Diagnostic: no EMMS instruction before call

[gnzlbg]

The Intel compiler has an extremely useful warning (https://software.intel.com/en-us/articles/cdiag964) that triggers if MMX code interfaces with x87 code without an EMMS instruction before the x87 code.

Example (playground):

pub fn init(_x: i32);
fn main() {
    unsafe {
        let m0 = _mm_cvtsi32_si64(1);
        let m1 = _mm_cvtsi32_si64(2);
        let mut m2 = _mm_cvtsi32_si64(4);

        m2 = _mm_add_pi8(m0, m1);
        let value = _mm_cvtsi64_si32(m2);

        // Without a call to _mm_empty()
        // this program has UB
        //_mm_empty();
        init(value);
    }
}

Without the call to _mm_empty no emms instruction is emitted, and the program has undefined behavior.

It would be nice if rustc could emit the following error message:

init(value);
^^^^^^^^^^ error: no EMMS instruction before call

note: Use the EMMS instruction (e.g.  by calling the _mm_empty() intrinsic ) 
after the MMX instructions immediately before the x87 code to restore 
the Floating-point status on the CPU.

There are a couple of common ways to emit an EMMS instruction:

• using {core, std}::arch::{x86, x86_64}::{_mm_empty(), _m_empty()}
• using asm!("emms" : : : "volatile");

The diagnostic should not be emitted if an EMMS instruction is emitted right before a call to an x87 function.

---

Note, LLVM, by design, does not emit EMMS instructions when they are needed (https://bugs.llvm.org/show_bug.cgi?id=1838). Instead, the user generating LLVM-IR is responsible of doing so manually when necessary.

[the8472]

GCC just implemented rewiring the MMX intrinsics to 8byte SSE2 equivalents. Maybe rust could do the same to avoid this pitfall.

[gnzlbg]

@the8472 would you mind opening a separate issue to discuss that? This wouldn't help anybody trying to actually use the MMX registers.

Implementing the rewiring should be trivial though. We could have a -Z flag to toggle it, and rustc would just need to use normal vector types instead of x86_mmx. LLVM already does the rest (MMX registers are only actually used if the LLVM IR explicitly uses the x86_mmx type).

[mati865]

IMO with MMX support removed from Rust this should be closed.

[workingjubilee]

Agreed. It is no longer possible to invoke the relevant intrinsics on nightly, so Rust cannot cause the relevant interaction directly, and in particular the reproducer just results in a compiler error. This issue can be closed.