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We need to do this to prevent a miscompile which sinks an objc_retain past an objc_release that releases the object objc_retain retains. This happens because the top-down and bottom-up traversals each determines the insert point for retain or release individually without knowing where the other instruction is moved. For example, when the following IR is fed to the ARC optimizer, the top-down traversal decides to insert objc_retain right before objc_release and the bottom-up traversal decides to insert objc_release right after clang.arc.use. (IR before ARC optimizer) %11 = call i8* @objc_retain(i8* %10) call void (...) @clang.arc.use(%0* %5) call void @llvm.dbg.value(...) call void @objc_release(i8* %6) This reverses the order of objc_release and objc_retain, which causes the object to be destructed prematurely. (IR after ARC optimizer) call void (...) @clang.arc.use(%0* %5) call void @objc_release(i8* %6) call void @llvm.dbg.value(...) %11 = call i8* @objc_retain(i8* %10) rdar://problem/30530580 llvm-svn: 301289
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