beam_ssa_opt: Fix liveness optimization

Add more instructions to the list of functions that can be safely
removed if their values are not used. This is necessary for
correctness when doing more aggressive optimizations. Without this
change, the 'succeeded' instruction could be optimized away leaving
just the instruction followed by an unconditional branch, which the
beam_ssa_codegen does not know how to handle. Here is an example:

    _3 = bs_start_match _1
    br label 13

By adding bs_start_match to the list, the bs_start_match instruction
will be removed too. (If the result of bs_start_match is actually
used, the succeeded instruction would not be removed.)

While we are it, rename the misnamed function is_pure/1 to
no_side_effect/1 and move it to beam_ssa. is_pure/1 is a bad name
because bif:get has no side effect, but is not pure.

1 files changed, 2 insertions, 15 deletions

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.