From 3cdfdf95718ae329c5c1dc166b3becdd76369f5d Mon Sep 17 00:00:00 2001
From: Leonard Chan <leonardchan@google.com>
Date: Wed, 9 Sep 2026 23:07:03 +0000
Subject: [PATCH] fuchsia: dynamically enable fix-cortex-a53-835769 for generic
 CPUs

Previously, `+fix-cortex-a53-835769` was statically included in `base.features`
for `aarch64-unknown-fuchsia`. This caused the Cortex-A53 erratum workaround
to be unconditionally enabled regardless of the selected target CPU (e.g.,
when passing `-C target-cpu=cortex-a73`).

The behavior we want is to have it enabled by default only for the
generic CPU (armv8-a), and it should be togglable via the normal
`-Ctarget-features=` flag. This matches the target-feature logic in
clang.

AI: Note gemini was used to help verify this does match Clang's behavior
and write out the matrix of different test invocations. I reviewed this
code to the best of my ability before submitting.
---
 compiler/rustc_codegen_ssa/src/target_features.rs  | 14 +++++++++++++-
 .../src/spec/targets/aarch64_unknown_fuchsia.rs    |  2 +-
 .../aarch64-fuchsia-target-features.rs             | 13 ++++++++++++-
 3 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/compiler/rustc_codegen_ssa/src/target_features.rs b/compiler/rustc_codegen_ssa/src/target_features.rs
index 7dad0cc1732dd..606c817034560 100644
--- a/compiler/rustc_codegen_ssa/src/target_features.rs
+++ b/compiler/rustc_codegen_ssa/src/target_features.rs
@@ -10,7 +10,7 @@ use rustc_middle::ty::TyCtxt;
 use rustc_session::Session;
 use rustc_session::diagnostics::feature_err;
 use rustc_span::{Span, Symbol, edit_distance, sym};
-use rustc_target::spec::{Arch, SanitizerSet};
+use rustc_target::spec::{Arch, Os, SanitizerSet};
 use rustc_target::target_features::{RUSTC_SPECIFIC_FEATURES, Stability};
 use smallvec::SmallVec;
 
@@ -451,6 +451,18 @@ pub fn target_spec_to_backend_features<'a>(
         extend_backend_features("ptx70", true);
     }
 
+    // When targetting aarch64 Fuchsia, the +fix-cortex-a53-835769 should be applied by default
+    // whenever using the generic (armv8-a) CPU. This matches Clang's behavior for aarch64 Fuchsia.
+    if sess.target.os == Os::Fuchsia
+        && sess.target.arch == Arch::AArch64
+        && matches!(
+            sess.opts.cg.target_cpu.as_deref().unwrap_or(&sess.target.cpu),
+            "generic" | "cortex-a53"
+        )
+    {
+        extend_backend_features("fix-cortex-a53-835769", true);
+    }
+
     // Compute implied features
     parse_rust_feature_list(
         sess,
diff --git a/compiler/rustc_target/src/spec/targets/aarch64_unknown_fuchsia.rs b/compiler/rustc_target/src/spec/targets/aarch64_unknown_fuchsia.rs
index 8dede888055f7..d16110cfd66f9 100644
--- a/compiler/rustc_target/src/spec/targets/aarch64_unknown_fuchsia.rs
+++ b/compiler/rustc_target/src/spec/targets/aarch64_unknown_fuchsia.rs
@@ -5,7 +5,7 @@ use crate::spec::{
 pub(crate) fn target() -> Target {
     let mut base = base::fuchsia::opts();
     base.cpu = "generic".into();
-    base.features = "+v8a,+crc,+aes,+sha2,+neon,+fix-cortex-a53-835769".into();
+    base.features = "+v8a,+crc,+aes,+sha2,+neon".into();
     base.max_atomic_width = Some(128);
     base.stack_probes = StackProbeType::Inline;
     base.supported_sanitizers = SanitizerSet::ADDRESS
diff --git a/tests/codegen-llvm/aarch64-fuchsia-target-features.rs b/tests/codegen-llvm/aarch64-fuchsia-target-features.rs
index 32cdc2ce8a553..c9d1a4da4f569 100644
--- a/tests/codegen-llvm/aarch64-fuchsia-target-features.rs
+++ b/tests/codegen-llvm/aarch64-fuchsia-target-features.rs
@@ -1,8 +1,19 @@
 //@ add-minicore
+//@ revisions: DEFAULT GENERIC A53 A73 A73_ENABLE DEFAULT_DISABLE
 //@ compile-flags: --crate-type=rlib --target=aarch64-unknown-fuchsia
 //@ needs-llvm-components: aarch64
+//@ [GENERIC] compile-flags: -C target-cpu=generic
+//@ [A53] compile-flags: -C target-cpu=cortex-a53
+//@ [A73] compile-flags: -C target-cpu=cortex-a73
+//@ [A73_ENABLE] compile-flags: -C target-cpu=cortex-a73 -C target-feature=+fix-cortex-a53-835769
+//@ [DEFAULT_DISABLE] compile-flags: -C target-feature=-fix-cortex-a53-835769
 
-// CHECK: attributes #0 = { {{.*}}"target-features"="{{.*}}+fix-cortex-a53-835769{{.*}}" }
+// DEFAULT: attributes #0 = { {{.*}}"target-features"="{{.*}}+fix-cortex-a53-835769{{.*}}" }
+// GENERIC: attributes #0 = { {{.*}}"target-features"="{{.*}}+fix-cortex-a53-835769{{.*}}" }
+// A53: attributes #0 = { {{.*}}"target-features"="{{.*}}+fix-cortex-a53-835769{{.*}}" }
+// A73-NOT: fix-cortex-a53-835769
+// A73_ENABLE: attributes #0 = { {{.*}}"target-features"="{{.*}}+fix-cortex-a53-835769{{.*}}" }
+// DEFAULT_DISABLE: attributes #0 = { {{.*}}"target-features"="{{.*}}-fix-cortex-a53-835769" }
 
 #![feature(no_core, lang_items)]
 #![no_core]