various minor target feature cleanups

Author: RalfJung

compiler/rustc_codegen_gcc/src/gcc_util.rs

@@ -6,6 +6,7 @@ use smallvec::{SmallVec, smallvec};
 
 fn gcc_features_by_flags(sess: &Session, features: &mut Vec<String>) {
     target_features::retpoline_features_by_flags(sess, features);
+    // FIXME: LLVM also sets +reserve-x18 here under some conditions.
 }
 
 /// The list of GCC features computed from CLI flags (`-Ctarget-cpu`, `-Ctarget-feature`,
@@ -59,8 +60,6 @@ pub(crate) fn global_gcc_features(sess: &Session, diagnostics: bool) -> Vec<Stri
 
     gcc_features_by_flags(sess, &mut features);
 
-    // FIXME: LLVM also sets +reserve-x18 here under some conditions.
-
     features
 }
 

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -625,6 +625,15 @@ pub(crate) fn target_cpu(sess: &Session) -> &str {
 /// The target features for compiler flags other than `-Ctarget-features`.
 fn llvm_features_by_flags(sess: &Session, features: &mut Vec<String>) {
     target_features::retpoline_features_by_flags(sess, features);
+
+    // -Zfixed-x18
+    if sess.opts.unstable_opts.fixed_x18 {
+        if sess.target.arch != "aarch64" {
+            sess.dcx().emit_fatal(errors::FixedX18InvalidArch { arch: &sess.target.arch });
+        } else {
+            features.push("+reserve-x18".into());
+        }
+    }
 }
 
 /// The list of LLVM features computed from CLI flags (`-Ctarget-cpu`, `-Ctarget-feature`,
@@ -736,19 +745,10 @@ pub(crate) fn global_llvm_features(
                 )
             },
         );
-
-        llvm_features_by_flags(sess, &mut features);
     }
 
-    // -Zfixed-x18
-    // FIXME: merge with `llvm_features_by_flags`.
-    if sess.opts.unstable_opts.fixed_x18 {
-        if sess.target.arch != "aarch64" {
-            sess.dcx().emit_fatal(errors::FixedX18InvalidArch { arch: &sess.target.arch });
-        } else {
-            features.push("+reserve-x18".into());
-        }
-    }
+    // We add this in the "base target" so that these show up in `sess.unstable_target_features`.
+    llvm_features_by_flags(sess, &mut features);
 
     features
 }

compiler/rustc_codegen_ssa/src/target_features.rs

@@ -12,9 +12,7 @@ use rustc_session::Session;
 use rustc_session::lint::builtin::AARCH64_SOFTFLOAT_NEON;
 use rustc_session::parse::feature_err;
 use rustc_span::{Span, Symbol, sym};
-use rustc_target::target_features::{
-    self, RUSTC_SPECIAL_FEATURES, RUSTC_SPECIFIC_FEATURES, Stability,
-};
+use rustc_target::target_features::{self, RUSTC_SPECIFIC_FEATURES, Stability};
 use smallvec::SmallVec;
 
 use crate::errors;
@@ -176,8 +174,18 @@ fn parse_rust_feature_flag<'a>(
 
     for feature in sess.opts.cg.target_feature.split(',') {
         if let Some(base_feature) = feature.strip_prefix('+') {
+            // Skip features that are not target features, but rustc features.
+            if RUSTC_SPECIFIC_FEATURES.contains(&base_feature) {
+                return;
+            }
+
             callback(base_feature, sess.target.implied_target_features(base_feature), true)
         } else if let Some(base_feature) = feature.strip_prefix('-') {
+            // Skip features that are not target features, but rustc features.
+            if RUSTC_SPECIFIC_FEATURES.contains(&base_feature) {
+                return;
+            }
+
             // If `f1` implies `f2`, then `!f2` implies `!f1` -- this is standard logical
             // contraposition. So we have to find all the reverse implications of `base_feature` and
             // disable them, too.
@@ -229,15 +237,7 @@ pub fn cfg_target_feature(
         .target
         .rust_target_features()
         .iter()
-        .filter(|(feature, _, _)| {
-            // Skip checking special features, those are not known to the backend.
-            if RUSTC_SPECIAL_FEATURES.contains(feature) {
-                // FIXME: `true` here means we'll always think the feature is enabled.
-                // Does that really make sense?
-                return true;
-            }
-            target_base_has_feature(feature)
-        })
+        .filter(|(feature, _, _)| target_base_has_feature(feature))
         .map(|(feature, _, _)| Symbol::intern(feature))
         .collect();
 
