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This patch implements Chandler's idea [0] for supporting languages that require support for infinite loops with side effects, such as Rust, providing part of a solution to bug 965 [1]. Specifically, it adds an `llvm.sideeffect()` intrinsic, which has no actual effect, but which appears to optimization passes to have obscure side effects, such that they don't optimize away loops containing it. It also teaches several optimization passes to ignore this intrinsic, so that it doesn't significantly impact optimization in most cases. As discussed on llvm-dev [2], this patch is the first of two major parts. The second part, to change LLVM's semantics to have defined behavior on infinite loops by default, with a function attribute for opting into potential-undefined-behavior, will be implemented and posted for review in a separate patch. [0] http://lists.llvm.org/pipermail/llvm-dev/2015-July/088103.html [1] https://bugs.llvm.org/show_bug.cgi?id=965 [2] http://lists.llvm.org/pipermail/llvm-dev/2017-October/118632.html Differential Revision: https://reviews.llvm.org/D38336 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317729 91177308-0d34-0410-b5e6-96231b3b80d8