2 files changed, 34 insertions, 15 deletions

diff --git a/lib/compiler/src/beam_ssa.erl b/lib/compiler/src/beam_ssa.erl
index 2ce00990e0..fc13ba06de 100644
--- a/lib/compiler/src/beam_ssa.erl
+++ b/lib/compiler/src/beam_ssa.erl
@@ -28,6 +28,7 @@
          linearize/1,
          mapfold_instrs_rpo/4,
          normalize/1,
+         no_side_effect/1,
          predecessors/1,
          rename_vars/3,
          rpo/1,rpo/2,
@@ -152,6 +153,37 @@ clobbers_xregs(#b_set{op=Op}) ->
         _ -> false
     end.
 
+%% no_side_effect(#b_set{}) -> true|false.
+%%  Test whether this instruction has no side effect and thus is safe
+%%  not to execute if its value is not used. Note that even if `true`
+%%  is returned, the instruction could still be impure (e.g. bif:get).
+
+-spec no_side_effect(b_set()) -> boolean().
+
+no_side_effect(#b_set{op=Op}) ->
+    case Op of
+        {bif,_} -> true;
+        {float,get} -> true;
+        bs_init -> true;
+        bs_extract -> true;
+        bs_match -> true;
+        bs_start_match -> true;
+        bs_test_tail -> true;
+        bs_put -> true;
+        extract -> true;
+        get_hd -> true;
+        get_tl -> true;
+        get_tuple_element -> true;
+        has_map_field -> true;
+        is_nonempty_list -> true;
+        is_tagged_tuple -> true;
+        put_map -> true;
+        put_list -> true;
+        put_tuple -> true;
+        succeeded -> true;
+        _ -> false
+    end.
+
 -spec predecessors(Blocks) -> #{BlockNumber:=[Predecessor]} when
       Blocks :: block_map(),
       BlockNumber :: label(),
diff --git a/lib/compiler/src/beam_ssa_opt.erl b/lib/compiler/src/beam_ssa_opt.erl
index 257e036719..5f3777072a 100644
--- a/lib/compiler/src/beam_ssa_opt.erl
+++ b/lib/compiler/src/beam_ssa_opt.erl
@@ -769,14 +769,14 @@ live_opt_is([#b_set{op=succeeded,dst=#b_var{name=SuccDst}=SuccDstVar,
                     end
             end
     end;
-live_opt_is([#b_set{op=Op,dst=#b_var{name=Dst}}=I|Is], Live0, Acc) ->
+live_opt_is([#b_set{dst=#b_var{name=Dst}}=I|Is], Live0, Acc) ->
     case gb_sets:is_member(Dst, Live0) of
         true ->
             Live1 = gb_sets:union(Live0, gb_sets:from_ordset(beam_ssa:used(I))),
             Live = gb_sets:delete_any(Dst, Live1),
             live_opt_is(Is, Live, [I|Acc]);
         false ->
-            case is_pure(Op) of
+            case beam_ssa:no_side_effect(I) of
                 true ->
                     live_opt_is(Is, Live0, Acc);
                 false ->
@@ -787,19 +787,6 @@ live_opt_is([#b_set{op=Op,dst=#b_var{name=Dst}}=I|Is], Live0, Acc) ->
 live_opt_is([], Live, Acc) ->
     {Acc,Live}.
 
-is_pure({bif,_}) -> true;
-is_pure({float,get}) -> true;
-is_pure(bs_extract) -> true;
-is_pure(extract) -> true;
-is_pure(get_hd) -> true;
-is_pure(get_tl) -> true;
-is_pure(get_tuple_element) -> true;
-is_pure(is_nonempty_list) -> true;
-is_pure(is_tagged_tuple) -> true;
-is_pure(put_list) -> true;
-is_pure(put_tuple) -> true;
-is_pure(_) -> false.
-
 live_opt_unused(#b_set{op=get_map_element}=Set) ->
     {replace,Set#b_set{op=has_map_field}};
 live_opt_unused(_) -> keep.