target_feature cfg only obeys `-C target-feature="+feature"`

[rkruppe]

This program on a 32 bit Linux outputs no SSE :( even though our default target does enable SSE and SSE2:

#![feature(cfg_target_feature)]

#[cfg(target_feature="sse")]
fn main() {
    println!("yay, SSE!");
}

#[cfg(not(target_feature="sse"))]
fn main() {
    println!("no SSE :(");
}

I'll save myself the hassle of copying rustc -vV from the VM I tested this on, because the relevant code hasn't been touched since cfg_target_feature was introduced in 4f44258 (July 2015).

This is because the feature detection is really naive, neither asking LLVM for details nor even looking at rustc's own target definitions. @huonw mentioned this in #27731 (comment) but IMHO this deserves an issue of its own.

[huonw]

Yeah, this should be looking at the target spec in use (i.e. even if it is a custom one, not only knowing about the built-in ones), and it should also correctly turn off features when requested (e.g. -C target-feature="-sse").

neither asking LLVM for details

(I tried to do this, but couldn't find a way to actually get it to work. This would be far-and-away the nicest way if someone could make it work.)

[Aatch]

Argh, LLVM apparently doesn't want anybody to get/query CPU features. This means it'll require a patch to LLVM to do properly, not a big one, since the functionality we need is basically already there (is feature X enabled), but it's not exposed at present.

[rkruppe]

Querying LLVM would be perfect, but since we're committed to supporting some official releases (not sure which ones exactly) this won't be a solution in the short term. Still, if someone wrote that patch and got it upstream now, this issue could be solved properly in the future (whenever we bump our minimum requirement to, let's say, 3.9).

[ranma42]

I think it might be possible to extract the features from LLVM without any changes to its code base.
Given an llvm::TargetMachine they are made available through its getMCSubtargetInfo() method. The MCSubtargetInfo objects exposes the raw features through the getFeatureBits () method and (given the CPU, which is available from the same object though getCPU()), they can be associated to the corresponding strings using llvm::SubtargetFeatures.

[huonw]

Fixed by #31709, I believe. Thanks @ranma42!