Rollup merge of #127506 - liushuyu:s390x-target-features, r=davidtwco

rustc_target: add known safe s390x target features

This pull request adds known safe target features for s390x (aka IBM Z systems).
Currently, these features are unstable since stabilizing the target features requires submitting proposals.

The `vector` feature was added in IBM Z13 (`arch11`), and this is a SIMD feature for the newer IBM Z systems.
The `backchain` attribute is the IBM Z way of adding frame pointers like unwinding capabilities (the "frame-pointer" switch on IBM Z and IBM POWER platforms will add _emulated_ frame pointers to the binary, which profilers can't use for unwinding the stack).

Both attributes can be applied at the LLVM module or function levels. However, the `backchain` attribute has to be enabled for all the functions in the call stack to get a successful unwind process.

1 files changed, 13 insertions, 1 deletions

diff --git a/compiler/rustc_codegen_llvm/src/llvm_util.rs b/compiler/rustc_codegen_llvm/src/llvm_util.rs
index 98dc8ac86d2..4d56d1d3b1a 100644
--- a/compiler/rustc_codegen_llvm/src/llvm_util.rs
+++ b/compiler/rustc_codegen_llvm/src/llvm_util.rs
@@ -14,7 +14,7 @@ use rustc_session::config::{PrintKind, PrintRequest};
 use rustc_session::Session;
 use rustc_span::symbol::Symbol;
 use rustc_target::spec::{MergeFunctions, PanicStrategy};
-use rustc_target::target_features::RUSTC_SPECIFIC_FEATURES;
+use rustc_target::target_features::{RUSTC_SPECIAL_FEATURES, RUSTC_SPECIFIC_FEATURES};
 
 use std::ffi::{c_char, c_void, CStr, CString};
 use std::fmt::Write;
@@ -321,6 +321,10 @@ pub fn target_features(sess: &Session, allow_unstable: bool) -> Vec<Symbol> {
             }
         })
         .filter(|feature| {
+            // skip checking special features, as LLVM may not understands them
+            if RUSTC_SPECIAL_FEATURES.contains(feature) {
+                return true;
+            }
             // check that all features in a given smallvec are enabled
             for llvm_feature in to_llvm_features(sess, feature) {
                 let cstr = SmallCStr::new(llvm_feature);
@@ -546,6 +550,7 @@ pub(crate) fn global_llvm_features(sess: &Session, diagnostics: bool) -> Vec<Str
 
     // -Ctarget-features
     let supported_features = sess.target.supported_target_features();
+    let (llvm_major, _, _) = get_version();
     let mut featsmap = FxHashMap::default();
     let feats = sess
         .opts
@@ -604,6 +609,13 @@ pub(crate) fn global_llvm_features(sess: &Session, diagnostics: bool) -> Vec<Str
             if RUSTC_SPECIFIC_FEATURES.contains(&feature) {
                 return None;
             }
+
+            // if the target-feature is "backchain" and LLVM version is greater than 18
+            // then we also need to add "+backchain" to the target-features attribute.
+            // otherwise, we will only add the naked `backchain` attribute to the attribute-group.
+            if feature == "backchain" && llvm_major < 18 {
+                return None;
+            }
             // ... otherwise though we run through `to_llvm_features` when
             // passing requests down to LLVM. This means that all in-language
             // features also work on the command line instead of having two