[5.2] SILCombine: fix a miscompile in the alloc_stack optimization which causes a use-after-free. #29890
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Seems that I have to update a sil_combine test in the 5.2 branch |
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airspeedswift
approved these changes
Feb 17, 2020
…uses a use-after-free. A "copy_addr [take] %src to [initialization] %alloc_stack" is replaced by a "destroy_addr %src" if the alloc_stack is otherwise dead. This is okay as long as the "moved" object is kept alive otherwise. This can break if a retain of %src is moved after the copy_addr. It cannot happen with OSSA. So as soon as we have OSSA, we can remove the check again. rdar://problem/59496027
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This is a cherry-pick from #29882
Explanation: This fixes a miscompile in the SILCombine optimization. When trying to remove a "dead" alloc_stack, the optimization didn't take into account that release-instructions could be moved over copy-instructions. This makes the alloc_stack the only instance which keeps an object alive. Removing the alloc_stack resulted in a release of an object, followed by a retain: exactly in the wrong order. The compiled code crashed.
Scope: It's difficult to scope. The bug is in the compiler since a very long time. The fact that we didn't see it earlier indicates that this miscompile is unlikely to actually happen. On the other hand it can really be almost any kind of swift source code which can trigger this bug.
Risk: Low. It's basically an additional bail-out condition for the optimization.
Issue: rdar://problem/59496027
Testing: I added a regression test.
Reviewer: @aschwaighofer