Fix unsafe optimization of stack trace building

The `sys_core_fold` pass of the compiler would optimize
away the building of the stacktrace in code such as:

    try
        ...
    catch
        C:R:Stk ->
           erlang:raise(C, {R,Stk}, Stk)
    end

That optimization is unsafe and would cause a crash in a later compiler
pass.

2 files changed, 39 insertions, 7 deletions

diff --git a/lib/compiler/src/sys_core_fold.erl b/lib/compiler/src/sys_core_fold.erl
index 3a65b40fa5..1681d97efb 100644
--- a/lib/compiler/src/sys_core_fold.erl
+++ b/lib/compiler/src/sys_core_fold.erl
@@ -2635,12 +2635,20 @@ opt_build_stacktrace(#c_let{vars=[#c_var{name=Cooked}],
         #c_call{module=#c_literal{val=erlang},
                 name=#c_literal{val=raise},
                 args=[Class,Exp,#c_var{name=Cooked}]} ->
-            %% The stacktrace is only used in a call to erlang:raise/3.
-            %% There is no need to build the stacktrace. Replace the
-            %% call to erlang:raise/3 with the the raw_raise/3 instruction,
-            %% which will use a raw stacktrace.
-            #c_primop{name=#c_literal{val=raw_raise},
-                      args=[Class,Exp,RawStk]};
+            case core_lib:is_var_used(Cooked, #c_cons{hd=Class,tl=Exp}) of
+                true ->
+                    %% Not safe. The stacktrace is used in the class or
+                    %% reason.
+                    Let;
+                false ->
+                    %% The stacktrace is only used in the last
+                    %% argument for erlang:raise/3. There is no need
+                    %% to build the stacktrace. Replace the call to
+                    %% erlang:raise/3 with the the raw_raise/3
+                    %% instruction, which will use a raw stacktrace.
+                    #c_primop{name=#c_literal{val=raw_raise},
+                              args=[Class,Exp,RawStk]}
+            end;
         #c_let{vars=[#c_var{name=V}],arg=Arg,body=B0} when V =/= Cooked ->
             case core_lib:is_var_used(Cooked, Arg) of
                 false ->
diff --git a/lib/compiler/test/trycatch_SUITE.erl b/lib/compiler/test/trycatch_SUITE.erl
index 1b7ef4ddb0..8f9cd9ab1e 100644
--- a/lib/compiler/test/trycatch_SUITE.erl
+++ b/lib/compiler/test/trycatch_SUITE.erl
@@ -1189,7 +1189,8 @@ bad_raise(Expr) ->
 test_raise(Expr) ->
     test_raise_1(Expr),
     test_raise_2(Expr),
-    test_raise_3(Expr).
+    test_raise_3(Expr),
+    test_raise_4(Expr).
 
 test_raise_1(Expr) ->
     erase(exception),
@@ -1263,5 +1264,28 @@ do_test_raise_3(Expr) ->
             erlang:raise(exit, {exception,C,E}, Stk)
     end.
 
+test_raise_4(Expr) ->
+    try
+        do_test_raise_4(Expr)
+    catch
+        exit:{exception,C,E,Stk}:Stk ->
+            try
+                Expr()
+            catch
+                C:E:S ->
+                    [StkTop|_] = S,
+                    [StkTop|_] = Stk
+            end
+    end.
+
+do_test_raise_4(Expr) ->
+    try
+        Expr()
+    catch
+        C:E:Stk ->
+            %% Here the stacktrace must be built.
+            erlang:raise(exit, {exception,C,E,Stk}, Stk)
+    end.
+
 
 id(I) -> I.