Avoid inserting end_cow_mutation_addr in some common cases #85482
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Based on #85481 |
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meg-gupta
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Nov 13, 2025
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We introduce
end_cow_mutation_addrat the scope ends of a lifetime dependent value's parent in cases where the lifetime dependent value maybe a MutableSpan and the parent has a modify scope.This is necessary to avoid miscompiles on the mutableSpan property of Array/ArraySlice/ContiguousArray to indicate the end of mutation.
The analysis used to insert
end_cow_mutation_addris conservative and may introduce unnecessaryend_cow_mutation_addrinstructions when generic types are involved which can indirectly block bounds check optimizations.Improve this analysis by avoiding insertion of
end_cow_mutation_addrwhen the parent maybe trivial, ~Copyable or builtin type. Also avoid insertingend_cow_mutation_addrby adding an early exit inmayHaveMutableSpanfor Escapable and Copyable types.This avoids an unnecessary
end_cow_mutation_addron themark_dependencebase of the MutableSpan's unsafe addressor based subscript result.