[llvm-dev] Basic block merging

[David Jones via llvm-dev]

Under certain circumstances, my compiler outputs basic blocks having the
same function:

bb_97:                                            ; preds = %bb_1
  %476 = getelementptr inbounds %LMtop.I0.ARType, %LMtop.I0.ARType* %0, i64
0, i32 6
  %477 = bitcast i8** %476 to %LBstd.Cprocess.CRType**
  %478 = load %LBstd.Cprocess.CRType*, %LBstd.Cprocess.CRType** %477, align
8
  %479 = getelementptr inbounds %LBstd.Cprocess.CRType,
%LBstd.Cprocess.CRType* %478, i64 0, i32 3
  %480 = bitcast i8** %479 to %LMtop.I0.ARType**
  store %LMtop.I0.ARType* %0, %LMtop.I0.ARType** %480, align 8
  br label %bb_106

bb_98:                                            ; preds = %bb_1
  %481 = getelementptr inbounds %LMtop.I0.ARType, %LMtop.I0.ARType* %0, i64
0, i32 6
  %482 = bitcast i8** %481 to %LBstd.Cprocess.CRType**
  %483 = load %LBstd.Cprocess.CRType*, %LBstd.Cprocess.CRType** %482, align
8
  %484 = getelementptr inbounds %LBstd.Cprocess.CRType,
%LBstd.Cprocess.CRType* %483, i64 0, i32 3
  %485 = bitcast i8** %484 to %LMtop.I0.ARType**
  store %LMtop.I0.ARType* %0, %LMtop.I0.ARType** %485, align 8
  br label %bb_106

These blocks have provably the same function - the instructions of one
correspond perfectly to instructions in the other.

I would have expected some optimization to detect this, and merge the
blocks by forwarding all jumps to one, to jump to the other.

I notice a "merge functions" pass, which does this if two functions can be
proven to have identical effect.

Is there a pass that merges blocks? Is it enabled by default?  If so, then
is there something non-obvious in the code that would be defeating this
optimization?
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