Add the no-builtins attribute to functions when no_builtins is applied at the crate level.

When `no_builtins` is applied at the crate level, we should add the
`no-builtins` attribute to each function to ensure it takes effect in LTO.

compiler/rustc_codegen_llvm/src/attributes.rs

@@ -335,6 +335,10 @@ pub fn from_fn_attrs<'ll, 'tcx>(
     to_add.extend(probestack_attr(cx));
     to_add.extend(stackprotector_attr(cx));
 
+    if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::NO_BUILTINS) {
+        to_add.push(llvm::CreateAttrString(cx.llcx, "no-builtins"));
+    }
+
     if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::COLD) {
         to_add.push(AttributeKind::Cold.create_attr(cx.llcx));
     }

compiler/rustc_codegen_ssa/src/codegen_attrs.rs

@@ -1,4 +1,4 @@
-use rustc_ast::{ast, MetaItemKind, NestedMetaItem};
+use rustc_ast::{ast, attr, MetaItemKind, NestedMetaItem};
 use rustc_attr::{list_contains_name, InlineAttr, InstructionSetAttr, OptimizeAttr};
 use rustc_errors::struct_span_err;
 use rustc_hir as hir;
@@ -60,6 +60,14 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: LocalDefId) -> CodegenFnAttrs {
         codegen_fn_attrs.flags |= CodegenFnAttrFlags::TRACK_CALLER;
     }
 
+    // When `no_builtins` is applied at the crate level, we should add the
+    // `no-builtins` attribute to each function to ensure it takes effect in LTO.
+    let crate_attrs = tcx.hir().attrs(rustc_hir::CRATE_HIR_ID);
+    let no_builtins = attr::contains_name(crate_attrs, sym::no_builtins);
+    if no_builtins {
+        codegen_fn_attrs.flags |= CodegenFnAttrFlags::NO_BUILTINS;
+    }
+
     let supported_target_features = tcx.supported_target_features(LOCAL_CRATE);
 
     let mut inline_span = None;

compiler/rustc_middle/src/middle/codegen_fn_attrs.rs

@@ -100,6 +100,8 @@ bitflags! {
         const REALLOCATOR               = 1 << 18;
         /// `#[rustc_allocator_zeroed]`: a hint to LLVM that the function only allocates zeroed memory.
         const ALLOCATOR_ZEROED          = 1 << 19;
+        /// `#[no_builtins]`: indicates that disable implicit builtin knowledge of functions for the function.
+        const NO_BUILTINS               = 1 << 20;
     }
 }
 

tests/codegen/no_builtins-at-crate.rs

@@ -0,0 +1,24 @@
+// compile-flags: -C opt-level=1
+
+#![no_builtins]
+#![crate_type = "lib"]
+
+// CHECK: define
+// CHECK-SAME: @__aeabi_memcpy
+// CHECK-SAME: #0
+#[no_mangle]
+pub unsafe extern "C" fn __aeabi_memcpy(dest: *mut u8, src: *const u8, size: usize) {
+    // CHECK: call
+    // CHECK-SAME: @memcpy(
+    memcpy(dest, src, size);
+}
+
+// CHECK: declare
+// CHECK-SAME: @memcpy
+// CHECK-SAME: #0
+extern "C" {
+    pub fn memcpy(dest: *mut u8, src: *const u8, n: usize) -> *mut u8;
+}
+
+// CHECK: attributes #0
+// CHECK-SAME: "no-builtins"

tests/run-make/no-builtins-attribute/Makefile

@@ -0,0 +1,9 @@
+include ../tools.mk
+
+# We want to check if `no-builtins` is also added to the function declarations in the used crate.
+
+all:
+	$(RUSTC) no_builtins.rs --emit=link
+	$(RUSTC) main.rs --emit=llvm-ir
+
+	cat "$(TMPDIR)"/main.ll | "$(LLVM_FILECHECK)" filecheck.main.txt

tests/run-make/no-builtins-attribute/filecheck.main.txt

@@ -0,0 +1,5 @@
+CHECK: declare void @foo()
+CHECK-SAME: #[[ATTR_3:[0-9]+]]
+
+CHECK: attributes #[[ATTR_3]] 
+CHECK-SAME: no-builtins

tests/run-make/no-builtins-attribute/main.rs

@@ -0,0 +1,10 @@
+extern crate no_builtins;
+
+#[no_mangle]
+fn call_foo() {
+    no_builtins::foo();
+}
+
+fn main() {
+    call_foo();
+}

tests/run-make/no-builtins-attribute/no_builtins.rs

@@ -0,0 +1,5 @@
+#![crate_type = "lib"]
+#![no_builtins]
+
+#[no_mangle]
+pub fn foo() {}