@@ -332,11 +332,6 @@ pub fn flag_to_backend_features<'a, const N: usize>(
             }
         },
         |base_feature, new_features, enable| {
-            // Skip features that are meant for rustc, not the backend.
-            if RUSTC_SPECIFIC_FEATURES.contains(&base_feature) {
-                return;
-            }
-
             rust_features.extend(
                 UnordSet::from(new_features).to_sorted_stable_ord().iter().map(|&&s| (enable, s)),
             );

compiler/rustc_target/src/target_features.rs

@@ -11,11 +11,6 @@ use crate::spec::{FloatAbi, RustcAbi, Target};
 /// These exist globally and are not in the target-specific lists below.
 pub const RUSTC_SPECIFIC_FEATURES: &[&str] = &["crt-static"];
 
-/// Features that require special handling when passing to LLVM:
-/// these are target-specific (i.e., must also be listed in the target-specific list below)
-/// but do not correspond to an LLVM target feature.
-pub const RUSTC_SPECIAL_FEATURES: &[&str] = &["backchain"];
-
 /// Stability information for target features.
 #[derive(Debug, Copy, Clone)]
 pub enum Stability {
@@ -275,12 +270,7 @@ static AARCH64_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
     ("rcpc3", Unstable(sym::aarch64_unstable_target_feature), &["rcpc2"]),
     // FEAT_RDM
     ("rdm", Stable, &["neon"]),
-    // This is needed for inline assembly, but shouldn't be stabilized as-is
-    // since it should be enabled globally using -Zfixed-x18, not
-    // #[target_feature].
-    // Note that cfg(target_feature = "reserve-x18") is currently not set for
-    // targets that reserve x18 by default.
-    ("reserve-x18", Unstable(sym::aarch64_unstable_target_feature), &[]),
+    ("reserve-x18", Forbidden { reason: "use `-Zfixed-x18` compiler flag instead" }, &[]),
     // FEAT_SB
     ("sb", Stable, &[]),
     // FEAT_SHA1 & FEAT_SHA256
@@ -455,19 +445,17 @@ static X86_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
     ("rdseed", Stable, &[]),
     (
         "retpoline-external-thunk",
-        Stability::Forbidden {
-            reason: "use `retpoline-external-thunk` target modifier flag instead",
-        },
+        Stability::Forbidden { reason: "use `-Zretpoline-external-thunk` compiler flag instead" },
         &[],
     ),
     (
         "retpoline-indirect-branches",
-        Stability::Forbidden { reason: "use `retpoline` target modifier flag instead" },
+        Stability::Forbidden { reason: "use `-Zretpoline` compiler flag instead" },
         &[],
     ),
     (
         "retpoline-indirect-calls",
-        Stability::Forbidden { reason: "use `retpoline` target modifier flag instead" },
+        Stability::Forbidden { reason: "use `-Zretpoline` compiler flag instead" },
         &[],
     ),
     ("rtm", Unstable(sym::rtm_target_feature), &[]),

