The current splitting algorithm works in three stages:
- Identify cold blocks, then
- Use forward/backward propagation to mark hot blocks, then
- 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
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.
- 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.
- Outline more than once per function, outline large single basic blocks, & try to remove unconditional branches in outlined functions.