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[CodeGen] Add a new pass for PostRA sink
Summary: This pass sinks COPY instructions into a successor block, if the COPY is not used in the current block and the COPY is live-in to a single successor (i.e., doesn't require the COPY to be duplicated). This avoids executing the the copy on paths where their results aren't needed. This also exposes additional opportunites for dead copy elimination and shrink wrapping. These copies were either not handled by or are inserted after the MachineSink pass. As an example of the former case, the MachineSink pass cannot sink COPY instructions with allocatable source registers; for AArch64 these type of copy instructions are frequently used to move function parameters (PhyReg) into virtual registers in the entry block.. For the machine IR below, this pass will sink %w19 in the entry into its successor (%bb.1) because %w19 is only live-in in %bb.1. ``` %bb.0: %wzr = SUBSWri %w1, 1 %w19 = COPY %w0 Bcc 11, %bb.2 %bb.1: Live Ins: %w19 BL @fun %w0 = ADDWrr %w0, %w19 RET %w0 %bb.2: %w0 = COPY %wzr RET %w0 ``` As we sink %w19 (CSR in AArch64) into %bb.1, the shrink-wrapping pass will be able to see %bb.0 as a candidate. With this change I observed 12% more shrink-wrapping candidate and 13% more dead copies deleted in spec2000/2006/2017 on AArch64. Reviewers: qcolombet, MatzeB, thegameg, mcrosier, gberry, hfinkel, john.brawn, twoh, RKSimon, sebpop, kparzysz Reviewed By: sebpop Subscribers: evandro, sebpop, sfertile, aemerson, mgorny, javed.absar, kristof.beyls, llvm-commits Differential Revision: https://reviews.llvm.org/D41463 llvm-svn: 328237
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Jun Bum Lim
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Mar 22, 2018
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