Adjust `-Ctarget-cpu=native` handling in cg_llvm

When cg_llvm encounters the `-Ctarget-cpu=native` it computes an
explciit set of features that applies to the target in order to
correctly compile code for the host CPU (because e.g. `skylake` alone is
not sufficient to tell if some of the instructions are available or
not).

However there were a couple of issues with how we did this. Firstly, the
order in which features were overriden wasn't quite right – conceptually
you'd expect `-Ctarget-cpu=native` option to override the features that
are implicitly set by the target definition. However due to how other
`-Ctarget-cpu` values are handled we must adopt the following order
of priority:

* Features from -Ctarget-cpu=*; are overriden by
* Features implied by --target; are overriden by
* Features from -Ctarget-feature; are overriden by
* function specific features.

Another problem was in that the function level `target-features`
attribute would overwrite the entire set of the globally enabled
features, rather than just the features the
`#[target_feature(enable/disable)]` specified. With something like
`-Ctarget-cpu=native` we'd end up in a situation wherein a function
without `#[target_feature(enable)]` annotation would have a broader
set of features compared to a function with one such attribute. This
turned out to be a cause of heavy run-time regressions in some code
using these function-level attributes in conjunction with
`-Ctarget-cpu=native`, for example.

With this PR rustc is more careful about specifying the entire set of
features for functions that use `#[target_feature(enable/disable)]` or
`#[instruction_set]` attributes.

Sadly testing the original reproducer for this behaviour is quite
impossible – we cannot rely on `-Ctarget-cpu=native` to be anything in
particular on developer or CI machines.

1 files changed, 10 insertions, 20 deletions

diff --git a/compiler/rustc_codegen_llvm/src/attributes.rs b/compiler/rustc_codegen_llvm/src/attributes.rs
index 09ece6164eb..64ebe585dd8 100644
--- a/compiler/rustc_codegen_llvm/src/attributes.rs
+++ b/compiler/rustc_codegen_llvm/src/attributes.rs
@@ -152,18 +152,6 @@ fn set_probestack(cx: &CodegenCx<'ll, '_>, llfn: &'ll Value) {
     }
 }
 
-pub fn llvm_target_features(sess: &Session) -> impl Iterator<Item = &str> {
-    const RUSTC_SPECIFIC_FEATURES: &[&str] = &["crt-static"];
-
-    let cmdline = sess
-        .opts
-        .cg
-        .target_feature
-        .split(',')
-        .filter(|f| !RUSTC_SPECIFIC_FEATURES.iter().any(|s| f.contains(s)));
-    sess.target.features.split(',').chain(cmdline).filter(|l| !l.is_empty())
-}
-
 pub fn apply_target_cpu_attr(cx: &CodegenCx<'ll, '_>, llfn: &'ll Value) {
     let target_cpu = SmallCStr::new(llvm_util::target_cpu(cx.tcx.sess));
     llvm::AddFunctionAttrStringValue(
@@ -301,20 +289,22 @@ pub fn from_fn_attrs(cx: &CodegenCx<'ll, 'tcx>, llfn: &'ll Value, instance: ty::
     // The target doesn't care; the subtarget reads our attribute.
     apply_tune_cpu_attr(cx, llfn);
 
-    let features = llvm_target_features(cx.tcx.sess)
-        .map(|s| s.to_string())
-        .chain(codegen_fn_attrs.target_features.iter().map(|f| {
+    let function_features = codegen_fn_attrs
+        .target_features
+        .iter()
+        .map(|f| {
             let feature = &f.as_str();
             format!("+{}", llvm_util::to_llvm_feature(cx.tcx.sess, feature))
-        }))
+        })
         .chain(codegen_fn_attrs.instruction_set.iter().map(|x| match x {
             InstructionSetAttr::ArmA32 => "-thumb-mode".to_string(),
             InstructionSetAttr::ArmT32 => "+thumb-mode".to_string(),
         }))
-        .collect::<Vec<String>>()
-        .join(",");
-
-    if !features.is_empty() {
+        .collect::<Vec<String>>();
+    if !function_features.is_empty() {
+        let mut global_features = llvm_util::llvm_global_features(cx.tcx.sess);
+        global_features.extend(function_features.into_iter());
+        let features = global_features.join(",");
         let val = CString::new(features).unwrap();
         llvm::AddFunctionAttrStringValue(
             llfn,