[PATCH] D80707: [legacyPM] Do not compute preserved analysis if there's no local change

[Josh Stone via Phabricator via llvm-commits]

cuviper added a comment.

I applied this to the Rust fork to measure it in rust#72703 <https://github.com/rust-lang/rust/pull/72703>, and here are the perf results <https://perf.rust-lang.org/compare.html?start=45127211566c53bac386b66909a830649182ab7a&end=5f884a00ab7292844b781231b742372b81bcd4ef>. It looks like a reasonable gain -- no real effect on check/debug builds, but around 1% fewer instructions executed for optimized builds.

I can't vouch for correctness, except that it didn't cause any obvious problems in rustc itself, nor any test failures in the CI build.


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https://reviews.llvm.org/D80707