Rollup merge of #113716 - DianQK:add-no_builtins-to-function, r=pnkfelix

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.**

This is also the reason why no_builtins does not take effect in LTO as mentioned in #35540.

Now, `#![no_builtins]` should be similar to `-fno-builtin` in clang/gcc, see https://clang.godbolt.org/z/z4j6Wsod5.

Next, we should make `#![no_builtins]` participate in LTO again. That makes sense, as LTO also takes into consideration function-level instruction optimizations, such as the MachineOutliner. More importantly, when a user writes a large `#![no_builtins]` crate, they would like this crate to participate in LTO as well.

We should also add a function-level no_builtins attribute to allow users to have more control over it. This is similar to Clang's `__attribute__((no_builtin))` feature, see https://clang.godbolt.org/z/Wod6KK6eq. Before implementing this feature, maybe we should discuss whether to support more fine-grained control, such as `__attribute__((no_builtin("memcpy")))`.

Related discussions:
- #109821
- #35540

Next (a separate pull request?):
- [ ] Revert #35637
- [ ] Add a function-level `no_builtin` attribute?

1 files changed, 9 insertions, 1 deletions

diff --git a/compiler/rustc_codegen_ssa/src/codegen_attrs.rs b/compiler/rustc_codegen_ssa/src/codegen_attrs.rs
index d6c23012762..0c7b8a79612 100644
--- a/compiler/rustc_codegen_ssa/src/codegen_attrs.rs
+++ b/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;