[PATCH] D46422: [LCSSA] Iteratively remove unused PHI nodes in formLCSSAForInstructions

[Bjorn Pettersson via Phabricator via llvm-commits]

bjope added a comment.

In https://reviews.llvm.org/D46422#1088686, @dberlin wrote:

> Is there an intermediate IR dump you can send me of where you end up with
>  phis using phis that are useless?
>
> (I'd also like to understand if you ever get cycles of that happening as
>  well)


This is what the function looks like if dumping it just before we start to remove unused PHI:s.
The PHI nodes that are removed are marked with `<--------------`

  define void @test() {
  entry:
    br label %gazank
  
  gazank:                                           ; preds = %gazonk, %entry
    %value = phi i16 [ 0, %entry ], [ undef, %gazonk ]
    br i1 undef, label %gazink, label %qqq
  
  gazink:                                           ; preds = %gazank
    br i1 undef, label %gazonk, label %infinite.loop.pred
  
  gazonk:                                           ; preds = %gazink
    br i1 undef, label %exit1, label %gazank
  
  qqq:                                              ; preds = %www, %gazank
    %value.lcssa = phi i16 [ %value.lcssa, %www ], [ %value, %gazank ]
    br i1 undef, label %www, label %exit2
  
  www:                                              ; preds = %qqq
    br i1 undef, label %qqq, label %foo.pred
  
  foo.pred:                                         ; preds = %www
    %value.lcssa.lcssa5 = phi i16 [ %value.lcssa, %www ]
    br label %foo
  
  foo:                                              ; preds = %unreachable1, %bar, %foo.pred
    %value3 = phi i16 [ undef, %unreachable1 ], [ %value3, %bar ], [ %value.lcssa.lcssa5, %foo.pred ]
    br i1 undef, label %bar, label %exit1.pred
  
  bar:                                              ; preds = %foo
    br i1 undef, label %foo, label %exit2.pred
  
  unreachable1:                                     ; No predecessors!
    br i1 undef, label %foo, label %exit2.pred
  
  exit1.pred:                                       ; preds = %foo
    %value3.lcssa = phi i16 [ %value3, %foo ]                                <-----------------
    br label %exit1
  
  exit1:                                            ; preds = %exit1.pred, %gazonk
    %value.lcssa2 = phi i16 [ %value3.lcssa, %exit1.pred ], [ %value, %gazonk ]     <-----------------
    ret void
  
  exit2.pred:                                       ; preds = %unreachable1, %bar
    %value3.lcssa4 = phi i16 [ undef, %unreachable1 ], [ %value3, %bar ]       <-----------------
    br label %exit2
  
  exit2:                                            ; preds = %exit2.pred, %qqq
    %value.lcssa.lcssa = phi i16 [ %value3.lcssa4, %exit2.pred ], [ %value.lcssa, %qqq ]       <-----------------
    ret void
  
  infinite.loop.pred:                               ; preds = %gazink
    %value.lcssa1 = phi i16 [ %value, %gazink ]
    br label %infinite.loop
  
  infinite.loop:                                    ; preds = %infinite.loop, %infinite.loop.pred
    %dead = phi i16 [ %value.lcssa1, %infinite.loop.pred ], [ 0, %infinite.loop ]
    br label %infinite.loop
  }


Repository:
  rL LLVM

https://reviews.llvm.org/D46422