[PATCH] D36504: [CodeGenPrepare] Convert uncond. branch to return into a return to help with shrink-wrapping

[Nemanja Ivanovic via Phabricator via llvm-commits]

nemanjai created this revision.

Currently, CGP will get rid of mostly empty blocks by merging them with their successors. However, in a case where a block `BB` simply branches unconditionally to a block that has no code other than a return, it can be helpful if `BB` just returned since shrink-wrapping might have another candidate for the epilogue.

Example:

  ...
  for.cond.cleanup.loopexit:                        ; preds = %for.body
    br label %for.cond.cleanup
  
  for.cond.cleanup:                                 ; preds = %for.cond.cleanup.loopexit, %entry
    %Ret.0.lcssa = phi i32 [ 0, %entry ], [ %0, %for.cond.cleanup.loopexit ]
    ret i32 %Ret.0.lcssa
  ...

Block `for.cond.cleanup.loopexit` can be used for the epilogue if flow has gone through `for.body` whereas block `for.cond.cleanup` can be used for the epilogue if flow never went into the loop.

The patch certainly increases the amount of shrink-wrapping we do. I'm currently running spec on PPC and will post the net change once the results are ready.


Repository:
  rL LLVM

https://reviews.llvm.org/D36504

Files:
  lib/CodeGen/CodeGenPrepare.cpp
  test/CodeGen/MIR/Generic/early-ret-bb.mir
  test/CodeGen/PowerPC/early-exit-shrink-wrap.ll

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