Preserve the .debug_gdb_scripts section

Make sure that compiler and linker don't optimize the section's contents
away by adding the global holding the data to "llvm.used". The volatile
load in the main shim is retained because "llvm.used", which translates
to SHF_GNU_RETAIN on ELF targets, requires a reasonably recent linker;
emitting the volatile load ensures compatibility with older linkers, at
least when libstd is used.

Pretty printers in dylib dependencies are now emitted by the main crate
instead of the dylib; apart from matching how rlibs are handled, this
approach has the advantage that `omit_gdb_pretty_printer_section` keeps
working with dylib dependencies.

Author: sebastianpoeplau

compiler/rustc_codegen_gcc/src/debuginfo.rs

@@ -254,7 +254,8 @@ impl<'gcc, 'tcx> DebugInfoCodegenMethods<'tcx> for CodegenCx<'gcc, 'tcx> {
         // TODO(antoyo): implement.
     }
 
-    fn debuginfo_finalize(&self) {
+    fn debuginfo_finalize(&mut self) {
+        // TODO: emit section `.debug_gdb_scripts`.
         self.context.set_debug_info(true)
     }
 

compiler/rustc_codegen_llvm/src/base.rs

@@ -109,11 +109,16 @@ pub(crate) fn compile_codegen_unit(
             }
 
             // Finalize code coverage by injecting the coverage map. Note, the coverage map will
-            // also be added to the `llvm.compiler.used` variable, created next.
+            // also be added to the `llvm.compiler.used` variable, created below.
             if cx.sess().instrument_coverage() {
                 cx.coverageinfo_finalize();
             }
 
+            // Finalize debuginfo. This adds to `llvm.used`, created below.
+            if cx.sess().opts.debuginfo != DebugInfo::None {
+                cx.debuginfo_finalize();
+            }
+
             // Create the llvm.used and llvm.compiler.used variables.
             if !cx.used_statics.is_empty() {
                 cx.create_used_variable_impl(c"llvm.used", &cx.used_statics);
@@ -130,11 +135,6 @@ pub(crate) fn compile_codegen_unit(
                     llvm::LLVMDeleteGlobal(old_g);
                 }
             }
-
-            // Finalize debuginfo
-            if cx.sess().opts.debuginfo != DebugInfo::None {
-                cx.debuginfo_finalize();
-            }
         }
 
         ModuleCodegen::new_regular(cgu_name.to_string(), llvm_module)

compiler/rustc_codegen_llvm/src/debuginfo/gdb.rs

@@ -1,5 +1,7 @@
 // .debug_gdb_scripts binary section.
 
+use std::ffi::CString;
+
 use rustc_codegen_ssa::base::collect_debugger_visualizers_transitive;
 use rustc_codegen_ssa::traits::*;
 use rustc_hir::attrs::AttributeKind;
@@ -33,7 +35,12 @@ pub(crate) fn insert_reference_to_gdb_debug_scripts_section_global(bx: &mut Buil
 pub(crate) fn get_or_insert_gdb_debug_scripts_section_global<'ll>(
     cx: &CodegenCx<'ll, '_>,
 ) -> &'ll Value {
-    let c_section_var_name = c"__rustc_debug_gdb_scripts_section__";
+    let c_section_var_name = CString::new(format!(
+        "__rustc_debug_gdb_scripts_section_{}_{:08x}",
+        cx.tcx.crate_name(LOCAL_CRATE),
+        cx.tcx.stable_crate_id(LOCAL_CRATE),
+    ))
+    .unwrap();
     let section_var_name = c_section_var_name.to_str().unwrap();
 
     let section_var = unsafe { llvm::LLVMGetNamedGlobal(cx.llmod, c_section_var_name.as_ptr()) };
@@ -89,17 +96,10 @@ pub(crate) fn needs_gdb_debug_scripts_section(cx: &CodegenCx<'_, '_>) -> bool {
     let omit_gdb_pretty_printer_section =
         find_attr!(cx.tcx.hir_krate_attrs(), AttributeKind::OmitGdbPrettyPrinterSection);
 
