ICE while using inline assembly

[SethDusek]

So I was messing around with the inline assembler, when I ran into an ICE

Code: https://gist.github.com/9dfeeb4d7cda8aeb1d25c7ea89ee277e

   Compiling asmswitch v0.1.0 (file:///home/kamal/rust/asmswitch)
     Running `rustc src/main.rs --crate-name asmswitch --crate-type bin -g -C metadata=71a43ba090afc09b --out-dir /home/kamal/rust/asmswitch/target/debug --emit=dep-info,link -L dependency=/home/kamal/rust/asmswitch/target/debug/deps`
error: Could not compile `asmswitch`.

Caused by:
  process didn't exit successfully: `rustc src/main.rs --crate-name asmswitch --crate-type bin -g -C metadata=71a43ba090afc09b --out-dir /home/kamal/rust/asmswitch/target/debug --emit=dep-info,link -L dependency=/home/kamal/rust/asmswitch/target/debug/deps` (exit code: 1)

[TimNN]

(I'm not getting an ICE but a crash), backtrace on OS X / nightly-2016-09-29:

* thread #2: tid = 0xd98a10, 0x0000000101d9fed4 librustc_llvm-6eb85298.dylib`llvm::X86TargetLowering::isZExtFree(llvm::SDValue, llvm::EVT) const + 4, name = 'rustc', stop reason = EXC_BAD_ACCESS (code=1, address=0x3a)
  * frame #0: 0x0000000101d9fed4 librustc_llvm-6eb85298.dylib`llvm::X86TargetLowering::isZExtFree(llvm::SDValue, llvm::EVT) const + 4
    frame #1: 0x0000000101f900d1 librustc_llvm-6eb85298.dylib`llvm::RegsForValue::getCopyToRegs(llvm::SDValue, llvm::SelectionDAG&, llvm::SDLoc const&, llvm::SDValue&, llvm::SDValue*, llvm::Value const*, llvm::ISD::NodeType) const + 689
    frame #2: 0x0000000101fb0fed librustc_llvm-6eb85298.dylib`llvm::SelectionDAGBuilder::visitInlineAsm(llvm::ImmutableCallSite) + 8605
    frame #3: 0x0000000101f93d48 librustc_llvm-6eb85298.dylib`llvm::SelectionDAGBuilder::visit(llvm::Instruction const&) + 1144
    frame #4: 0x0000000102025248 librustc_llvm-6eb85298.dylib`llvm::SelectionDAGISel::SelectBasicBlock(llvm::ilist_iterator<llvm::Instruction const>, llvm::ilist_iterator<llvm::Instruction const>, bool&) + 40
    frame #5: 0x00000001020249e2 librustc_llvm-6eb85298.dylib`llvm::SelectionDAGISel::SelectAllBasicBlocks(llvm::Function const&) + 10146
    frame #6: 0x0000000102021174 librustc_llvm-6eb85298.dylib`llvm::SelectionDAGISel::runOnMachineFunction(llvm::MachineFunction&) + 1684
    frame #7: 0x0000000101d26134 librustc_llvm-6eb85298.dylib`(anonymous namespace)::X86DAGToDAGISel::runOnMachineFunction(llvm::MachineFunction&) + 20
    frame #8: 0x000000010221f7e5 librustc_llvm-6eb85298.dylib`llvm::MachineFunctionPass::runOnFunction(llvm::Function&) + 293
    frame #9: 0x0000000102b189d3 librustc_llvm-6eb85298.dylib`llvm::FPPassManager::runOnFunction(llvm::Function&) + 515
    frame #10: 0x0000000102b18c2b librustc_llvm-6eb85298.dylib`llvm::FPPassManager::runOnModule(llvm::Module&) + 43
    frame #11: 0x0000000102b191b3 librustc_llvm-6eb85298.dylib`llvm::legacy::PassManagerImpl::run(llvm::Module&) + 1075
    frame #12: 0x000000010151b240 librustc_llvm-6eb85298.dylib`LLVMRustWriteOutputFile + 368
    frame #13: 0x00000001002164ef librustc_trans-6eb85298.dylib`rustc_trans::back::write::write_output_file::h7e8cf29735c33835 + 95
    frame #14: 0x00000001002be680 librustc_trans-6eb85298.dylib`rustc_trans::back::write::optimize_and_codegen::_$u7b$$u7b$closure$u7d$$u7d$::h368ad232bd936447 + 672
    frame #15: 0x000000010021de92 librustc_trans-6eb85298.dylib`rustc_trans::back::write::execute_work_item::h11850bd68a6e29be + 6434
    frame #16: 0x0000000100218c7b librustc_trans-6eb85298.dylib`rustc_trans::back::write::run_passes::h17498b78d8227324 + 3659
    frame #17: 0x00000001000ba57a librustc_driver-6eb85298.dylib`rustc_driver::driver::phase_5_run_llvm_passes::h11938d38ef5db27c + 298
    frame #18: 0x00000001000a54c3 librustc_driver-6eb85298.dylib`rustc_driver::driver::compile_input::h5b63ccd49eeeb98b + 7699
    frame #19: 0x00000001000cd179 librustc_driver-6eb85298.dylib`rustc_driver::run_compiler::h5441e14d75f46685 + 2041
    frame #20: 0x000000010000dba1 librustc_driver-6eb85298.dylib`std::panicking::try::do_call::h75a39c92a39808cb + 161
    frame #21: 0x00000001041bbfbb libstd-6eb85298.dylib`__rust_maybe_catch_panic + 27
    frame #22: 0x000000010002d585 librustc_driver-6eb85298.dylib`_$LT$F$u20$as$u20$alloc..boxed

