Fix #570: Honor #![no_builtins] attribute by passing -fno-builtin to GCC

Author: harin-ramesh

src/base.rs

@@ -8,7 +8,8 @@ use rustc_codegen_ssa::ModuleCodegen;
 use rustc_codegen_ssa::base::maybe_create_entry_wrapper;
 use rustc_codegen_ssa::mono_item::MonoItemExt;
 use rustc_codegen_ssa::traits::DebugInfoCodegenMethods;
-use rustc_hir::attrs::Linkage;
+use rustc_hir::attrs::{AttributeKind, Linkage};
+use rustc_hir::find_attr;
 use rustc_middle::dep_graph;
 #[cfg(feature = "master")]
 use rustc_middle::mir::mono::Visibility;
@@ -136,6 +137,15 @@ pub fn compile_codegen_unit(
         // NOTE: Rust relies on LLVM doing wrapping on overflow.
         context.add_command_line_option("-fwrapv");
 
+        // NOTE: We need to honor the `#![no_builtins]` attribute to prevent GCC from
+        // replacing code patterns (like loops) with calls to builtins (like memset).
+        // This is important for crates like `compiler_builtins` that implement these functions.
+        // See https://github.com/rust-lang/rustc_codegen_gcc/issues/570
+        let crate_attrs = tcx.hir_attrs(rustc_hir::CRATE_HIR_ID);
+        if find_attr!(crate_attrs, AttributeKind::NoBuiltins) {
+            context.add_command_line_option("-fno-builtin");
+        }
+
         if let Some(model) = tcx.sess.code_model() {
             use rustc_target::spec::CodeModel;