beam_bounds: Make 'bnot' converge faster

Fixes #7145

lib/compiler/src/beam_bounds.erl

@@ -46,7 +46,28 @@
 
 bounds('bnot', R0) ->
     case R0 of
-        {A,B} ->
+        {A, B} when is_integer(A), is_integer(B), A =/= B ->
+            %% While it's easy to get an exact range, doing so can make certain
+            %% chains of operations slow to converge, e.g.
+            %%
+            %%     f(0) -> -1; f(N) -> abs(bnot f(N)).
+            %%
+            %% Where the range increases by 1 every time we pass through,
+            %% making it more or less impossible to reach a fixpoint.
+            %%
+            %% We therefore widen the range a bit quicker to ensure that we
+            %% converge on 'any' within a reasonable time frame, hoping that
+            %% the range will still be tight enough in the cases where we
+            %% don't feed the result into itself.
+            case {abs(A) bsr ?NUM_BITS, abs(B) bsr ?NUM_BITS} of
+                {0, 0} ->
+                    Min = min(-B - 1, -(B bsl 1) - 1),
+                    Max = max(-A - 1, -(A bsl 1) - 1),
+                    normalize({Min, Max});
+                {_, _} ->
+                    any
+            end;
+        {A, B} ->
             R = {inf_add(inf_neg(B), -1), inf_add(inf_neg(A), -1)},
             normalize(R);
         _ ->

lib/compiler/test/beam_bounds_SUITE.erl

@@ -197,6 +197,8 @@ bnot_bounds(_Config) ->
     {'-inf',-8} = beam_bounds:bounds('bnot', {7,'+inf'}),
     {'-inf',9} = beam_bounds:bounds('bnot', {-10,'+inf'}),
 
+    -1 = bnot_bounds_2(0),
+
     ok.
 
 bnot_bounds_1(R) ->
@@ -211,6 +213,10 @@ bnot_bounds_1(R) ->
             ct:fail(bad_min_or_max)
         end.
 
+%% GH-7145: 'bnot' converged too slowly, effectively hanging the compiler.
+bnot_bounds_2(0) -> -1;
+bnot_bounds_2(N) -> abs(bnot bnot_bounds_2(N)).
+
 bsr_bounds(_Config) ->
     test_noncommutative('bsr', {-12,12}, {0,7}),