Unknown inline asm constraints cause LLVM assertion failures

[main--]

#![feature(asm)]

fn main() {
    let x = 42u;
    unsafe { asm!("" :: "g"(x)); }
}

rustc: /build/buildd/rust-nightly-201408070406~8fe73f1~trusty/src/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp:6668: void llvm::SelectionDAGBuilder::visitInlineAsm(llvm::ImmutableCallSite): Assertion `(OpInfo.ConstraintType == TargetLowering::C_RegisterClass || OpInfo.ConstraintType == TargetLowering::C_Register) && "Unknown constraint type!"' failed.
Aborted (core dumped)

This is caused by any constraints that rustc can't handle (even something obviously invalid like ö). But shouldn't g work?

[huonw]

As you can see (and as implied by the feature gate) Inline asm! is very experimental: we currently essentially pass all the input directly to LLVM, including the constraints, so if LLVM doesn't support g then we don't either (for reference: those docs are for GCC).

[main--]

I didn't expect 100% bug-free inline asm, but I was told to report all LLVM assertion failures, so that's what I did.

And my experience with clang was always that LLVM has no/little docs but it doesn't matter because you can just use the GCC docs and everything will work. Apparently just clang emulates gcc and not LLVM, so this doesn't apply to rust. Sorry about that.

But are there even any LLVM docs about inline asm? Google always leads me to the GCC docs.

[pczarn]

In GCC constraints, g is equivalent to rmi. Inline asm! doesn't currently bother with constraints unsupported by LLVM.

[huonw]

@main-- sorry for confusion, my comment was just addressing your question "But shouldn't g work?".

But are there even any LLVM docs about inline asm? Google always leads me to the GCC docs.

I don't think there are any. AIUI, the only docs is the source, but I don't personally know where in LLVM it is implemented (it's annoyed me in the past too).

[nodakai]

@huonw see "ISelLowering" files such as:

• https://github.com/rust-lang/llvm/blob/master/lib/Target/X86/X86ISelLowering.cpp#L18183
• https://github.com/rust-lang/llvm/blob/master/lib/Target/ARM/ARMISelLowering.cpp#L10083

[steveklabnik]

The inline asm docs explicitly link to the LLVM docs, and say that we use something "like GCC" but not the same as gcc.