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[CodeLayout] Faster basic block reordering, ext-tsp (#68617)
Aggressive inlining might produce huge functions with >10K of basic blocks. Since BFI treats _all_ blocks and jumps as "hot" having non-negative (but perhaps small) weight, the current implementation can be slow, taking minutes to produce an layout. This change introduces a few modifications that significantly (up to 50x on some instances) speeds up the computation. Some notable changes: - reduced the maximum chain size to 512 (from the prior 4096); - introduced MaxMergeDensityRatio param to avoid merging chains with very different densities; - dropped a couple of params that seem unnecessary. Looking at some "offline" metrics (e.g., the number of created fall-throughs), there shouldn't be problems; in fact, I do see some metrics go up. But it might be hard/impossible to measure perf difference for such small changes. I did test the performance clang-14 binary and do not record a perf or i-cache-related differences. My 5 benchmarks, with ext-tsp runtime (the lower the better) and "tsp-score" (the higher the better). **Before**: - benchmark 1: num functions: 13,047 reordering running time is 2.4 seconds score: 125503458 (128.3102%) - benchmark 2: num functions: 16,438 reordering running time is 3.4 seconds score: 12613997277 (129.7495%) - benchmark 3: num functions: 12,359 reordering running time is 1.9 seconds score: 1315881613 (105.8991%) - benchmark 4: num functions: 96,588 reordering running time is 7.3 seconds score: 89513906284 (100.3413%) - benchmark 5: num functions: 1 reordering running time is 372 seconds score: 21292505965077 (99.9979%) - benchmark 6: num functions: 71,155 reordering running time is 314 seconds score: 29795381626270671437824 (102.7519%) **After**: - benchmark 1: reordering running time is 2.2 seconds score: 125510418 (128.3130%) - benchmark 2: reordering running time is 2.6 seconds score: 12614502162 (129.7525%) - benchmark 3: reordering running time is 1.6 seconds score: 1315938168 (105.9024%) - benchmark 4: reordering running time is 4.9 seconds score: 89518095837 (100.3454%) - benchmark 5: reordering running time is 4.8 seconds score: 21292295939119 (99.9971%) - benchmark 6: reordering running time is 104 seconds score: 29796710925310302879744 (102.7565%)
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