libsystem_kernel.dylib`__pthread_kill + 10
libsystem_kernel.dylib`__pthread_kill:
->  0x7fffc99b9d42 <+10>: jae    0x7fffc99b9d4c            ; <+20>
    0x7fffc99b9d44 <+12>: movq   %rax, %rdi
    0x7fffc99b9d47 <+15>: jmp    0x7fffc99b2caf            ; cerror_nocancel
    0x7fffc99b9d4c <+20>: retq   
(lldb) bt
* thread #2, name = 'rustc', stop reason = signal SIGABRT
  * frame #0: 0x00007fffc99b9d42 libsystem_kernel.dylib`__pthread_kill + 10
    frame #1: 0x00007fffc9aa75bf libsystem_pthread.dylib`pthread_kill + 90
    frame #2: 0x00007fffc991f420 libsystem_c.dylib`abort + 129
    frame #3: 0x00007fffc98e6893 libsystem_c.dylib`__assert_rtn + 320
    frame #4: 0x0000000103f45f84 librustc_llvm-a020c9c42ea0ddd7.dylib`llvm::ConstantExpr::getTrunc(llvm::Constant*, llvm::Type*, bool) + 484
    frame #5: 0x0000000100fd13ec librustc_trans-2a23ec83470b8da3.dylib`rustc_trans::mir::constant::MirConstContext::const_lvalue::hcabbdbff5e8ff317 + 2668
    frame #6: 0x0000000100fd2422 librustc_trans-2a23ec83470b8da3.dylib`rustc_trans::mir::constant::MirConstContext::const_rvalue::h19d240f0e896d085 + 418
    frame #7: 0x0000000100fcfad0 librustc_trans-2a23ec83470b8da3.dylib`rustc_trans::mir::constant::MirConstContext::trans::ha4a4f90ba0446139 + 1840
    frame #8: 0x0000000100fd6749 librustc_trans-2a23ec83470b8da3.dylib`rustc_trans::mir::constant::_$LT$impl$u20$rustc_trans..mir..MirContext$LT$$u27$a$C$$u20$$u27$tcx$GT$$GT$::trans_constant::hfab8591e311015c1 + 697
    frame #9: 0x0000000100fd9f91 librustc_trans-2a23ec83470b8da3.dylib`rustc_trans::mir::operand::_$LT$impl$u20$rustc_trans..mir..MirContext$LT$$u27$a$C$$u20$$u27$tcx$GT$$GT$::trans_operand::he38cb0ddc21903f4 + 49
    frame #10: 0x0000000100fdb6f1 librustc_trans-2a23ec83470b8da3.dylib`rustc_trans::mir::rvalue::_$LT$impl$u20$rustc_trans..mir..MirContext$LT$$u27$a$C$$u20$$u27$tcx$GT$$GT$::trans_rvalue_operand::hee4d435691371db0 + 129
    frame #11: 0x0000000100fc6944 librustc_trans-2a23ec83470b8da3.dylib`rustc_trans::mir::block::_$LT$impl$u20$rustc_trans..mir..MirContext$LT$$u27$a$C$$u20$$u27$tcx$GT$$GT$::trans_block::he55852e7fc7fd602 + 4404
    frame #12: 0x0000000100fc351c librustc_trans-2a23ec83470b8da3.dylib`rustc_trans::mir::trans_mir::h2b1fef795fef4867 + 16268
    frame #13: 0x0000000100fe875e librustc_trans-2a23ec83470b8da3.dylib`rustc_trans::trans_item::TransItem::define::h66a94f3ed0cbd750 + 2414
    frame #14: 0x0000000100f8190b librustc_trans-2a23ec83470b8da3.dylib`rustc_trans::base::trans_crate::hc8538e6b8fc181e8 + 12955
    frame #15: 0x000000010019706f librustc_driver-45e2d96c004bc54e.dylib`rustc_driver::driver::phase_4_translate_to_llvm::h8794d833f6109935 + 1567
<snip>

Triggered from this line:

                            if projected_ty.is_bool() {
                                unsafe {
                                    val = llvm::LLVMConstTrunc(val, Type::i1(self.ccx).to_ref());
                                }
                            }

The LLVM assertion:

Constant *ConstantExpr::getTrunc(Constant *C, Type *Ty, bool OnlyIfReduced) {
  ...
  assert(C->getType()->getScalarSizeInBits() > Ty->getScalarSizeInBits()&&
"SrcTy must be larger than DestTy for Trunc!");

[kennytm]

Can't repro on the master version.

Probably already fixed. I suggest you wait for tomorrow's nightly.

[TimNN]

Still fails on master for me, did you build with LLVM Assertions, @kennytm?

[kennytm]

@TimNN Ah llvm.assertions = false by default. So right it is likely still not fixed. Sorry.

[TimNN]

Introduced in the rollup #41717 which leads me to highly suspect #41625 (cc @nikomatsakis).

[TimNN]

The problem appears to be that in https://github.com/rust-lang/rust/blob/master/src/librustc_trans/mir/constant.rs#L419 ConstantExpr::getTrunc is called with a bool constant and bool type as an argument, which causes the assertion because 1 > 1 is obviously false.

I don't know enough about that part of the compiler / LLVM to decide what the fix here should be.

[nikomatsakis]

Off-hand, I see no reason that #41625 would be "at fault" here, or would even change the MIR being generated, but I guess I can't rule that out. It might also have just exposed an underlying problem, I'm not sure. Certainly it looks like the most suspicious member of #41717

[TimNN]

Here is the diff of --emit=mir -Zno-trans: https://gist.github.com/TimNN/0bf68095617926e0046fa7e5205e5c5e

[TimNN]

Ah, so with -Zmir-opt-level=0, the assertion also occurs before #41625 (and also (long) before the LLVM 4.0 upgrade).

[TimNN]

With -Zmir-opt-level=0 this was introduced between nightly-2017-01-08 and nightly-2017-01-12 (Changes).

[TimNN]

I've looked over the PRs merged in that period and suspect this was a pre-existing bug which was exposed by #38837. Other potentially interesting PRs in that range: #38813, #38989

[nikomatsakis]

Seems like the right fix is just to skip the call to truncate if the thing is already boolean. @eddyb will take it.