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We've long supported readcyclecounter on PPC64, but it is easier there (the read of the 64-bit time-base register can be accomplished via a single instruction). This now provides an implementation for PPC32 as well. On PPC32, the time-base register is still 64 bits, but can only be read 32 bits at a time via two separate SPRs. The ISA manual explains how to do this properly (it involves re-reading the upper bits and looping if the counter has wrapped while being read). This requires PPC to implement a custom integer splitting legalization for the READCYCLECOUNTER node, turning it into a target-specific SDAG node, which then gets turned into a pseudo-instruction, which is then expanded to the necessary sequence (which has three SPR reads, the comparison and the branch). Thanks to Paul Hargrove for pointing out to me that this was still unimplemented. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223161 91177308-0d34-0410-b5e6-96231b3b80d8
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