beam_ssa_pre_codegen: Fix an internal consistency failure

The following function:

    is_two_tuple(Arg) ->
        case is_tuple(Arg) of
            false -> false;
            true -> tuple_size(Arg) == 2
        end.

would cause an internal consistency failure:

    Internal consistency check failed - please report this bug.
    Instruction: {bif,tuple_size,{f,0},[{x,0}],{z,0}}
    Error:       {invalid_store,{z,0},{integer,[]}}:

2 files changed, 19 insertions, 5 deletions

diff --git a/lib/compiler/src/beam_ssa_pre_codegen.erl b/lib/compiler/src/beam_ssa_pre_codegen.erl
index 32232e9b9f..56fe9b4793 100644
--- a/lib/compiler/src/beam_ssa_pre_codegen.erl
+++ b/lib/compiler/src/beam_ssa_pre_codegen.erl
@@ -1994,13 +1994,15 @@ reserve_zregs(Blocks, Intervals, Res) ->
     beam_ssa:fold_rpo(F, [0], Res, Blocks).
 
 reserve_zreg([#b_set{op={bif,tuple_size},dst=Dst},
-              #b_set{op={bif,'=:='},args=[Dst,Val]}], _Last, ShortLived, A0) ->
-    case Val of
-        #b_literal{val=Arity} when Arity bsr 32 =:= 0 ->
+              #b_set{op={bif,'=:='},args=[Dst,Val]}], Last, ShortLived, A0) ->
+    case {Val,Last} of
+        {#b_literal{val=Arity},#b_br{}} when Arity bsr 32 =:= 0 ->
             %% These two instructions can be combined to a test_arity
             %% instruction provided that the arity variable is short-lived.
             reserve_zreg_1(Dst, ShortLived, A0);
-        _ ->
+        {_,_} ->
+            %% Either the arity is too big, or the boolean value from
+            %% '=:=' will be returned.
             A0
     end;
 reserve_zreg([#b_set{op={bif,tuple_size},dst=Dst}],
diff --git a/lib/compiler/test/beam_ssa_SUITE.erl b/lib/compiler/test/beam_ssa_SUITE.erl
index e32e3eebfc..15cf9bcbf3 100644
--- a/lib/compiler/test/beam_ssa_SUITE.erl
+++ b/lib/compiler/test/beam_ssa_SUITE.erl
@@ -92,7 +92,13 @@ start_it([_|_]=MFA) ->
     end.
 
 tuple_matching(_Config) ->
-    do_tuple_matching({tag,42}).
+    do_tuple_matching({tag,42}),
+
+    true = is_two_tuple({a,b}),
+    false = is_two_tuple({a,b,c}),
+    false = is_two_tuple(atom),
+
+    ok.
 
 do_tuple_matching(Arg) ->
     Res = do_tuple_matching_1(Arg),
@@ -118,6 +124,12 @@ do_tuple_matching_3(Tuple) when is_tuple(Tuple) ->
             {ok,element(2, Tuple)}
     end.
 
+is_two_tuple(Arg) ->
+    case is_tuple(Arg) of
+        false -> false;
+        true -> tuple_size(Arg) == 2
+    end.
+
 -record(reporter_state, {res,run_config}).
 -record(run_config, {report_interval=0}).