Merge pull request #7157 from jhogberg/john/compiler/fix-bnot-converg…

…ence beam_bounds: Make 'bnot' converge faster
erlang · Apr 26, 2023 · bab771b · bab771b
2 parents ab803ea + 8e5b1fb
commit bab771b
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Showing 3 changed files with 35 additions and 2 deletions.
diff --git 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);
         _ ->

diff --git a/lib/compiler/src/beam_validator.erl b/lib/compiler/src/beam_validator.erl
@@ -2830,7 +2830,13 @@ unpack_typed_arg(#tr{r=Reg,t=Type}, Vst) ->
     %% The validator is not yet clever enough to do proper range analysis like
     %% the main type pass, so our types will be a bit cruder here, but they
     %% should at the very least not be in direct conflict.
-    true = none =/= beam_types:meet(get_movable_term_type(Reg, Vst), Type),
+    Current = get_movable_term_type(Reg, Vst),
+    case beam_types:meet(Current, Type) of
+        none ->
+            throw({bad_typed_register, Current, Type});
+        _ ->
+            ok
+    end,
     Reg;
 unpack_typed_arg(Arg, _Vst) ->
     Arg.

diff --git a/lib/compiler/test/beam_bounds_SUITE.erl b/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}),