beam_dead: Remove code that cannot be covered in forward/4

Since the optimizations in forward/4 already depends on some
assumptions on how code is generated anyway, document the
assumptions in a comment and remove the uncoverable code.

1 files changed, 13 insertions, 22 deletions

diff --git a/lib/compiler/src/beam_dead.erl b/lib/compiler/src/beam_dead.erl
index bb93110176..135c7757c5 100644
--- a/lib/compiler/src/beam_dead.erl
+++ b/lib/compiler/src/beam_dead.erl
@@ -246,30 +246,24 @@ forward([{select_val,Reg,_,{list,List}}=I|Is], D0, Lc, Acc) ->
     D = update_value_dict(List, Reg, D0),
     forward(Is, D, Lc, [I|Acc]);
 forward([{label,Lbl}=LblI,{block,[{set,[Dst],[Lit],move}|BlkIs]}=Blk|Is], D, Lc, Acc) ->
+    %% Assumption: The target labels in a select_val/3 instruction
+    %% cannot be reached in any other way than through the select_val/3
+    %% instruction (i.e. there can be no fallthrough to such label and
+    %% it cannot be referenced by, for example, a jump/1 instruction).
     Block = case gb_trees:lookup({Lbl,Dst}, D) of
-		{value,Lit} ->
-		    %% The move instruction seems to be redundant, but also make
-		    %% sure that the instruction preceeding the label
-		    %% cannot fall through to the move instruction.
-		    case is_unreachable_after(Acc) of
-			false -> Blk;	  %Must keep move instruction.
-			true ->  {block,BlkIs}	%Safe to remove move instruction.
-		    end;
-		_ -> Blk		       %Keep move instruction.
+		{value,Lit} -> {block,BlkIs}; %Safe to remove move instruction.
+		_ -> Blk		      %Must keep move instruction.
 	    end,
     forward([Block|Is], D, Lc, [LblI|Acc]);
 forward([{label,Lbl}=LblI|[{move,Lit,Dst}|Is1]=Is0], D, Lc, Acc) ->
+    %% Assumption: The target labels in a select_val/3 instruction
+    %% cannot be reached in any other way than through the select_val/3
+    %% instruction (i.e. there can be no fallthrough to such label and
+    %% it cannot be referenced by, for example, a jump/1 instruction).
     Is = case gb_trees:lookup({Lbl,Dst}, D) of
-	     {value,Lit} ->
-		 %% The move instruction seems to be redundant, but also make
-		 %% sure that the instruction preceeding the label
-		 %% cannot fall through to the move instruction.
-		 case is_unreachable_after(Acc) of
-		     false -> Is0;	  %Must keep move instruction.
-		     true -> Is1     %Safe to remove move instruction.
-		  end;
-	     _ -> Is0		       %Keep move instruction.
-	  end,
+	     {value,Lit} -> Is1;     %Safe to remove move instruction.
+	     _ -> Is0		     %Keep move instruction.
+	 end,
     forward(Is, D, Lc, [LblI|Acc]);
 forward([{test,is_eq_exact,_,[Dst,Src]}=I,
 	 {block,[{set,[Dst],[Src],move}|Bl]}|Is], D, Lc, Acc) ->
@@ -306,9 +300,6 @@ update_value_dict([Lit,{f,Lbl}|T], Reg, D0) ->
     update_value_dict(T, Reg, D);
 update_value_dict([], _, D) -> D.
 
-is_unreachable_after([I|_]) ->
-    beam_jump:is_unreachable_after(I).
-
 %%%
 %%% Scan instructions in reverse execution order and remove dead code.
 %%%