[PATCH] D28934: Write a new SSAUpdater

[Daniel Berlin via Phabricator via llvm-commits]

dberlin added a comment.

Updating the patch in a second, i templated the trivial phi removal, fixed some bug, started on unit tests enough for others to play with it.

It should be a pretty direct conversion from the old ssaupdater (setAvailableValue->writeVariable, getvalueAtEndOfBlock->readVariableAfterDef, getvalueAtMiddleOfBlock->readVariableBeforeDef)

A few notes from more testing:

1. The old updater crashes if you call getValueAtEndOfBlock for a currently empty block (you have to insert the instruction first) (I can make the new one work on incomplete cfgs, actually, if we want)
2. The only cases we generate spurious phis now is in the case of irreducible control flow. This will go away once i implement the SCC algorithm.

Something like this:

  define void @F(i32, i32) {
  if:
    br i1 true, label %loopmain, label %loopstart
  
  loopstart:                                        ; preds = %loopmain, %if
    br label %loopmain
  
  loopmain:                                         ; preds = %loopstart, %if
    br i1 true, label %loopstart, label %afterloop
  
  afterloop:                                        ; preds = %loopmain
    ret i32 %0
  }

Will become

  define void @F(i32, i32) {
  if:
    br i1 true, label %loopmain, label %loopstart
  
  loopstart:                                        ; preds = %loopmain, %if
    %update1 = phi i32 [ %update, %loopmain ], [ %0, %if ]
    br label %loopmain
  
  loopmain:                                         ; preds = %loopstart, %if
    %update = phi i32 [ %update1, %loopstart ], [ %0, %if ]
    br i1 true, label %loopstart, label %afterloop
  
  afterloop:                                        ; preds = %loopmain
    ret i32 %0
  }




https://reviews.llvm.org/D28934