-    // To ensure the section `__rustc_debug_gdb_scripts_section__` will not create
-    // ODR violations at link time, this section will not be emitted for rlibs since
-    // each rlib could produce a different set of visualizers that would be embedded
-    // in the `.debug_gdb_scripts` section. For that reason, we make sure that the
-    // section is only emitted for leaf crates.
+    // We collect pretty printers transitively for all crates, so we make sure
+    // that the section is only emitted for leaf crates.
     let embed_visualizers = cx.tcx.crate_types().iter().any(|&crate_type| match crate_type {
-        CrateType::Executable
-        | CrateType::Dylib
-        | CrateType::Cdylib
-        | CrateType::Staticlib
-        | CrateType::Sdylib => {
+        CrateType::Executable | CrateType::Cdylib | CrateType::Staticlib | CrateType::Sdylib => {
             // These are crate types for which we will embed pretty printers since they
             // are treated as leaf crates.
             true
@@ -110,9 +110,11 @@ pub(crate) fn needs_gdb_debug_scripts_section(cx: &CodegenCx<'_, '_>) -> bool {
             // want to slow down the common case.
             false
         }
-        CrateType::Rlib => {
-            // As per the above description, embedding pretty printers for rlibs could
-            // lead to ODR violations so we skip this crate type as well.
+        CrateType::Rlib | CrateType::Dylib => {
+            // Don't embed pretty printers for these crate types; the compiler
+            // can see the `#[debug_visualizer]` attributes when using the
+            // library, and emitting `.debug_gdb_scripts` regardless would
+            // break `#![omit_gdb_pretty_printer_section]`.
             false
         }
     });

compiler/rustc_codegen_llvm/src/debuginfo/mod.rs

@@ -30,7 +30,7 @@ use tracing::debug;
 
 use self::metadata::{UNKNOWN_COLUMN_NUMBER, UNKNOWN_LINE_NUMBER, file_metadata, type_di_node};
 use self::namespace::mangled_name_of_instance;
-use self::utils::{DIB, create_DIArray, is_node_local_to_unit};
+use self::utils::{DIB, create_DIArray, debug_context, is_node_local_to_unit};
 use crate::builder::Builder;
 use crate::common::{AsCCharPtr, CodegenCx};
 use crate::llvm;
@@ -131,20 +131,28 @@ impl<'ll, 'tcx> CodegenUnitDebugContext<'ll, 'tcx> {
 }
 
 /// Creates any deferred debug metadata nodes
-pub(crate) fn finalize(cx: &CodegenCx<'_, '_>) {
-    if let Some(dbg_cx) = &cx.dbg_cx {
-        debug!("finalize");
-
-        if gdb::needs_gdb_debug_scripts_section(cx) {
-            // Add a .debug_gdb_scripts section to this compile-unit. This will
-            // cause GDB to try and load the gdb_load_rust_pretty_printers.py file,
-            // which activates the Rust pretty printers for binary this section is
-            // contained in.
-            gdb::get_or_insert_gdb_debug_scripts_section_global(cx);
-        }
+pub(crate) fn finalize(cx: &mut CodegenCx<'_, '_>) {
+    if cx.dbg_cx.is_none() {
+        return;
+    }
+
+    debug!("finalize");
 
-        dbg_cx.finalize(cx.sess());
+    if gdb::needs_gdb_debug_scripts_section(cx) {
+        // Add a .debug_gdb_scripts section to this compile-unit. This will
+        // cause GDB to try and load the gdb_load_rust_pretty_printers.py file,
+        // which activates the Rust pretty printers for binary this section is
+        // contained in.
+        let section_var = gdb::get_or_insert_gdb_debug_scripts_section_global(cx);
+
+        // Make sure that the linker doesn't optimize the global away. Adding
+        // it to `llvm.used` has the advantage that it works even in no_std
+        // binaries, where we don't have a main shim and thus don't emit a
+        // volatile load to preserve the global.
+        cx.add_used_global(section_var);
     }
+
+    debug_context(cx).finalize(cx.sess());
 }
 
 impl<'ll> Builder<'_, 'll, '_> {
@@ -614,7 +622,7 @@ impl<'ll, 'tcx> DebugInfoCodegenMethods<'tcx> for CodegenCx<'ll, 'tcx> {
         metadata::extend_scope_to_file(self, scope_metadata, file)
     }
 
