Auto merge of #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 https://github.com/rust-lang/rust/issues/64291 and https://github.com/rust-lang/rust/issues/137560

3 files changed, 18 insertions, 6 deletions

diff --git a/compiler/rustc_codegen_ssa/src/back/write.rs b/compiler/rustc_codegen_ssa/src/back/write.rs
index 2ec203458a3..216a18e72ed 100644
--- a/compiler/rustc_codegen_ssa/src/back/write.rs
+++ b/compiler/rustc_codegen_ssa/src/back/write.rs
@@ -546,9 +546,12 @@ fn copy_all_cgu_workproducts_to_incr_comp_cache_dir(
         if let Some(path) = &module.bytecode {
             files.push((OutputType::Bitcode.extension(), path.as_path()));
         }
-        if let Some((id, product)) =
-            copy_cgu_workproduct_to_incr_comp_cache_dir(sess, &module.name, files.as_slice())
-        {
+        if let Some((id, product)) = copy_cgu_workproduct_to_incr_comp_cache_dir(
+            sess,
+            &module.name,
+            files.as_slice(),
+            &module.links_from_incr_cache,
+        ) {
             work_products.insert(id, product);
         }
     }
@@ -934,7 +937,9 @@ fn execute_copy_from_cache_work_item<B: ExtraBackendMethods>(
 ) -> WorkItemResult<B> {
     let incr_comp_session_dir = cgcx.incr_comp_session_dir.as_ref().unwrap();
 
-    let load_from_incr_comp_dir = |output_path: PathBuf, saved_path: &str| {
+    let mut links_from_incr_cache = Vec::new();
+
+    let mut load_from_incr_comp_dir = |output_path: PathBuf, saved_path: &str| {
         let source_file = in_incr_comp_dir(incr_comp_session_dir, saved_path);
         debug!(
             "copying preexisting module `{}` from {:?} to {}",
@@ -943,7 +948,10 @@ fn execute_copy_from_cache_work_item<B: ExtraBackendMethods>(
             output_path.display()
         );
         match link_or_copy(&source_file, &output_path) {
-            Ok(_) => Some(output_path),
+            Ok(_) => {
+                links_from_incr_cache.push(source_file);
+                Some(output_path)
+            }
             Err(error) => {
                 cgcx.create_dcx().handle().emit_err(errors::CopyPathBuf {
                     source_file,
@@ -966,7 +974,7 @@ fn execute_copy_from_cache_work_item<B: ExtraBackendMethods>(
             load_from_incr_comp_dir(dwarf_obj_out, saved_dwarf_object_file)
         });
 
-    let load_from_incr_cache = |perform, output_type: OutputType| {
+    let mut load_from_incr_cache = |perform, output_type: OutputType| {
         if perform {
             let saved_file = module.source.saved_files.get(output_type.extension())?;
             let output_path = cgcx.output_filenames.temp_path(output_type, Some(&module.name));
@@ -986,6 +994,7 @@ fn execute_copy_from_cache_work_item<B: ExtraBackendMethods>(
     }
 
     WorkItemResult::Finished(CompiledModule {
+        links_from_incr_cache,
         name: module.name,
         kind: ModuleKind::Regular,
         object,
diff --git a/compiler/rustc_codegen_ssa/src/base.rs b/compiler/rustc_codegen_ssa/src/base.rs
index 396febcc637..a739743be8c 100644
--- a/compiler/rustc_codegen_ssa/src/base.rs
+++ b/compiler/rustc_codegen_ssa/src/base.rs
@@ -658,6 +658,7 @@ pub fn codegen_crate<B: ExtraBackendMethods>(
                 bytecode: None,
                 assembly: None,
                 llvm_ir: None,
+                links_from_incr_cache: Vec::new(),
             }
         })
     });
diff --git a/compiler/rustc_codegen_ssa/src/lib.rs b/compiler/rustc_codegen_ssa/src/lib.rs
index 44b8cc459cf..93c34a2f576 100644
--- a/compiler/rustc_codegen_ssa/src/lib.rs
+++ b/compiler/rustc_codegen_ssa/src/lib.rs
@@ -122,6 +122,7 @@ impl<M> ModuleCodegen<M> {
             bytecode,
             assembly,
             llvm_ir,
+            links_from_incr_cache: Vec::new(),
         }
     }
 }
@@ -135,6 +136,7 @@ pub struct CompiledModule {
     pub bytecode: Option<PathBuf>,
     pub assembly: Option<PathBuf>, // --emit=asm
     pub llvm_ir: Option<PathBuf>,  // --emit=llvm-ir, llvm-bc is in bytecode
+    pub links_from_incr_cache: Vec<PathBuf>,
 }
 
 impl CompiledModule {