Make target-cpu=native detect individual features

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -164,7 +164,8 @@ pub fn target_machine_factory(
 
     let code_model = to_llvm_code_model(sess.code_model());
 
-    let features = attributes::llvm_target_features(sess).collect::<Vec<_>>();
+    let mut features = llvm_util::handle_native_features(sess);
+    features.extend(attributes::llvm_target_features(sess).map(|s| s.to_owned()));
     let mut singlethread = sess.target.singlethread;
 
     // On the wasm target once the `atomics` feature is enabled that means that

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -1708,6 +1708,10 @@ extern "C" {
         PM: &PassManager<'_>,
     );
 
+    pub fn LLVMGetHostCPUFeatures() -> *mut c_char;
+
+    pub fn LLVMDisposeMessage(message: *mut c_char);
+
     // Stuff that's in llvm-wrapper/ because it's not upstream yet.
 
     /// Opens an object file.

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -8,7 +8,7 @@ use rustc_session::config::PrintRequest;
 use rustc_session::Session;
 use rustc_span::symbol::Symbol;
 use rustc_target::spec::{MergeFunctions, PanicStrategy};
-use std::ffi::CString;
+use std::ffi::{CStr, CString};
 
 use std::slice;
 use std::str;
@@ -221,6 +221,37 @@ pub fn target_cpu(sess: &Session) -> &str {
     handle_native(name)
 }
 
+pub fn handle_native_features(sess: &Session) -> Vec<String> {
+    match sess.opts.cg.target_cpu {
+        Some(ref s) => {
+            if s != "native" {
+                return vec![];
+            }
+
+            let features_string = unsafe {
+                let ptr = llvm::LLVMGetHostCPUFeatures();
+                let features_string = if !ptr.is_null() {
+                    CStr::from_ptr(ptr)
+                        .to_str()
+                        .unwrap_or_else(|e| {
+                            bug!("LLVM returned a non-utf8 features string: {}", e);
+                        })
+                        .to_owned()
+                } else {
+                    bug!("could not allocate host CPU features, LLVM returned a `null` string");
+                };
+
+                llvm::LLVMDisposeMessage(ptr);
+
+                features_string
+            };
+
+            features_string.split(",").map(|s| s.to_owned()).collect()
+        }
+        None => vec![],
+    }
+}
+
 pub fn tune_cpu(sess: &Session) -> Option<&str> {
     match sess.opts.debugging_opts.tune_cpu {
         Some(ref s) => Some(handle_native(&**s)),