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When allocating live intervals in linear order and all of them are local to a single basic block you get an optimal coloring. This is also true if you reverse the order, but it is not true if you sort live ranges beginnings in reverse order, change to sort live range endings in reverse order. Take the following live ranges for example: |---| |--------| |----------| |-------| They get colored suboptimally with 3 registers if you sort the live range starting points in reverse order (but optimally with live range begins in order, or live range ends in reverse order). Apparently the previous strategy was intentional because of allocation time considerations. I am having a hard time replicating these effects, while I see substantial improvements in allocation quality with this change. No testcase as none of the (in tree) targets use reverse order mode. Differential Revision: http://reviews.llvm.org/D8625 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233742 91177308-0d34-0410-b5e6-96231b3b80d8
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