[TimNN]

This was introduced between nightly-2016-03-09 and nightly-2016-03-11 (Changes).

I suspect this was an effect of #31710 - I would assume that the use of a function name in inline assembly would now require some special handling to turn it into the function address / pointer.

cc @eddyb.

[TimNN]

Indeed, it appears as if any ZST in the position of foo in the provided example triggers a crash.

[eddyb]

Indeed, need to cast foo to unsafe fn() -> usize now as per #19925.
transmute got a warning cycle but inline assembly is unstable anyway.

The best we can do is some sanity checking for asm! similar to variadic C functions.
cc @nikomatsakis

[TimNN]

(I wanted to add nightly-to-nightly, which we didn't have, but I hit enter too soon and accidentally tagged this as stable-to-nightly (which I then removed))

[nagisa]

Should be A-inline-assembly.

[tari]

I get no crash (or abort) when using LLVM compiled in Debug mode with assertions enabled, rustc 7b33876 on x86_64 Linux. The generated code looks correct.

I assume the ZST issue is the cause, but it sounds like there's an LLVM bug involved as well.

[aidanhs]

Copying my example from the duplicate issue I raised (which doesn't really add anything except rust playground links):

---

This is fine:

#![feature(asm)]
fn abcd() {
    println!("lo");
}
fn main() {
    unsafe{asm!(".global abc;.set abc, ${0:P}" : : "i"(abcd as fn()))};
    println!("hi");
    unsafe { abc() };
}
extern {
    fn abc();
}

https://is.gd/2hOCz4
Output when running compiled binary:

hi
lo

This is not (removed as fn()):

#![feature(asm)]
fn abcd() {
    println!("lo");
}
fn main() {
    unsafe{asm!(".global abc;.set abc, ${0:P}" : : "i"(abcd))};
    println!("hi");
    unsafe { abc() };
}
extern {
    fn abc();
}

https://is.gd/09bozZ
Output when compiling:

rustc 1.15.0-nightly (2217bd771 2016-11-25)
rustc: /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/llvm/include/llvm/Support/Casting.h:81: static bool llvm::isa_impl_cl<To, From*>::doit(const From*) [with To = llvm::ConstantSDNode; From = llvm::SDNode]: Assertion `Val && "isa<> used on a null pointer"' failed.
Aborted (core dumped)

[Amanieu]

This issue does not apply to the new asm! (RFC 2850) which automatically coerces ZST fn type to function pointers.

The legacy llvm_asm! is deprecated and is no longer maintained.