[SILGenCleanup] Complete lifetimes of defs backwards reachable from dead-end blocks. #78105
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Add a helper similar to findExitingBlocks.
In preparation for adding more users.
It was previously `isVisited`. And mark `visit` private, it needn't be part of the interface for any current clients.
Don't form a set of unreachable blocks, we only need to check whether any given block is unreachable, which can be done via `ReachableBlocks::isVisited`.
The method walks over the blocks of the function testing each for reachability and returns false on the first that is not.
Replace a SmallVector.
In preparation for completing lifetimes in multiple ranges (namely post-orders of subgraphs of the function), extract a complete-in-range method.
Enable clients to check whether the worklist has anything in it without popping a block to find out.
For a function to have complete lifetimes, the lifetime of every def in the function which is backwards-reachable from a dead-end block must be completed. Previously, every def in every block which appears in the post-order of the function's blocks is completed. This was not enough: defs in "unreachable blocks" (i.e. those which aren't forwards-reachable from the function entry) must be completed too, and such blocks do not appear in the function's post-order. Here, defs in unreachable blocks are be completed too. Such defs must also be completed in relative post-order. To do this, roots for the non-entry post-orders must be found. rdar://141197164
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For a function to have complete lifetimes, the lifetime of every def in the function which is backwards-reachable from a dead-end block must be completed.
Previously, every def in every block which appears in the post-order of the function's blocks is completed. This was not enough: defs in "unreachable blocks" (i.e. those which aren't forwards-reachable from the function entry) must be completed too, and such blocks do not appear in the function's post-order.
Here, defs in unreachable blocks are be completed too. Such defs must also be completed in relative post-order. To do this, roots for the non-entry post-orders must be found.
rdar://141197164