-    fn debuginfo_finalize(&self) {
+    fn debuginfo_finalize(&mut self) {
         finalize(self)
     }
 

compiler/rustc_codegen_ssa/src/base.rs

@@ -609,15 +609,7 @@ pub fn collect_debugger_visualizers_transitive(
 ) -> BTreeSet<DebuggerVisualizerFile> {
     tcx.debugger_visualizers(LOCAL_CRATE)
         .iter()
-        .chain(
-            tcx.crates(())
-                .iter()
-                .filter(|&cnum| {
-                    let used_crate_source = tcx.used_crate_source(*cnum);
-                    used_crate_source.rlib.is_some() || used_crate_source.rmeta.is_some()
-                })
-                .flat_map(|&cnum| tcx.debugger_visualizers(cnum)),
-        )
+        .chain(tcx.crates(()).iter().flat_map(|&cnum| tcx.debugger_visualizers(cnum)))
         .filter(|visualizer| visualizer.visualizer_type == visualizer_type)
         .cloned()
         .collect::<BTreeSet<_>>()

compiler/rustc_codegen_ssa/src/traits/debuginfo.rs

@@ -50,7 +50,7 @@ pub trait DebugInfoCodegenMethods<'tcx>: BackendTypes {
         scope_metadata: Self::DIScope,
         file: &SourceFile,
     ) -> Self::DIScope;
-    fn debuginfo_finalize(&self);
+    fn debuginfo_finalize(&mut self);
 
     // FIXME(eddyb) find a common convention for all of the debuginfo-related
     // names (choose between `dbg`, `debug`, `debuginfo`, `debug_info` etc.).

tests/codegen-llvm/gdb_debug_script_load.rs

@@ -9,6 +9,8 @@
 #![feature(lang_items)]
 #![no_std]
 
+// CHECK: @llvm.used = {{.+}} @__rustc_debug_gdb_scripts_section
+
 #[panic_handler]
 fn panic_handler(_: &core::panic::PanicInfo) -> ! {
     loop {}
@@ -22,7 +24,7 @@ extern "C" fn rust_eh_personality() {
 // Needs rustc to generate `main` as that's where the magic load is inserted.
 // IOW, we cannot write this test with `#![no_main]`.
 // CHECK-LABEL: @main
-// CHECK: load volatile i8, {{.+}} @__rustc_debug_gdb_scripts_section__
+// CHECK: load volatile i8, {{.+}} @__rustc_debug_gdb_scripts_section
 
 #[lang = "start"]
 fn lang_start<T: 'static>(

tests/run-make/symbols-all-mangled/rmake.rs

@@ -35,13 +35,7 @@ fn symbols_check_archive(path: &str) {
             continue; // All compiler-builtins symbols must remain unmangled
         }
 
-        if name.contains("rust_eh_personality") {
-            continue; // Unfortunately LLVM doesn't allow us to mangle this symbol
-        }
-
-        if name.contains(".llvm.") {
-            // Starting in LLVM 21 we get various implementation-detail functions which
-            // contain .llvm. that are not a problem.
+        if symbol_ok_everywhere(name) {
             continue;
         }
 
@@ -71,13 +65,7 @@ fn symbols_check(path: &str) {
             continue;
         }
 
-        if name.contains("rust_eh_personality") {
-            continue; // Unfortunately LLVM doesn't allow us to mangle this symbol
-        }
-
-        if name.contains(".llvm.") {
-            // Starting in LLVM 21 we get various implementation-detail functions which
-            // contain .llvm. that are not a problem.
+        if symbol_ok_everywhere(name) {
             continue;
         }
 
@@ -88,3 +76,22 @@ fn symbols_check(path: &str) {
 fn strip_underscore_if_apple(symbol: &str) -> &str {
     if cfg!(target_vendor = "apple") { symbol.strip_prefix("_").unwrap() } else { symbol }
 }
+
+fn symbol_ok_everywhere(name: &str) -> bool {
+    if name.contains("rust_eh_personality") {
+        return true; // Unfortunately LLVM doesn't allow us to mangle this symbol
+    }
+
+    if name.contains(".llvm.") {
+        // Starting in LLVM 21 we get various implementation-detail functions which
+        // contain .llvm. that are not a problem.
+        return true;
+    }
+
+    if name.starts_with("__rustc_debug_gdb_scripts_section") {
+        // These symbols are fine; they're made unique by the crate ID.
+        return true;
+    }
+
+    return false;
+}