[6.1] Fix DCE for ownership forwarding instructions #78063
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meg-gupta
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DCE deletes ownership forwarding instructions when it doesn’t have useful users. It inserts destroy_value/end_borrow for its operands to compensate their lifetimes. DCE also deletes branches when its successor blocks does not have useful instructions. It deletes blocks and creates a jump to the nearest post dominating block. When DCE needs to delete a forwarding instruction in a dead block, it cannot just create lifetime ends of its operands at its position. Use LifetimeCompletion utility in such cases. rdar://140428721
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tbkka
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Dec 10, 2024
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Explanation: DCE deletes ownership forwarding instructions when it doesn’t have useful users. It inserts destroy_value/end_borrow for its operands to compensate their lifetimes.
DCE also deletes branches when its successor blocks does not have useful instructions. It deletes blocks and creates a jump to the nearest post dominating block.
When DCE needs to delete a forwarding instruction in a dead block, it cannot just create lifetime ends of its operands at its position. Use LifetimeCompletion utility in such cases.
Scope: Affects DCE running on ownership ssa
Risk: Medium.
Testing: Swift CI testing
Reviewed by: @nate-chandler
Original PR: #78053
rdar://140428721