tests/assembly/s390x-backchain-toggle.rs

@@ -1,5 +1,5 @@
 //@ add-core-stubs
-//@ revisions: enable-backchain disable-backchain
+//@ revisions: enable-backchain disable-backchain default-backchain
 //@ assembly-output: emit-asm
 //@ compile-flags: -Copt-level=3 --crate-type=lib --target=s390x-unknown-linux-gnu
 //@ needs-llvm-components: systemz
@@ -26,6 +26,8 @@ extern "C" fn test_backchain() -> i32 {
     // enable-backchain: stg [[REG1]], 0(%r15)
     // disable-backchain: aghi %r15, -160
     // disable-backchain-NOT: stg %r{{.*}}, 0(%r15)
+    // default-backchain: aghi %r15, -160
+    // default-backchain-NOT: stg %r{{.*}}, 0(%r15)
     unsafe {
         extern_func();
     }
@@ -35,6 +37,7 @@ extern "C" fn test_backchain() -> i32 {
     // Make sure that the expected return value is written into %r2 (return register):
     // enable-backchain-NEXT: lghi %r2, 1
     // disable-backchain: lghi %r2, 0
+    // default-backchain: lghi %r2, 0
     #[cfg(target_feature = "backchain")]
     {
         1

tests/ui/check-cfg/target_feature.stderr

@@ -211,7 +211,6 @@ LL |     cfg!(target_feature = "_UNEXPECTED_VALUE");
 `reference-types`
 `relax`
 `relaxed-simd`
-`reserve-x18`
 `rtm`
 `sb`
 `scq`

tests/ui/target-feature/retpoline-target-feature-flag.by_feature1.stderr

@@ -1,4 +1,4 @@
-warning: target feature `retpoline-external-thunk` cannot be enabled with `-Ctarget-feature`: use `retpoline-external-thunk` target modifier flag instead
+warning: target feature `retpoline-external-thunk` cannot be enabled with `-Ctarget-feature`: use `-Zretpoline-external-thunk` compiler flag instead
    |
    = note: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #116344 <https://github.com/rust-lang/rust/issues/116344>

tests/ui/target-feature/retpoline-target-feature-flag.by_feature2.stderr

@@ -1,4 +1,4 @@
-warning: target feature `retpoline-indirect-branches` cannot be enabled with `-Ctarget-feature`: use `retpoline` target modifier flag instead
+warning: target feature `retpoline-indirect-branches` cannot be enabled with `-Ctarget-feature`: use `-Zretpoline` compiler flag instead
    |
    = note: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #116344 <https://github.com/rust-lang/rust/issues/116344>

tests/ui/target-feature/retpoline-target-feature-flag.by_feature3.stderr

@@ -1,4 +1,4 @@
-warning: target feature `retpoline-indirect-calls` cannot be enabled with `-Ctarget-feature`: use `retpoline` target modifier flag instead
+warning: target feature `retpoline-indirect-calls` cannot be enabled with `-Ctarget-feature`: use `-Zretpoline` compiler flag instead
    |
    = note: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #116344 <https://github.com/rust-lang/rust/issues/116344>

tests/ui/target-feature/retpoline-target-feature-flag.rs

@@ -16,6 +16,6 @@
 #![no_core]
 extern crate minicore;
 
-//[by_feature1]~? WARN target feature `retpoline-external-thunk` cannot be enabled with `-Ctarget-feature`: use `retpoline-external-thunk` target modifier flag instead
-//[by_feature2]~? WARN target feature `retpoline-indirect-branches` cannot be enabled with `-Ctarget-feature`: use `retpoline` target modifier flag instead
-//[by_feature3]~? WARN target feature `retpoline-indirect-calls` cannot be enabled with `-Ctarget-feature`: use `retpoline` target modifier flag instead
+//[by_feature1]~? WARN target feature `retpoline-external-thunk` cannot be enabled with `-Ctarget-feature`
+//[by_feature2]~? WARN target feature `retpoline-indirect-branches` cannot be enabled with `-Ctarget-feature`
+//[by_feature3]~? WARN target feature `retpoline-indirect-calls` cannot be enabled with `-Ctarget-feature`