Split critical edges leaving an indirectbr when a simple case applies.
Declared in <llvm/Transforms/Utils/BasicBlockUtils.h>
bool
SplitIndirectBrCriticalEdges(
Function& F,
bool IgnoreBlocksWithoutPHI,
BranchProbabilityInfo* BPI = nullptr,
BlockFrequencyInfo* BFI = nullptr,
DomTreeUpdater* DTU = nullptr);Split critical edges where the source of the edge is an indirectbr instruction. This isn't always possible, but we can handle some easy cases. This is useful because MI is unable to split such critical edges, which means it will not be able to sink instructions along those edges. This is especially painful for indirect branches with many successors, where we end up having to prepare all outgoing values in the origin block.
Our normal algorithm for splitting critical edges requires us to update the outgoing edges of the edge origin block, but for an indirectbr this is hard, since it would require finding and updating the block addresses the indirect branch uses. But if a block only has a single indirectbr predecessor, with the others being regular branches, we can do it in a different way. Say we have A -> D, B -> D, I -> D where only I -> D is an indirectbr. We can split D into D0 and D1, where D0 contains only the PHIs from D, and D1 is the D block body. We can then duplicate D0 as D0A and D0B, and create the following structure: A -> D0A, B -> D0A, I -> D0B, D0A -> D1, D0B -> D1 If BPI and BFI aren't non-null, BPI/BFI will be updated accordingly. When IgnoreBlocksWithoutPHI is set to true critical edges leading to a block without phi-instructions will not be split.
True if any critical edges were split.
| Name | Description |
|---|---|
| F | Function whose indirectbr critical edges may be split. |
| IgnoreBlocksWithoutPHI | Skip destinations that contain no PHI nodes. |
| BPI | Optional branch-probability info to update. |
| BFI | Optional block-frequency info to update. |
| DTU | Optional dominator-tree updater for CFG changes. |