cg_llvm: split dwarf filename and comp dir

llvm-dwp concatenates `DW_AT_comp_dir` with `DW_AT_GNU_dwo_name` (only
when `DW_AT_comp_dir` exists), which can result in it failing to find
the DWARF object files.

In earlier testing, `DW_AT_comp_dir` wasn't present in the final
object and the current directory was the output directory.

When running tests through compiletest, the working directory of the
compilation is different from output directory and that resulted in
`DW_AT_comp_dir` being in the object file (and set to the current
working directory, rather than the output directory), and
`DW_AT_GNU_dwo_name` being set to the full path (rather than just
the filename), so llvm-dwp was failing.

This commit changes the compilation directory provided to LLVM to match
the output directory, where DWARF objects are output; and ensures that
only the filename is used for `DW_AT_GNU_dwo_name`.

Signed-off-by: David Wood <david@davidtw.co>

compiler/rustc_codegen_llvm/src/back/lto.rs

@@ -734,7 +734,7 @@ pub unsafe fn optimize_thin_module(
     let module_name = &thin_module.shared.module_names[thin_module.idx];
     let split_dwarf_file = cgcx
         .output_filenames
-        .split_dwarf_file(cgcx.split_dwarf_kind, Some(module_name.to_str().unwrap()));
+        .split_dwarf_filename(cgcx.split_dwarf_kind, Some(module_name.to_str().unwrap()));
     let tm_factory_config = TargetMachineFactoryConfig { split_dwarf_file };
     let tm =
         (cgcx.tm_factory)(tm_factory_config).map_err(|e| write::llvm_err(&diag_handler, &e))?;

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -95,7 +95,7 @@ pub fn create_informational_target_machine(sess: &Session) -> &'static mut llvm:
 pub fn create_target_machine(tcx: TyCtxt<'_>, mod_name: &str) -> &'static mut llvm::TargetMachine {
     let split_dwarf_file = tcx
         .output_filenames(LOCAL_CRATE)
-        .split_dwarf_file(tcx.sess.opts.debugging_opts.split_dwarf, Some(mod_name));
+        .split_dwarf_filename(tcx.sess.opts.debugging_opts.split_dwarf, Some(mod_name));
     let config = TargetMachineFactoryConfig { split_dwarf_file };
     target_machine_factory(&tcx.sess, tcx.backend_optimization_level(LOCAL_CRATE))(config)
         .unwrap_or_else(|err| llvm_err(tcx.sess.diagnostic(), &err).raise())

compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs

@@ -993,12 +993,14 @@ pub fn compile_unit_metadata(
     let producer = format!("clang LLVM ({})", rustc_producer);
 
     let name_in_debuginfo = name_in_debuginfo.to_string_lossy();
-    let work_dir = tcx.sess.working_dir.0.to_string_lossy();
     let flags = "\0";
+
+    let out_dir = &tcx.output_filenames(LOCAL_CRATE).out_directory;
     let split_name = tcx
         .output_filenames(LOCAL_CRATE)
-        .split_dwarf_file(tcx.sess.opts.debugging_opts.split_dwarf, Some(codegen_unit_name))
+        .split_dwarf_filename(tcx.sess.opts.debugging_opts.split_dwarf, Some(codegen_unit_name))
         .unwrap_or_default();
+    let out_dir = out_dir.to_str().unwrap();
     let split_name = split_name.to_str().unwrap();
 
     // FIXME(#60020):
@@ -1024,8 +1026,8 @@ pub fn compile_unit_metadata(
             debug_context.builder,
             name_in_debuginfo.as_ptr().cast(),
             name_in_debuginfo.len(),
-            work_dir.as_ptr().cast(),
-            work_dir.len(),
+            out_dir.as_ptr().cast(),
+            out_dir.len(),
             llvm::ChecksumKind::None,
             ptr::null(),
             0,

compiler/rustc_codegen_llvm/src/lib.rs

@@ -356,7 +356,7 @@ impl ModuleLlvm {
 
             let split_dwarf_file = cgcx
                 .output_filenames
-                .split_dwarf_file(cgcx.split_dwarf_kind, Some(name.to_str().unwrap()));
+                .split_dwarf_filename(cgcx.split_dwarf_kind, Some(name.to_str().unwrap()));
             let tm_factory_config = TargetMachineFactoryConfig { split_dwarf_file };
 
             let tm = match (cgcx.tm_factory)(tm_factory_config) {

compiler/rustc_session/src/config.rs

@@ -617,9 +617,20 @@ impl OutputFilenames {
         path
     }
 
+    /// Returns the name of the Split DWARF file - this can differ depending on which Split DWARF
+    /// mode is being used, which is the logic that this function is intended to encapsulate.
+    pub fn split_dwarf_filename(
+        &self,
+        split_dwarf_kind: SplitDwarfKind,
+        cgu_name: Option<&str>,
+    ) -> Option<PathBuf> {
+        self.split_dwarf_path(split_dwarf_kind, cgu_name)
+            .map(|path| path.strip_prefix(&self.out_directory).unwrap_or(&path).to_path_buf())
+    }
+
     /// Returns the path for the Split DWARF file - this can differ depending on which Split DWARF
     /// mode is being used, which is the logic that this function is intended to encapsulate.
-    pub fn split_dwarf_file(
+    pub fn split_dwarf_path(
         &self,
         split_dwarf_kind: SplitDwarfKind,
         cgu_name: Option<&str>,