beam_bounds: Make 'bnot' converge faster

Fixes #7145

1 files changed, 22 insertions, 1 deletions

diff --git a/lib/compiler/src/beam_bounds.erl b/lib/compiler/src/beam_bounds.erl
index 6f3902596e..6ac5141821 100644
--- a/lib/compiler/src/beam_bounds.erl
+++ b/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);
         _ ->