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The sub-registers explicitly listed in SubRegs in the .td files form a tree. In a complicated register bank, it is possible to have sub-register relationships across sub-trees. For example, the ARM NEON double vector Q0_Q1 is a tree: Q0_Q1 = [Q0, Q1], Q0 = [D0, D1], Q1 = [D2, D3] But we also define the DPair register D1_D2 = [D1, D2] which is fully contained in Q0_Q1. This patch teaches TableGen to find such sub-register relationships, and assign sub-register indices to them. In the example, TableGen will create a dsub_1_dsub_2 sub-register index, and add D1_D2 as a sub-register of Q0_Q1. This will eventually enable the coalescer to handle copies of skewed sub-registers. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@156587 91177308-0d34-0410-b5e6-96231b3b80d8
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