LLVM assertion: Can only indirectify direct input operands!

[Amanieu]

The following code triggers an LLVM assert:

#![feature(asm)]
unsafe fn test(x: *mut u32) {
    asm!("": "=m" (*x));
}

rustc: /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp:6012: void llvm::SelectionDAGBuilder::visitInlineAsm(llvm::ImmutableCallSite): Assertion `(OpInfo.isMultipleAlternative || (OpInfo.Type == InlineAsm::isInput)) && "Can only indirectify direct input operands!"' failed.

One thing of note in the LLVM output is this line:

%5 = call i32 asm "", "=m,~{dirflag},~{fpsr},~{flags}"(), !dbg !199, !srcloc !201

Note that the operand to the asm is left empty for some reason, which is probably wrong.

[Amanieu]

Here is what clang generates for equivalent code, it seems to translate "m" and "=m" into "*m" and "=*m":

call void asm "", "=*m"(i32* %2) #1, !srcloc !3

Using the =*m constraint directly from rust is difficult because type checking treats it as an output even though the parameter is an input pointer value.

[brson]

Still repros.

[steveklabnik]

playpen repro: https://is.gd/rUbnON

[eddyb]

@Amanieu By _ you mean *?

[Amanieu]

@eddyb yes

[eddyb]

@Amanieu We should update the compiler to treat =*m as write to *output, I guess.
Maybe MIR borrowck will be able to handle it better shrug.

[istankovic]

Still a problem with rustc 1.26.0-nightly (adf2135ad 2018-03-17).