x86-retpoline flag (target modifier) to enable retpoline-related target features

Author: azhogin

Diff for: compiler/rustc_codegen_gcc/src/gcc_util.rs

@@ -102,7 +102,9 @@ pub(crate) fn global_gcc_features(sess: &Session, diagnostics: bool) -> Vec<Stri
                     sess.dcx().emit_warn(unknown_feature);
                 }
                 Some(&(_, stability, _)) => {
-                    if let Err(reason) = stability.toggle_allowed() {
+                    if let Err(reason) = stability.toggle_allowed(
+                        |flag| sess.opts.target_feature_flag_enabled(flag)
+                    ) {
                         sess.dcx().emit_warn(ForbiddenCTargetFeature {
                             feature,
                             enabled: if enable { "enabled" } else { "disabled" },

Diff for: compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -731,7 +731,9 @@ pub(crate) fn global_llvm_features(
                         sess.dcx().emit_warn(unknown_feature);
                     }
                     Some((_, stability, _)) => {
-                        if let Err(reason) = stability.toggle_allowed() {
+                        if let Err(reason) = stability.toggle_allowed(
+                            |flag| sess.opts.target_feature_flag_enabled(flag)
+                        ) {
                             sess.dcx().emit_warn(ForbiddenCTargetFeature {
                                 feature,
                                 enabled: if enable { "enabled" } else { "disabled" },

Diff for: compiler/rustc_codegen_ssa/src/target_features.rs

@@ -64,7 +64,9 @@ pub(crate) fn from_target_feature_attr(
 
             // Only allow target features whose feature gates have been enabled
             // and which are permitted to be toggled.
-            if let Err(reason) = stability.toggle_allowed() {
+            if let Err(reason) = stability.toggle_allowed(
+                |flag| tcx.sess.opts.target_feature_flag_enabled(flag)
+            ) {
                 tcx.dcx().emit_err(errors::ForbiddenTargetFeatureAttr {
                     span: item.span(),
                     feature,

Diff for: compiler/rustc_session/src/config.rs

@@ -2499,6 +2499,8 @@ pub fn build_session_options(early_dcx: &mut EarlyDiagCtxt, matches: &getopts::M
 
     let prints = collect_print_requests(early_dcx, &mut cg, &unstable_opts, matches);
 
+    Options::fill_target_features_by_flags(&unstable_opts, &mut cg);
+
     let cg = cg;
 
     let sysroot_opt = matches.opt_str("sysroot").map(|m| PathBuf::from(&m));

Diff for: compiler/rustc_session/src/options.rs

@@ -286,6 +286,28 @@ macro_rules! top_level_options {
                 mods.sort_by(|a, b| a.opt.cmp(&b.opt));
                 mods
             }
+
+            pub fn target_feature_flag_enabled(&self, flag: &str) -> bool {
+                match flag {
+                    "x86-retpoline" => self.unstable_opts.x86_retpoline,
+                    _ => false,
+                }
+            }
+
+            pub fn fill_target_features_by_flags(
+                unstable_opts: &UnstableOptions, cg: &mut CodegenOptions
+            ) {
+                if unstable_opts.x86_retpoline {
+                    if !cg.target_feature.is_empty() {
+                        cg.target_feature.push(',');
+                    }
+                    cg.target_feature.push_str(
+                        "+retpoline-external-thunk,\
+                        +retpoline-indirect-branches,\
+                        +retpoline-indirect-calls"
+                    );
+                }
+            }
         }
     );
 }
@@ -2560,6 +2582,9 @@ written to standard error output)"),
         "use spec-compliant C ABI for `wasm32-unknown-unknown` (default: legacy)"),
     write_long_types_to_disk: bool = (true, parse_bool, [UNTRACKED],
         "whether long type names should be written to files instead of being printed in errors"),
+    x86_retpoline: bool = (false, parse_bool, [TRACKED TARGET_MODIFIER],
+        "enable retpoline-external-thunk, retpoline-indirect-branches and retpoline-indirect-calls \
+        target features (default: no)"),
     // tidy-alphabetical-end
 
     // If you add a new option, please update:

