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This is a second attempt to land this patch. The first one resulted in a crash of clang sanitizer buildbot. The fix is here and regression test is added. This is a last fix for the corner case of PR32214. Actually this is not really corner case in general. We should not do a loop rotation if we create an additional branch due to it. Consider the case where we have a loop chain H, M, B, C , where H is header with viable fallthrough from pre-header and exit from the loop M - some middle block B - backedge to Header but with exit from the loop also. C - some cold block of the loop. Let's H is determined as a best exit. If we do a loop rotation M, B, C, H we can introduce the extra branch. Let's compute the change in number of branches: +1 branch from pre-header to header -1 branch from header to exit +1 branch from header to middle block if there is such -1 branch from cold bock to header if there is one So if C is not a predecessor of H then we introduce extra branch. This change actually prohibits rotation of the loop if both true Best Exit has next element in chain as successor. Last element in chain is not a predecessor of first element of chain. Reviewers: iteratee, xur, sammccall, chandlerc Reviewed By: iteratee Subscribers: llvm-commits Differential Revision: https://reviews.llvm.org/D34745 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307631 91177308-0d34-0410-b5e6-96231b3b80d8
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