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". This
eliminates the need for a volatile load in the main shim; since the LLVM
codegen backend is the only implementer of the corresponding trait
function, remove it entirely.

Author: sebastianpoeplau

compiler/rustc_codegen_gcc/src/debuginfo.rs

@@ -36,10 +36,6 @@ impl<'a, 'gcc, 'tcx> DebugInfoBuilderMethods for Builder<'a, 'gcc, 'tcx> {
         _variable_alloca.set_location(_dbg_loc);
     }
 
-    fn insert_reference_to_gdb_debug_scripts_section_global(&mut self) {
-        // TODO(antoyo): insert reference to gdb debug scripts section global.
-    }
-
     /// FIXME(tempdragon): Currently, this function is not yet implemented. It seems that the
     /// debug name and the mangled name should both be included in the LValues.
     /// Besides, a function to get the rvalue type(m_is_lvalue) should also be included.
@@ -254,7 +250,8 @@ 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_ast::attr;
 use rustc_codegen_ssa::base::collect_debugger_visualizers_transitive;
 use rustc_codegen_ssa::traits::*;
@@ -9,31 +11,21 @@ use rustc_middle::middle::debugger_visualizer::DebuggerVisualizerType;
 use rustc_session::config::{CrateType, DebugInfo};
 use rustc_span::sym;
 
-use crate::builder::Builder;
 use crate::common::CodegenCx;
 use crate::llvm;
 use crate::value::Value;
 
-/// Inserts a side-effect free instruction sequence that makes sure that the
-/// .debug_gdb_scripts global is referenced, so it isn't removed by the linker.
-pub(crate) fn insert_reference_to_gdb_debug_scripts_section_global(bx: &mut Builder<'_, '_, '_>) {
-    if needs_gdb_debug_scripts_section(bx) {
-        let gdb_debug_scripts_section = get_or_insert_gdb_debug_scripts_section_global(bx);
-        // Load just the first byte as that's all that's necessary to force
-        // LLVM to keep around the reference to the global.
-        let volatile_load_instruction = bx.volatile_load(bx.type_i8(), gdb_debug_scripts_section);
-        unsafe {
-            llvm::LLVMSetAlignment(volatile_load_instruction, 1);
-        }
-    }
-}
-
 /// Allocates the global variable responsible for the .debug_gdb_scripts binary
 /// section.
 pub(crate) fn get_or_insert_gdb_debug_scripts_section_global<'ll>(
-    cx: &CodegenCx<'ll, '_>,
+    cx: &mut 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()) };
@@ -80,6 +72,8 @@ pub(crate) fn get_or_insert_gdb_debug_scripts_section_global<'ll>(
             // This should make sure that the whole section is not larger than
             // the string it contains. Otherwise we get a warning from GDB.
             llvm::LLVMSetAlignment(section_var, 1);
+            // Make sure that the linker doesn't optimize the global away.
+            cx.add_used_global(section_var);
             section_var
         }
     })

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 +131,22 @@ 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.
+        gdb::get_or_insert_gdb_debug_scripts_section_global(cx);
     }
+
+    debug_context(cx).finalize(cx.sess());
 }
 
 impl<'ll> Builder<'_, 'll, '_> {
@@ -215,10 +217,6 @@ impl<'ll> DebugInfoBuilderMethods for Builder<'_, 'll, '_> {
         }
     }
 
-    fn insert_reference_to_gdb_debug_scripts_section_global(&mut self) {
-        gdb::insert_reference_to_gdb_debug_scripts_section_global(self)
-    }
-
     fn set_var_name(&mut self, value: &'ll Value, name: &str) {
         // Avoid wasting time if LLVM value names aren't even enabled.
         if self.sess().fewer_names() {
@@ -614,7 +612,7 @@ 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

@@ -528,8 +528,6 @@ pub fn maybe_create_entry_wrapper<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         let llbb = Bx::append_block(cx, llfn, "top");
         let mut bx = Bx::build(cx, llbb);
 
-        bx.insert_reference_to_gdb_debug_scripts_section_global();
-
         let isize_ty = cx.type_isize();
         let ptr_ty = cx.type_ptr();
         let (arg_argc, arg_argv) = get_argc_argv(&mut bx);

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.).
@@ -81,6 +81,5 @@ pub trait DebugInfoBuilderMethods: BackendTypes {
     );
     fn set_dbg_loc(&mut self, dbg_loc: Self::DILocation);
     fn clear_dbg_loc(&mut self);
-    fn insert_reference_to_gdb_debug_scripts_section_global(&mut self);
     fn set_var_name(&mut self, value: Self::Value, name: &str);
 }

tests/codegen/gdb_debug_script_load.rs

@@ -4,34 +4,14 @@
 //@ ignore-wasm
 //@ ignore-emscripten
 
-//@ compile-flags: -g -C no-prepopulate-passes -Cpanic=abort
+//@ compile-flags: -g -Cpanic=abort
 
-#![feature(lang_items)]
 #![no_std]
+#![no_main]
 
 #[panic_handler]
 fn panic_handler(_: &core::panic::PanicInfo) -> ! {
     loop {}
 }
 
-#[no_mangle]
-extern "C" fn rust_eh_personality() {
-    loop {}
-}
-
-// 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__
-
-#[lang = "start"]
-fn lang_start<T: 'static>(
-    _main: fn() -> T,
-    _argc: isize,
-    _argv: *const *const u8,
-    _sigpipe: u8,
-) -> isize {
-    return 0;
-}
-
-fn main() {}
+// CHECK: @llvm.used = {{.+}} @__rustc_debug_gdb_scripts_section