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Note that this is disabled by default and still requires a patch to handleMove() which is not upstreamed yet. If the TrackLaneMasks policy/strategy is enabled the MachineScheduler will build a schedule graph where definitions of independent subregisters are no longer serialised. Implementation comments: - Without lane mask tracking a sub register def also counts as a use (except for the first one with the read-undef flag set), with lane mask tracking enabled this is no longer the case. - Pressure Diffs where previously maintained per definition of a vreg with the help of the SSA information contained in the LiveIntervals. With lanemask tracking enabled we cannot do this anymore and instead change the pressure diffs for all uses of the vreg as it becomes live/dead. For this changed style to work correctly we ignore uses of instructions that define the same register again: They won't affect register pressure. - With lanemask tracking we remove all read-undef flags from sub register defs when building the graph and re-add them later when all vreg lanes have become dead. Differential Revision: http://reviews.llvm.org/D14969 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@258259 91177308-0d34-0410-b5e6-96231b3b80d8