Fix #570: Honor #![no_builtins] attribute by passing -fno-builtin to GCC (#854)

Author: harin-ramesh

.github/workflows/ci.yml

@@ -119,7 +119,7 @@ jobs:
 
     - name: Run tests
       run: |
-        ./y.sh test --release --clean --build-sysroot ${{ matrix.commands }}
+        ./y.sh test --release --clean --build-sysroot --no-builtins-tests ${{ matrix.commands }}
 
   duplicates:
     runs-on: ubuntu-24.04

build_system/src/test.rs

@@ -43,6 +43,7 @@ fn get_runners() -> Runners {
     runners.insert("--extended-regex-tests", ("Run extended regex tests", extended_regex_tests));
     runners.insert("--mini-tests", ("Run mini tests", mini_tests));
     runners.insert("--cargo-tests", ("Run cargo tests", cargo_tests));
+    runners.insert("--no-builtins-tests", ("Test #![no_builtins] attribute", no_builtins_tests));
     runners
 }
 
@@ -317,6 +318,65 @@ fn maybe_run_command_in_vm(
     Ok(())
 }
 
+/// Compile a source file to an object file and check if it contains a memset reference.
+fn object_has_memset(
+    env: &Env,
+    args: &TestArg,
+    src_file: &str,
+    obj_file_name: &str,
+) -> Result<bool, String> {
+    let cargo_target_dir = Path::new(&args.config_info.cargo_target_dir);
+    let obj_file = cargo_target_dir.join(obj_file_name);
+    let obj_file_str = obj_file.to_str().expect("obj_file to_str");
+
+    let mut command = args.config_info.rustc_command_vec();
+    command.extend_from_slice(&[
+        &src_file,
+        &"--emit",
+        &"obj",
+        &"-O",
+        &"--target",
+        &args.config_info.target_triple,
+        &"-o",
+    ]);
+    command.push(&obj_file_str);
+    run_command_with_env(&command, None, Some(env))?;
+
+    let nm_output = run_command_with_env(&[&"nm", &obj_file_str], None, Some(env))?;
+    let nm_stdout = String::from_utf8_lossy(&nm_output.stdout);
+
+    Ok(nm_stdout.contains("memset"))
+}
+
+fn no_builtins_tests(env: &Env, args: &TestArg) -> Result<(), String> {
+    // Test that the #![no_builtins] attribute prevents GCC from replacing
+    // code patterns (like loops) with calls to builtins (like memset).
+    // See https://github.com/rust-lang/rustc_codegen_gcc/issues/570
+
+    // Test 1: WITH #![no_builtins] - memset should NOT be present
+    println!("[TEST] no_builtins attribute (with #![no_builtins])");
+    let has_memset =
+        object_has_memset(env, args, "tests/no_builtins/no_builtins.rs", "no_builtins_test.o")?;
+    if has_memset {
+        return Err("no_builtins test FAILED: Found 'memset' in object file.\n\
+             The #![no_builtins] attribute should prevent GCC from replacing \n\
+             code patterns with builtin calls."
+            .to_string());
+    }
+
+    // Test 2: WITHOUT #![no_builtins] - memset SHOULD be present
+    println!("[TEST] no_builtins attribute (without #![no_builtins])");
+    let has_memset =
+        object_has_memset(env, args, "tests/no_builtins/with_builtins.rs", "with_builtins_test.o")?;
+    if !has_memset {
+        return Err("no_builtins test FAILED: 'memset' NOT found in object file.\n\
+             Without #![no_builtins], GCC should replace the loop with memset."
+            .to_string());
+    }
+
+    Ok(())
+}
+
 fn std_tests(env: &Env, args: &TestArg) -> Result<(), String> {
     let cargo_target_dir = Path::new(&args.config_info.cargo_target_dir);
     // FIXME: create a function "display_if_not_quiet" or something along the line.
@@ -1248,6 +1308,7 @@ fn run_all(env: &Env, args: &TestArg) -> Result<(), String> {
     test_libcore(env, args)?;
     extended_sysroot_tests(env, args)?;
     cargo_tests(env, args)?;
+    no_builtins_tests(env, args)?;
     test_rustc(env, args)?;
 
     Ok(())

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 +137,17 @@ 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).
+        // The `-fno-tree-loop-distribute-patterns` flag disables the loop distribution pass
+        // that transforms loops into calls to library functions (memset, memcpy, etc.).
+        // See GCC handling for more details:
+        // https://github.com/rust-lang/gcc/blob/efdd0a7290c22f5438d7c5380105d353ee3e8518/gcc/c-family/c-opts.cc#L953
+        let crate_attrs = tcx.hir_attrs(rustc_hir::CRATE_HIR_ID);
+        if find_attr!(crate_attrs, AttributeKind::NoBuiltins) {
+            context.add_command_line_option("-fno-tree-loop-distribute-patterns");
+        }
+
         if let Some(model) = tcx.sess.code_model() {
             use rustc_target::spec::CodeModel;
 

tests/no_builtins/no_builtins.rs

@@ -0,0 +1,24 @@
+// Test that the #![no_builtins] attribute is honored.
+// When this attribute is present, GCC should not replace code patterns
+// (like loops) with calls to builtins (like memset).
+// See https://github.com/rust-lang/rustc_codegen_gcc/issues/570
+//
+// This test is verified by the build system test `--no-builtins-tests` which
+// compiles this file and checks that `memset` is not referenced in the object file.
+
+#![no_std]
+#![no_builtins]
+#![crate_type = "lib"]
+
+// This function implements a byte-setting loop that GCC would typically
+// optimize into a memset call. With #![no_builtins], GCC should preserve
+// the loop instead of replacing it with a builtin call.
+#[no_mangle]
+#[inline(never)]
+pub unsafe fn set_bytes(mut s: *mut u8, c: u8, n: usize) {
+    let end = s.add(n);
+    while s < end {
+        *s = c;
+        s = s.add(1);
+    }
+}

tests/no_builtins/with_builtins.rs

@@ -0,0 +1,21 @@
+// Test that without #![no_builtins], GCC DOES replace code patterns with builtins.
+// This is the counterpart to no_builtins.rs - we verify that memset IS emitted
+// when the no_builtins attribute is NOT present.
+//
+// This test is verified by the build system test `--no-builtins-tests` which
+// compiles this file and checks that `memset` IS referenced in the object file.
+
+#![no_std]
+#![crate_type = "lib"]
+
+// This function implements a byte-setting loop that GCC should optimize
+// into a memset call when no_builtins is NOT set.
+#[no_mangle]
+#[inline(never)]
+pub unsafe fn set_bytes(mut s: *mut u8, c: u8, n: usize) {
+    let end = s.add(n);
+    while s < end {
+        *s = c;
+        s = s.add(1);
+    }
+}