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  4. May 23, 2008
    • Dan Gohman's avatar
      Tidy up BasicBlock::getFirstNonPHI, and change a bunch of places to · f96e1371
      Dan Gohman authored
      use it instead of duplicating its functionality.
      
      llvm-svn: 51499
      f96e1371
    • Matthijs Kooijman's avatar
      Restucture a part of the SimplifyCFG pass and include a testcase. · aef2b819
      Matthijs Kooijman authored
      The SimplifyCFG pass looks at basic blocks that contain only phi nodes,
      followed by an unconditional branch. In a lot of cases, such a block (BB) can
      be merged into their successor (Succ).
      
      This merging is performed by TryToSimplifyUncondBranchFromEmptyBlock. It does
      this by taking all phi nodes in the succesor block Succ and expanding them to
      include the predecessors of BB. Furthermore, any phi nodes in BB are moved to
      Succ and expanded to include the predecessors of Succ as well.
      
      Before attempting this merge, CanPropagatePredecessorsForPHIs checks to see if
      all phi nodes can be properly merged. All functional changes are made to
      this function, only comments were updated in
      TryToSimplifyUncondBranchFromEmptyBlock.
      
      In the original code, CanPropagatePredecessorsForPHIs looks quite convoluted
      and more like stack of checks added to handle different kinds of situations
      than a comprehensive check. In particular the first check in the function did
      some value checking for the case that BB and Succ have a common predecessor,
      while the last check in the function simply rejected all cases where BB and
      Succ have a common predecessor. The first check was still useful in the case
      that BB did not contain any phi nodes at all, though, so it was not completely
      useless.
      
      Now, CanPropagatePredecessorsForPHIs is restructured to to look a lot more
      similar to the code that actually performs the merge. Both functions now look
      at the same phi nodes in about the same order.  Any conflicts (phi nodes with
      different values for the same source) that could arise from merging or moving
      phi nodes are detected. If no conflicts are found, the merge can happen.
      
      Apart from only restructuring the checks, two main changes in functionality
      happened.
      
      Firstly, the old code rejected blocks with common predecessors in most cases.
      The new code performs some extra checks so common predecessors can be handled
      in a lot of cases. Wherever common predecessors still pose problems, the
      blocks are left untouched.
      
      Secondly, the old code rejected the merge when values (phi nodes) from BB were
      used in any other place than Succ. However, it does not seem that there is any
      situation that would require this check. Even more, this can be proven.
      
      Consider that BB is a block containing of a single phi node "%a" and a branch
      to Succ. Now, since the definition of %a will dominate all of its uses, BB
      will dominate all blocks that use %a. Furthermore, since the branch from BB to
      Succ is unconditional, Succ will also dominate all uses of %a.
      
      Now, assume that one predecessor of Succ is not dominated by BB (and thus not
      dominated by Succ). Since at least one use of %a (but in reality all of them)
      is reachable from Succ, you could end up at a use of %a without passing
      through it's definition in BB (by coming from X through Succ). This is a
      contradiction, meaning that our original assumption is wrong. Thus, all
      predecessors of Succ must also be dominated by BB (and thus also by Succ).
      
      This means that moving the phi node %a from BB to Succ does not pose any
      problems when the two blocks are merged, and any use checks are not needed.
      
      llvm-svn: 51478
      aef2b819
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