Auto merge of rust-lang#128320 - saethlin:link-me-maybe, r=compiler-errors

Avoid no-op unlink+link dances in incr comp

Incremental compilation scales quite poorly with the number of CGUs. This PR improves one reason for that.

The incr comp process hard-links all the files from an old session into a new one, then it runs the backend, which may just hard-link the new session files into the output directory. Then codegen hard-links all the output files back to the new session directory.

This PR (perhaps unimaginatively) fixes the silliness that ensues in the last step. The old `link_or_copy` implementation would be passed pairs of paths which are already the same inode, then it would blindly delete the destination and re-create the hard-link that it just deleted. This PR lets us skip both those operations. We don't skip the other two hard-links.

`cargo +stage1 b && touch crates/core/main.rs && strace -cfw -elink,linkat,unlink,unlinkat cargo +stage1 b` before and then after on `ripgrep-13.0.0`:
```
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 52.56    0.024950          25       978       485 unlink
 34.38    0.016318          22       727           linkat
 13.06    0.006200          24       249           unlinkat
------ ----------- ----------- --------- --------- ----------------
100.00    0.047467          24      1954       485 total
```
```
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 42.83    0.014521          57       252           unlink
 38.41    0.013021          26       486           linkat
 18.77    0.006362          25       249           unlinkat
------ ----------- ----------- --------- --------- ----------------
100.00    0.033904          34       987           total
```

This reduces the number of hard-links that are causing perf troubles, noted in rust-lang#64291 and rust-lang#137560

Author: bors

src/driver/aot.rs

@@ -103,12 +103,14 @@ impl OngoingCodegen {
                             ("o", &module_regular.object.as_ref().unwrap()),
                             ("asm.o", &module_global_asm.object.as_ref().unwrap()),
                         ],
+                        &[],
                     )
                 } else {
                     rustc_incremental::copy_cgu_workproduct_to_incr_comp_cache_dir(
                         sess,
                         &module_regular.name,
                         &[("o", &module_regular.object.as_ref().unwrap())],
+                        &[],
                     )
                 };
                 if let Some((work_product_id, work_product)) = work_product {
@@ -381,6 +383,7 @@ fn emit_cgu(
             bytecode: None,
             assembly: None,
             llvm_ir: None,
+            links_from_incr_cache: Vec::new(),
         }),
         existing_work_product: None,
     })
@@ -437,6 +440,7 @@ fn emit_module(
         bytecode: None,
         assembly: None,
         llvm_ir: None,
+        links_from_incr_cache: Vec::new(),
     })
 }
 
@@ -460,22 +464,23 @@ fn reuse_workproduct_for_cgu(
             err
         ));
     }
+
     let obj_out_global_asm =
         crate::global_asm::add_file_stem_postfix(obj_out_regular.clone(), ".asm");
-    let has_global_asm = if let Some(asm_o) = work_product.saved_files.get("asm.o") {
+    let source_file_global_asm = if let Some(asm_o) = work_product.saved_files.get("asm.o") {
         let source_file_global_asm = rustc_incremental::in_incr_comp_dir_sess(&tcx.sess, asm_o);
         if let Err(err) = rustc_fs_util::link_or_copy(&source_file_global_asm, &obj_out_global_asm)
         {
             return Err(format!(
                 "unable to copy {} to {}: {}",
-                source_file_regular.display(),
-                obj_out_regular.display(),
+                source_file_global_asm.display(),
+                obj_out_global_asm.display(),
                 err
             ));
         }
-        true
+        Some(source_file_global_asm)
     } else {
-        false
+        None
     };
 
     Ok(ModuleCodegenResult {
@@ -487,15 +492,17 @@ fn reuse_workproduct_for_cgu(
             bytecode: None,
             assembly: None,
             llvm_ir: None,
+            links_from_incr_cache: vec![source_file_regular],
         },
-        module_global_asm: has_global_asm.then(|| CompiledModule {
+        module_global_asm: source_file_global_asm.map(|source_file| CompiledModule {
             name: cgu.name().to_string(),
             kind: ModuleKind::Regular,
             object: Some(obj_out_global_asm),
             dwarf_object: None,
             bytecode: None,
             assembly: None,
             llvm_ir: None,
+            links_from_incr_cache: vec![source_file],
         }),
         existing_work_product: Some((cgu.work_product_id(), work_product)),
     })
@@ -637,6 +644,7 @@ fn emit_metadata_module(tcx: TyCtxt<'_>, metadata: &EncodedMetadata) -> Compiled
         bytecode: None,
         assembly: None,
         llvm_ir: None,
+        links_from_incr_cache: Vec::new(),
     }
 }