compiler: Fully disable no_return optimizations in try blocks

Validation could fail when a function that never returned was used
in a try block (see attached test case). It's possible to solve
this without disabling the optimization as the generated code is
sound, but I'm not comfortable making such a large change this
close to the OTP 22 release.

2 files changed, 46 insertions, 4 deletions

diff --git a/lib/compiler/src/beam_ssa_type.erl b/lib/compiler/src/beam_ssa_type.erl
index c01ea4af91..06b42f1928 100644
--- a/lib/compiler/src/beam_ssa_type.erl
+++ b/lib/compiler/src/beam_ssa_type.erl
@@ -267,10 +267,29 @@ opt_is([#b_set{op=call,args=Args0,dst=Dst}=I0|Is],
     I1 = beam_ssa:normalize(I0#b_set{args=Args}),
     {Ts1,Ds,Fdb,I2} = opt_call(I1, D, Ts0, Ds0, Fdb0),
     case {map_get(Dst, Ts1),Is} of
-        {_,[#b_set{op=succeeded}]} ->
+        {Type,[#b_set{op=succeeded}]} when Type =/= none ->
             %% This call instruction is inside a try/catch
-            %% block. Don't attempt to optimize it.
+            %% block. Don't attempt to simplify it.
             opt_is(Is, Ts1, Ds, Fdb, D, Sub0, [I2|Acc]);
+        {none,[#b_set{op=succeeded}]} ->
+            %% This call instruction is inside a try/catch
+            %% block, but we know it will never return and
+            %% later optimizations may try to exploit that.
+            %%
+            %% For example, if we have an expression that
+            %% either returns this call or a tuple, we know
+            %% that the expression always returns a tuple
+            %% and can turn a later element/3 into
+            %% get_tuple_element.
+            %%
+            %% This is sound but difficult to validate in a
+            %% meaningful way as try/catch currently forces
+            %% us to maintain the illusion that the success
+            %% block is reachable even when its not, so we
+            %% disable the optimization to keep things
+            %% simple.
+            Ts = Ts1#{ Dst := any },
+            opt_is(Is, Ts, Ds, Fdb, D, Sub0, [I2|Acc]);
         {none,_} ->
             %% This call never returns. The rest of the
             %% instructions will not be executed.
diff --git a/lib/compiler/test/trycatch_SUITE.erl b/lib/compiler/test/trycatch_SUITE.erl
index 8f9cd9ab1e..539f9d69fa 100644
--- a/lib/compiler/test/trycatch_SUITE.erl
+++ b/lib/compiler/test/trycatch_SUITE.erl
@@ -27,7 +27,8 @@
 	 nested_horrid/1,last_call_optimization/1,bool/1,
 	 plain_catch_coverage/1,andalso_orelse/1,get_in_try/1,
 	 hockey/1,handle_info/1,catch_in_catch/1,grab_bag/1,
-         stacktrace/1,nested_stacktrace/1,raise/1]).
+         stacktrace/1,nested_stacktrace/1,raise/1,
+         no_return_in_try_block/1]).
 
 -include_lib("common_test/include/ct.hrl").
 
@@ -43,7 +44,8 @@ groups() ->
        nested_after,nested_horrid,last_call_optimization,
        bool,plain_catch_coverage,andalso_orelse,get_in_try,
        hockey,handle_info,catch_in_catch,grab_bag,
-       stacktrace,nested_stacktrace,raise]}].
+       stacktrace,nested_stacktrace,raise,
+       no_return_in_try_block]}].
 
 
 init_per_suite(Config) ->
@@ -1287,5 +1289,26 @@ do_test_raise_4(Expr) ->
             erlang:raise(exit, {exception,C,E,Stk}, Stk)
     end.
 
+no_return_in_try_block(Config) when is_list(Config) ->
+    1.0 = no_return_in_try_block_1(0),
+    1.0 = no_return_in_try_block_1(0.0),
+
+    gurka = no_return_in_try_block_1(gurka),
+    [] = no_return_in_try_block_1([]),
+
+    ok.
+
+no_return_in_try_block_1(H) ->
+    try
+        Float = if
+                    is_number(H) -> float(H);
+                    true -> no_return()
+                end,
+        Float + 1
+    catch
+        throw:no_return -> H
+    end.
+
+no_return() -> throw(no_return).
 
 id(I) -> I.