beam_dead: Optimize select_val instructions

In a select_val instruction, values associated with a label
which is the same as the failure label can be removed. We
already do this optimization in beam_clean, but it is better
do this sort of optimization before the beam_jump pass.

Also rewrite a select_val instruction with a single value
to is_eq_exact instruction followed by a jump instruction.

1 files changed, 25 insertions, 1 deletions

diff --git a/lib/compiler/src/beam_dead.erl b/lib/compiler/src/beam_dead.erl
index 0cb5040177..fcd108bd89 100644
--- a/lib/compiler/src/beam_dead.erl
+++ b/lib/compiler/src/beam_dead.erl
@@ -239,10 +239,18 @@ backward([{test,is_eq_exact,Fail,[Dst,{integer,Arity}]}=I|
 backward([{label,Lbl}=L|Is], D, Acc) ->
     backward(Is, beam_utils:index_label(Lbl, Acc, D), [L|Acc]);
 backward([{select,select_val,Reg,{f,Fail0},List0}|Is], D, Acc) ->
-    List = shortcut_select_list(List0, Reg, D, []),
+    List1 = shortcut_select_list(List0, Reg, D, []),
     Fail1 = shortcut_label(Fail0, D),
     Fail = shortcut_bs_test(Fail1, Is, D),
+    List = prune_redundant(List1, Fail),
     case List of
+	[] ->
+	    Jump = {jump,{f,Fail}},
+	    backward([Jump|Is], D, Acc);
+	[V,F] ->
+	    Test = {test,is_eq_exact,{f,Fail},[Reg,V]},
+	    Jump = {jump,F},
+	    backward([Jump,Test|Is], D, Acc);
 	[{atom,B1},F,{atom,B2},F] when B1 =:= not B2 ->
 	    Test = {test,is_boolean,{f,Fail},[Reg]},
 	    Jump = {jump,F},
@@ -307,6 +315,16 @@ backward([{test,Op,{f,To0},Ops0}|Is], D, Acc) ->
 	    %% An is_atom test before an is_boolean test (with the
 	    %% same failure label) is redundant.
 	    backward(Is, D, Acc);
+	{{test,is_atom,Fail,[R]},
+	 [{test,is_eq_exact,Fail,[R,{atom,_}]}|_]} ->
+	    %% An is_atom test before a comparison with an atom (with
+	    %% the same failure label) is redundant.
+	    backward(Is, D, Acc);
+	{{test,is_integer,Fail,[R]},
+	 [{test,is_eq_exact,Fail,[R,{integer,_}]}|_]} ->
+	    %% An is_integer test before a comparison with an integer
+	    %% (with the same failure label) is redundant.
+	    backward(Is, D, Acc);
 	{{test,_,_,_},_} ->
 	    %% Still a test instruction. Done.
 	    backward(Is, D, [I|Acc]);
@@ -366,6 +384,12 @@ shortcut_label(To0, D) ->
 shortcut_select_label(To, Reg, Lit, D) ->
     shortcut_rel_op(To, is_ne_exact, [Reg,Lit], D).
 
+prune_redundant([_,{f,Fail}|T], Fail) ->
+    prune_redundant(T, Fail);
+prune_redundant([V,F|T], Fail) ->
+    [V,F|prune_redundant(T, Fail)];
+prune_redundant([], _) -> [].
+
 %% Replace a comparison operator with a test instruction and a jump.
 %% For example, if we have this code:
 %%