Description
Every time I recompile my Rust project after a change it takes 2 or 3 seconds. I was wondering where that time is spent. I used the measureme/summarize tool to profile the rust compiler and it showed me that most time is spent in the codegen_copy_artifacts_from_incr_cache stage:
+--------------------------------------------------------+-----------+-----------------+----------+------------+-----------------------+---------------------------------+
| Item | Self time | % of total time | Time | Item count | Incremental load time | Incremental result hashing time |
+--------------------------------------------------------+-----------+-----------------+----------+------------+-----------------------+---------------------------------+
| codegen_copy_artifacts_from_incr_cache | 12.24s | 83.333 | 12.24s | 255 | 0.00ns | 0.00ns |
+--------------------------------------------------------+-----------+-----------------+----------+------------+-----------------------+---------------------------------+
| run_linker | 448.51ms | 3.054 | 448.51ms | 1 | 0.00ns | 0.00ns |
+--------------------------------------------------------+-----------+-----------------+----------+------------+-----------------------+---------------------------------+
| incr_comp_prepare_session_directory | 416.99ms | 2.840 | 416.99ms | 1 | 0.00ns | 0.00ns |
+--------------------------------------------------------+-----------+-----------------+----------+------------+-----------------------+---------------------------------+
| LLVM_passes | 414.15ms | 2.820 | 414.15ms | 1 | 0.00ns | 0.00ns |
+--------------------------------------------------------+-----------+-----------------+----------+------------+-----------------------+---------------------------------+
| copy_all_cgu_workproducts_to_incr_comp_cache_dir | 349.22ms | 2.378 | 349.22ms | 1 | 0.00ns | 0.00ns |
+--------------------------------------------------------+-----------+-----------------+----------+------------+-----------------------+---------------------------------+
| codegen_crate | 264.25ms | 1.800 | 375.35ms | 1 | 0.00ns | 0.00ns |
+--------------------------------------------------------+-----------+-----------------+----------+------------+-----------------------+---------------------------------+
...
The reported time is larger than the actual time, probably because of multithreading.
This stage involves creating hard links.
Process Monitor shows that there are many SetLinkInformationFile syscalls some of which take 100-200ms each:
I have observed this behaviour on many different projects.
Steps to reproduce in any Rust project of non-trivial size are:
cargo build- Make any small change, like changing a string literal.
cargo rustc -- -Z self-profilesummarize summarize <filename>.mm_profdata
Looks like creating hard links is sometimes very slow on Windows. Maybe some other strategy can be used to avoid creating so many hard links?
rustc 1.87.0-nightly (f8a913b13 2025-02-23)
binary: rustc
commit-hash: f8a913b1381e90379c7ca63ac2b88b9518936628
commit-date: 2025-02-23
host: x86_64-pc-windows-msvc
release: 1.87.0-nightly
LLVM version: 20.1.0