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.

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, '_>,
 ) -> &'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.
+            llvm::LLVMRustAppendToUsed(cx.llmod, section_var);
             section_var
         }
     })

compiler/rustc_codegen_llvm/src/debuginfo/mod.rs

@@ -215,10 +215,6 @@ 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() {

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -1859,6 +1859,7 @@ unsafe extern "C" {
         T: &'a Type,
     ) -> &'a Value;
     pub(crate) fn LLVMRustInsertPrivateGlobal<'a>(M: &'a Module, T: &'a Type) -> &'a Value;
+    pub(crate) fn LLVMRustAppendToUsed<'a>(M: &'a Module, V: &'a Value);
     pub(crate) fn LLVMRustGetNamedValue(
         M: &Module,
         Name: *const c_char,

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

@@ -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);
 }

compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp

@@ -35,6 +35,7 @@
 #include "llvm/Support/Signals.h"
 #include "llvm/Support/Timer.h"
 #include "llvm/Support/ToolOutputFile.h"
+#include "llvm/Transforms/Utils/ModuleUtils.h"
 #include <iostream>
 
 // for raw `write` in the bad-alloc handler
@@ -233,6 +234,10 @@ extern "C" LLVMValueRef LLVMRustInsertPrivateGlobal(LLVMModuleRef M,
                                  GlobalValue::PrivateLinkage, nullptr));
 }
 
+extern "C" void LLVMRustAppendToUsed(LLVMModuleRef M, LLVMValueRef V) {
+  appendToUsed(*unwrap(M), {unwrap<GlobalValue>(V)});
+}
+
 // Must match the layout of `rustc_codegen_llvm::llvm::ffi::AttributeKind`.
 enum class LLVMRustAttributeKind {
   AlwaysInline = 0,

tests/codegen/gdb_debug_script_load.rs

@@ -6,32 +6,12 @@
 
 //@ compile-flags: -g -C no-prepopulate-passes -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