cranelift: Split predecessor/successor lists #8503
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This establishes the property that the VCode's various lists of ranges each fully cover the index range of another list. Previously, the block_succ_range list covered the first half of block_succs_preds, and the block_pred_range list covered the second half. While I was in the area, I replaced the O(n log n) sort in compute_preds_from_succs with a linear-time counting sort, which uses less temporary storage and directly computes the ranges we want as a byproduct.
cfallin
approved these changes
Apr 29, 2024
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This is super-cool! IMHO this is the "Book" algorithm (in the Paul Erdos sense) for finding preds from succs. Clean and direct.
Out of curiosity have you measured the compile-time impact of this? I wonder if it'll show up in code with complex CFGs... we should take it in any case though.
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I hadn't measured, but according to Hyperfine, compiling the Spidermonkey benchmark from Sightglass is unchanged with this commit. I think that's unsurprising; the largest function has 2650 blocks and 6777 edges (before legalization and critical-edge splitting), and that's just not that many. But I'm still absurdly pleased with finding a situation where a linear-time sort is actually the best choice. |
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This establishes the property that the VCode's various lists of ranges each fully cover the index range of another list. Previously, the block_succ_range list covered the first half of block_succs_preds, and the block_pred_range list covered the second half.
While I was in the area, I replaced the O(n log n) sort in compute_preds_from_succs with a linear-time counting sort, which uses less temporary storage and directly computes the ranges we want as a byproduct.