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0c245b6f0f
With a previous patch, the VSX swap optimization is able to recognize the doubleword load-splat idiom that can be implemented using lxvdsx. However, that does not cover a doubleword splat where the source is a register. We can implement this using xxspltd (a special form of xxpermdi). This patch teaches the swap optimization pass about this idiom. As a prerequisite, it also permits swap optimization to succeed for all forms of SUBREG_TO_REG. Previously we were conservative and only allowed SUBREG_TO_REG when it copied a full register. However, on reflection any form of SUBREG_TO_REG is safe in and of itself, so long as an unsafe operation is not performed on its result. In particular, a widening SUBREG_TO_REG often occurs as an input to a doubleword splat idiom, particularly in auto-vectorized code. The doubleword splat idiom is an XXPERMDI operation where both source registers are identical, and the selection mask is either 0 (splat the first element) or 3 (splat the second element). To determine whether the registers are identical, we use the existing mechanism for looking through "copy-like" operations. That mechanism has a side effect of marking the XXPERMDI operation as using a physical register, which would invalidate its presence in a swap-optimized region. This is correct for the form of XXPERMDI that performs a swap and hence would be removed, but is not what we want for a doubleword-splat variety of XXPERMDI. Therefore we reset the physical-register flag on the XXPERMDI when it represents a splat. A simple test case is added to verify that we generate the splat and that we also remove the xxswapd instructions that would otherwise be associated with the load and store of another operand. llvm-svn: 241285 |
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