[4.0] runtime: Fix overflow of swift_unownedRetain reference counts #11293
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On 32bit platforms there are 7 bits reserved for the unowned retain count. This makes overflow a likely scenario. Implement overflow into the side table. rdar://33495003
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On 32bit platforms there are 7 bits reserved for the unowned retain count. This
makes overflow a likely scenario. Implement overflow into the side table.
rdar://33495003
• Explanation: Usage of unowned references cause a crash on 32bit if there are more than 127 unowned references to one object. This is because the current representation reserves 7 bits on 32bit platforms.
• Scope of Issue: A crash will happen on 32bit platforms if there are more than 127 unowned references to an object
• Origination: This bug was likely introduced when moving to the new reference count representation in swift 4
• Risk: Low. Only the implementation of unowned references was changed. Behavior should only change when overflow happens.
• Testing: A test case was added to the regression suite