48b36c9d59
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 |
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