| commit | 1c21d5cb9b8e48ab928919a6f358eba8ffd3b49c | [log] [tgz] |
|---|---|---|
| author | David Green <david.green@arm.com> | Mon Sep 15 07:32:00 2025 +0100 |
| committer | GitHub <noreply@github.com> | Mon Sep 15 07:32:00 2025 +0100 |
| tree | 35d713fa74a3900c43a5ff0b607ce504c0fd265c | |
| parent | 6931bad36c0ddae441d115814022fd5d3cbc554b [diff] |
[GlobalISel] Remove GI known bits cache (#157352) There is a cache on the known-bit computed by global-isel. It only works inside a single query to computeKnownBits, which limits its usefulness, and according to the tests can sometimes limit the effectiveness of known-bits queries. (Although some AMD tests look longer). Keeping the cache valid and clearing it at the correct times can also require being careful about the functions called inside known-bits queries. I measured compile-time of removing it and came up with: ``` 7zip 2.06405E+11 2.06436E+11 0.015018992 Bullet 1.01298E+11 1.01186E+11 -0.110236169 ClamAV 57942466667 57848066667 -0.16292023 SPASS 45444466667 45402966667 -0.091320249 consumer 35432466667 35381233333 -0.144594317 kimwitu++ 40858833333 40927933333 0.169118877 lencod 70022366667 69950633333 -0.102443457 mafft 38439900000 38413233333 -0.069372362 sqlite3 35822266667 35770033333 -0.145812474 tramp3d 82083133333 82045600000 -0.045726 Average -0.068828739 ``` The last column is % difference between with / without the cache. So in total it seems to be costing slightly more to keep the current known-bits cache than if it was removed. (Measured in instruction count, similar to llvm-compile-time-tracker). The hit rate wasn't terrible - higher than I expected. In the llvm-test-suite+external projects it was hit 4791030 times out of 91107008 queries, slightly more than 5%. Note that as globalisel increases in complexity, more known bits calls might be made and the numbers might shift. If that is the case it might be better to have a cache that works across calls, providing it doesn't make effectiveness worse.
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