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[AMDGPU] lower-switch in preISel as a workaround for legacy DA Summary: The default target of the switch instruction may sometimes be an "unreachable" block, when it is guaranteed that one of the cases is always taken. The dominator tree concludes that such a switch instruction does not have an immediate post dominator. This confuses divergence analysis, which is unable to propagate sync dependence to the targets of the switch instruction. As a workaround, the AMDGPU target now invokes lower-switch as a preISel pass. LowerSwitch is designed to handle the unreachable default target correctly, allowing the divergence analysis to locate the correct immediate dominator of the now-lowered switch. llvm-svn: 342956
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