Auto merge of rust-lang#128320 - saethlin:link-me-maybe, r=<try>

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
```

r? `@ghost`

Author: bors

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -20,6 +20,7 @@ use rustc_errors::{
     Style,
 };
 use rustc_fs_util::link_or_copy;
+use rustc_fs_util::LinkOrCopy;
 use rustc_hir::def_id::{CrateNum, LOCAL_CRATE};
 use rustc_incremental::{
     copy_cgu_workproduct_to_incr_comp_cache_dir, in_incr_comp_dir, in_incr_comp_dir_sess,
@@ -548,9 +549,12 @@ 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);
         }
     }
@@ -942,7 +946,9 @@ 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 {}",
@@ -951,7 +957,11 @@ 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(LinkOrCopy::Copy) => Some(output_path),
+            Ok(LinkOrCopy::Link) => {
+                links_from_incr_cache.push(source_file);
+                Some(output_path)
+            }
             Err(error) => {
                 cgcx.create_dcx().handle().emit_err(errors::CopyPathBuf {
                     source_file,
@@ -974,7 +984,7 @@ 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));
@@ -994,6 +1004,7 @@ fn execute_copy_from_cache_work_item<B: ExtraBackendMethods>(
     }
 
     WorkItemResult::Finished(CompiledModule {
+        links_from_incr_cache,
         name: module.name,
         kind: ModuleKind::Regular,
         object,

compiler/rustc_codegen_ssa/src/base.rs

@@ -629,6 +629,7 @@ pub fn codegen_crate<B: ExtraBackendMethods>(
                 bytecode: None,
                 assembly: None,
                 llvm_ir: None,
+                links_from_incr_cache: Vec::new(),
             }
         })
     });

compiler/rustc_codegen_ssa/src/lib.rs

@@ -93,6 +93,7 @@ impl<M> ModuleCodegen<M> {
             bytecode,
             assembly,
             llvm_ir,
+            links_from_incr_cache: Vec::new(),
         }
     }
 }
@@ -106,6 +107,7 @@ 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 {

compiler/rustc_fs_util/src/lib.rs

@@ -56,25 +56,31 @@ pub enum LinkOrCopy {
     Copy,
 }
 
-/// Copies `p` into `q`, preferring to use hard-linking if possible. If
-/// `q` already exists, it is removed first.
+/// Copies `p` into `q`, preferring to use hard-linking if possible.
 /// The result indicates which of the two operations has been performed.
 pub fn link_or_copy<P: AsRef<Path>, Q: AsRef<Path>>(p: P, q: Q) -> io::Result<LinkOrCopy> {
+    // Creating a hard-link will fail if the destination path already exists. We could defensively
+    // call remove_file in this function, but that pessimizes callers who can avoid such calls.
+    // Incremental compilation calls this function a lot, and is able to avoid calls that
+    // would fail the first hard_link attempt.
+
     let p = p.as_ref();
     let q = q.as_ref();
-    match fs::remove_file(q) {
-        Ok(()) => (),
-        Err(err) if err.kind() == io::ErrorKind::NotFound => (),
-        Err(err) => return Err(err),
-    }
 
-    match fs::hard_link(p, q) {
-        Ok(()) => Ok(LinkOrCopy::Link),
-        Err(_) => match fs::copy(p, q) {
-            Ok(_) => Ok(LinkOrCopy::Copy),
-            Err(e) => Err(e),
-        },
+    let err = match fs::hard_link(p, q) {
+        Ok(()) => return Ok(LinkOrCopy::Link),
+        Err(err) => err,
+    };
+
+    if err.kind() == io::ErrorKind::AlreadyExists {
+        fs::remove_file(q)?;
+        if fs::hard_link(p, q).is_ok() {
+            return Ok(LinkOrCopy::Link);
+        }
     }
+
+    // Hard linking failed, fall back to copying.
+    fs::copy(p, q).map(|_| LinkOrCopy::Copy)
 }
 
 #[cfg(unix)]

compiler/rustc_incremental/src/persist/work_product.rs

@@ -10,6 +10,7 @@ use rustc_middle::dep_graph::{WorkProduct, WorkProductId};
 use rustc_session::Session;
 use std::fs as std_fs;
 use std::path::Path;
+use std::path::PathBuf;
 use tracing::debug;
 
 /// Copies a CGU work product to the incremental compilation directory, so next compilation can
@@ -18,6 +19,7 @@ pub fn copy_cgu_workproduct_to_incr_comp_cache_dir(
     sess: &Session,
     cgu_name: &str,
     files: &[(&'static str, &Path)],
+    known_links: &[PathBuf],
 ) -> Option<(WorkProductId, WorkProduct)> {
     debug!(?cgu_name, ?files);
     sess.opts.incremental.as_ref()?;
@@ -26,6 +28,10 @@ pub fn copy_cgu_workproduct_to_incr_comp_cache_dir(
     for (ext, path) in files {
         let file_name = format!("{cgu_name}.{ext}");
         let path_in_incr_dir = in_incr_comp_dir_sess(sess, &file_name);
+        if known_links.contains(&path_in_incr_dir) {
+            let _ = saved_files.insert(ext.to_string(), file_name);
+            continue;
+        }
         match link_or_copy(path, &path_in_incr_dir) {
             Ok(_) => {
                 let _ = saved_files.insert(ext.to_string(), file_name);