AliasAnalysis: use a complexity limit for the isObjReleased function #68316
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We already use a complexity limit for other functions in AliasAnalysis. This is a workaround for quadratic complexity in ARCSequenceOpts. Fixes a compile time problem rdar://114352817
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Reviewed for correctness.
It took me a quite while to figure out what this change really means.
Summary:
swift::mayDecrementRefCount is the basic primitive for all ARC optimization: ARCSequenceOpts and ARCCodeMotion. These passes are quadratic in the number of live objects. Each live object uses a separate call to mayDecrementRefCount which is implemented in terms of AliasAnalysis::isObjectReleasedByInst, which calls AliasAnalysis::getOwnershipEffect.
AliasAnalysis::getOwnershipEffect calls into escape analysis to walk the SSA graph. Escape analysis has a normal budget to limit the graph traversal.
This changes AliasAnalysis::isObjectReleasedByInst to use 1/10 the normal escape analysis budget. This only affects the ARC passes that call into this entry point.
The deeper, underlying bug is that AliasAnalysis::getOwnershipEffect does not support multiple simultaneous queries. All roots could be queried with a single escape analysis walk.
We already use a complexity limit for other functions in AliasAnalysis. This is a workaround for quadratic complexity in ARCSequenceOpts.
Fixes a compile time problem
rdar://114352817