[cmake] Enable reverse iteration by default through build macro
Summary:
Reverse iteration can be turned on, by default, by setting -DLLVM_REVERSE_ITERATION:BOOL=ON during cmake.
With this enabled, we can uncover lots of cases of non-determinism in codegen by simply running our tests (without any other change).
We can then setup a buildbot which will have this turned on by default. Initially, a lot of unit tests will fail in this configuration.
Once we start fixing non-determinism issues, we can gradually make this a blocker for patches.
Reviewers: davide, dblaikie, mehdi_amini, dberlin
Reviewed By: dblaikie
Subscribers: probinson, mgorny, llvm-commits
Differential Revision: https://reviews.llvm.org/D33908
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@304757
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