Diff for: compiler/rustc_target/src/target_features.rs

@@ -34,6 +34,9 @@ pub enum Stability {
     /// particular for features are actually ABI configuration flags (not all targets are as nice as
     /// RISC-V and have an explicit way to set the ABI separate from target features).
     Forbidden { reason: &'static str },
+    /// This feature can not be set via `-Ctarget-feature` or `#[target_feature]`, it can only be set
+    /// by target modifier flag. Target modifier flags are tracked to be consistent in linked modules.
+    EnabledByTargetModifierFlag { reason: &'static str, flag: &'static str },
 }
 use Stability::*;
 
@@ -49,6 +52,7 @@ impl<CTX> HashStable<CTX> for Stability {
             Stability::Forbidden { reason } => {
                 reason.hash_stable(hcx, hasher);
             }
+            Stability::EnabledByTargetModifierFlag { .. } => {}
         }
     }
 }
@@ -74,15 +78,23 @@ impl Stability {
             Stability::Unstable(nightly_feature) => Some(nightly_feature),
             Stability::Stable { .. } => None,
             Stability::Forbidden { .. } => panic!("forbidden features should not reach this far"),
+            Stability::EnabledByTargetModifierFlag { .. } => None,
         }
     }
 
     /// Returns whether the feature may be toggled via `#[target_feature]` or `-Ctarget-feature`.
     /// (It might still be nightly-only even if this returns `true`, so make sure to also check
     /// `requires_nightly`.)
-    pub fn toggle_allowed(&self) -> Result<(), &'static str> {
+    pub fn toggle_allowed(&self, flag_enabled: impl Fn(&str) -> bool) -> Result<(), &'static str> {
         match self {
             Stability::Forbidden { reason } => Err(reason),
+            Stability::EnabledByTargetModifierFlag { reason, flag } => {
+                if !flag_enabled(*flag) {
+                    Err(reason)
+                } else {
+                    Ok(())
+                }
+            }
             _ => Ok(()),
         }
     }
@@ -414,6 +426,18 @@ const X86_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
     ("prfchw", Unstable(sym::prfchw_target_feature), &[]),
     ("rdrand", Stable, &[]),
     ("rdseed", Stable, &[]),
+    ("retpoline-external-thunk", Stability::EnabledByTargetModifierFlag {
+        reason: "use `x86-retpoline` target modifier flag instead",
+        flag: "x86-retpoline",
+    }, &[]),
+    ("retpoline-indirect-branches", Stability::EnabledByTargetModifierFlag {
+        reason: "use `x86-retpoline` target modifier flag instead",
+        flag: "x86-retpoline",
+    }, &[]),
+    ("retpoline-indirect-calls", Stability::EnabledByTargetModifierFlag {
+        reason: "use `x86-retpoline` target modifier flag instead",
+        flag: "x86-retpoline",
+    }, &[]),
     ("rtm", Unstable(sym::rtm_target_feature), &[]),
     ("sha", Stable, &["sse2"]),
     ("sha512", Unstable(sym::sha512_sm_x86), &["avx2"]),

Diff for: tests/ui/target-feature/retpoline-target-feature-flag.by_feature.stderr

@@ -0,0 +1,7 @@
+warning: target feature `retpoline-external-thunk` cannot be toggled with `-Ctarget-feature`: use `x86-retpoline` target modifier 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>
+
+warning: 1 warning emitted
+

Diff for: tests/ui/target-feature/retpoline-target-feature-flag.rs

@@ -0,0 +1,14 @@
+//@ revisions: by_flag by_feature
+//@ compile-flags: --target=x86_64-unknown-linux-gnu --crate-type=lib
+//@ needs-llvm-components: x86
+//@ [by_flag]compile-flags: -Zx86-retpoline
+//@ [by_feature]compile-flags: -Ctarget-feature=+retpoline-external-thunk
+//@ [by_flag]build-pass
+// For now this is just a warning.
+//@ [by_feature]build-pass
+#![feature(no_core, lang_items)]
+#![no_std]
+#![no_core]
+
+#[lang = "sized"]
+pub trait Sized {}