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The current splitting algorithm works in three stages: 1) Identify cold blocks, then 2) Use forward/backward propagation to mark hot blocks, then 3) Grow a SESE region of blocks *outside* of the set of hot blocks and start outlining. While testing this pass on Apple internal frameworks I noticed that some kinds of control flow (e.g. loops) are never outlined, even though they unconditionally lead to / follow cold blocks. I noticed two other issues related to how cold regions are identified: - An inconsistency can arise in the internal state of the hotness propagation stage, as a block may end up in both the ColdBlocks set and the HotBlocks set. Further inconsistencies can arise as these sets do not match what's in ProfileSummaryInfo. - It isn't necessary to limit outlining to single-exit regions. This patch teaches the splitting algorithm to identify maximal cold regions and outline them. A maximal cold region is defined as the set of blocks post-dominated by a cold sink block, or dominated by that sink block. This approach can successfully outline loops in the cold path. As a side benefit, it maintains less internal state than the current approach. Due to a limitation in CodeExtractor, blocks within the maximal cold region which aren't dominated by a single entry point (a so-called "max ancestor") are filtered out. Results: - X86 (LNT + -Os + externals): 134KB of TEXT were outlined compared to 47KB pre-patch, or a ~3x improvement. Did not see a performance impact across two runs. - AArch64 (LNT + -Os + externals + Apple-internal benchmarks): 149KB of TEXT were outlined. Ditto re: performance impact. - Outlining results improve marginally in the internal frameworks I tested. Follow-ups: - Outline more than once per function, outline large single basic blocks, & try to remove unconditional branches in outlined functions. Differential Revision: https://reviews.llvm.org/D53627 llvm-svn: 345209 |
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