From bc7986ec791ded521f3f75ab82645b6be0c39508 Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov@redhat.com>
Date: Thu, 18 Sep 2025 17:09:14 +0200
Subject: [PATCH] Add attributes for #[global_allocator] functions

Emit `#[rustc_allocator]` etc. attributes on the functions generated
by the `#[global_allocator]` macro, which will emit LLVM attributes
like `"alloc-family"`. If the module with the global allocator
participates in LTO, this ensures that the attributes typically
emitted on the allocator declarations are not lost if the
definition is imported.
---
 .../src/global_allocator.rs                   | 16 ++++++--
 .../global-allocator-attributes.rs            | 41 +++++++++++++++++++
 2 files changed, 54 insertions(+), 3 deletions(-)
 create mode 100644 tests/codegen-llvm/global-allocator-attributes.rs

diff --git a/compiler/rustc_builtin_macros/src/global_allocator.rs b/compiler/rustc_builtin_macros/src/global_allocator.rs
index f14b1920722f4..96bece2a368cd 100644
--- a/compiler/rustc_builtin_macros/src/global_allocator.rs
+++ b/compiler/rustc_builtin_macros/src/global_allocator.rs
@@ -85,7 +85,7 @@ impl AllocFnFactory<'_, '_> {
             body,
             define_opaque: None,
         }));
-        let item = self.cx.item(self.span, self.attrs(), kind);
+        let item = self.cx.item(self.span, self.attrs(method), kind);
         self.cx.stmt_item(self.ty_span, item)
     }
 
@@ -100,8 +100,18 @@ impl AllocFnFactory<'_, '_> {
         self.cx.expr_call(self.ty_span, method, args)
     }
 
-    fn attrs(&self) -> AttrVec {
-        thin_vec![self.cx.attr_word(sym::rustc_std_internal_symbol, self.span)]
+    fn attrs(&self, method: &AllocatorMethod) -> AttrVec {
+        let alloc_attr = match method.name {
+            sym::alloc => sym::rustc_allocator,
+            sym::dealloc => sym::rustc_deallocator,
+            sym::realloc => sym::rustc_reallocator,
+            sym::alloc_zeroed => sym::rustc_allocator_zeroed,
+            _ => unreachable!("Unknown allocator method!"),
+        };
+        thin_vec![
+            self.cx.attr_word(sym::rustc_std_internal_symbol, self.span),
+            self.cx.attr_word(alloc_attr, self.span)
+        ]
     }
 
     fn arg_ty(&self, input: &AllocatorMethodInput, args: &mut ThinVec<Param>) -> Box<Expr> {
diff --git a/tests/codegen-llvm/global-allocator-attributes.rs b/tests/codegen-llvm/global-allocator-attributes.rs
new file mode 100644
index 0000000000000..472ca77207500
--- /dev/null
+++ b/tests/codegen-llvm/global-allocator-attributes.rs
@@ -0,0 +1,41 @@
+//@ compile-flags: -C opt-level=3
+#![crate_type = "lib"]
+
+mod foobar {
+    use std::alloc::{GlobalAlloc, Layout};
+
+    struct Allocator;
+
+    unsafe impl GlobalAlloc for Allocator {
+        unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
+            // CHECK-LABEL: ; __rustc::__rust_alloc
+            // CHECK-NEXT: ; Function Attrs: {{.*}}allockind("alloc,uninitialized,aligned") allocsize(0){{.*}}
+            // CHECK-NEXT: define{{.*}} noalias{{.*}} ptr @{{.*}}__rust_alloc(i[[SIZE:[0-9]+]] {{.*}}%size, i[[SIZE]] allocalign{{.*}} %align)
+            panic!()
+        }
+
+        unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
+            // CHECK-LABEL: ; __rustc::__rust_dealloc
+            // CHECK-NEXT: ; Function Attrs: {{.*}}allockind("free"){{.*}}
+            // CHECK-NEXT: define{{.*}} void @{{.*}}__rust_dealloc(ptr allocptr{{.*}} %ptr, i[[SIZE]] {{.*}} %size, i[[SIZE]] {{.*}} %align)
+            panic!()
+        }
+
+        unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
+            // CHECK-LABEL: ; __rustc::__rust_realloc
+            // CHECK-NEXT: ; Function Attrs: {{.*}}allockind("realloc,aligned") allocsize(3){{.*}}
+            // CHECK-NEXT: define{{.*}} noalias{{.*}} ptr @{{.*}}__rust_realloc(ptr allocptr{{.*}} %ptr, i[[SIZE]] {{.*}} %size, i[[SIZE]] allocalign{{.*}} %align, i[[SIZE]] {{.*}} %new_size)
+            panic!()
+        }
+
+        unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
+            // CHECK-LABEL: ; __rustc::__rust_alloc_zeroed
+            // CHECK-NEXT: ; Function Attrs: {{.*}}allockind("alloc,zeroed,aligned") allocsize(0){{.*}}
+            // CHECK-NEXT: define{{.*}} noalias{{.*}} ptr @{{.*}}__rust_alloc_zeroed(i[[SIZE]] {{.*}} %size, i[[SIZE]] allocalign{{.*}} %align)
+            panic!()
+        }
+    }
+
+    #[global_allocator]
+    static GLOBAL: Allocator = Allocator;
+}