Merge branch 'bjorn/compiler/optimizations/OTP-11584'

* bjorn/compiler/optimizations/OTP-11584:
  Teach sys_core_fold:eval_case/2 to cope with handwritten Core Erlang
  sys_core_fold: Remove a redundant word in a comment

1 files changed, 17 insertions, 7 deletions

diff --git a/lib/compiler/src/sys_core_fold.erl b/lib/compiler/src/sys_core_fold.erl
index e302e2324d..500d431afe 100644
--- a/lib/compiler/src/sys_core_fold.erl
+++ b/lib/compiler/src/sys_core_fold.erl
@@ -1940,7 +1940,9 @@ opt_bool_case_guard(Arg, [#c_clause{pats=[#c_literal{val=false}]}=Fc,Tc]) ->
 %%  last clause is guaranteed to match so if there is only one clause
 %%  with a pattern containing only variables then rewrite to a let.
 
-eval_case(#c_case{arg=E,clauses=[#c_clause{pats=Ps0,body=B}]}, Sub) ->
+eval_case(#c_case{arg=E,clauses=[#c_clause{pats=Ps0,
+					   guard=#c_literal{val=true},
+					   body=B}]}=Case, Sub) ->
     Es = case cerl:is_c_values(E) of
 	     true -> cerl:values_es(E);
 	     false -> [E]
@@ -1955,7 +1957,7 @@ eval_case(#c_case{arg=E,clauses=[#c_clause{pats=Ps0,body=B}]}, Sub) ->
     %%
     %%   let <X,Y> = <SomeSideEffect(),SomeSideEffect()> in ...
     %%
-    %% because SomeSideEffect() would be called evaluated twice.
+    %% because SomeSideEffect() would be evaluated twice.
     %%
     %% Instead we must evaluate the case expression in an outer let
     %% like this:
@@ -1964,11 +1966,19 @@ eval_case(#c_case{arg=E,clauses=[#c_clause{pats=Ps0,body=B}]}, Sub) ->
     %%       let <X,Y> = <NewVar,NewVar> in ...
     %%
     Vs = make_vars([], length(Es)),
-    {true,Bs} = cerl_clauses:match_list(Ps0, Vs),
-    {Ps,As} = unzip(Bs),
-    InnerLet = cerl:c_let(Ps, core_lib:make_values(As), B),
-    Let = cerl:c_let(Vs, E, InnerLet),
-    expr(Let, sub_new(Sub));
+    case cerl_clauses:match_list(Ps0, Vs) of
+	{false,_} ->
+	    %% This can only happen if the Core Erlang code is
+	    %% handwritten or generated by another code generator
+	    %% than v3_core. Assuming that the Core Erlang program
+	    %% is correct, the clause will always match at run-time.
+	    Case;
+	{true,Bs} ->
+	    {Ps,As} = unzip(Bs),
+	    InnerLet = cerl:c_let(Ps, core_lib:make_values(As), B),
+	    Let = cerl:c_let(Vs, E, InnerLet),
+	    expr(Let, sub_new(Sub))
+    end;
 eval_case(Case, _) -> Case.
 
 %% case_opt(CaseArg, [Clause]) -> {CaseArg,[Clause]}.