make compiler slightly smarter about 0-value letrec-values bindings

which corresponds to code generated from experession that are
 mixed with internal definitions

collects/scribblings/guide/performance.scrbl

@@ -228,10 +228,11 @@ In the first case, the compiler likely does not know that
 available.
 
 This caveat about @racket[letrec] also applies to definitions of
-functions and constants within modules. A definition sequence in a
+functions and constants as internal definitions or in modules. A
-module body is analogous to a sequence of @racket[letrec] bindings,
+definition sequence in a module body is analogous to a sequence of
-and non-constant expressions in a module body can interfere with the
+@racket[letrec] bindings, and non-constant expressions in a module
-optimization of references to later bindings.
+body can interfere with the optimization of references to later
+bindings.
 
 @; ----------------------------------------------------------------------
 

src/racket/src/syntax.c

@@ -3707,6 +3707,23 @@ static int get_convert_arg_count(Scheme_Object *lift)
     return 0;
 }
 
+static Scheme_Object *drop_zero_value_return(Scheme_Object *expr)
+{
+  if (SAME_TYPE(SCHEME_TYPE(expr), scheme_sequence_type)) {
+    if (((Scheme_Sequence *)expr)->count == 2) {
+      if (SAME_TYPE(SCHEME_TYPE(((Scheme_Sequence *)expr)->array[1]), scheme_application_type)) {
+        if (((Scheme_App_Rec *)((Scheme_Sequence *)expr)->array[1])->num_args == 0) {
+          if (SAME_OBJ(scheme_values_func, ((Scheme_App_Rec *)((Scheme_Sequence *)expr)->array[1])->args[0])) {
+            return ((Scheme_Sequence *)expr)->array[0];
+          }
+        }
+      }
+    }
+  }
+
+  return NULL;
+}
+
 Scheme_Object *
 scheme_resolve_lets(Scheme_Object *form, Resolve_Info *info)
 {
@@ -3714,7 +3731,7 @@ scheme_resolve_lets(Scheme_Object *form, Resolve_Info *info)
   Scheme_Let_Header *head = (Scheme_Let_Header *)form;
   Scheme_Compiled_Let_Value *clv, *pre_body;
   Scheme_Let_Value *lv, *last = NULL;
-  Scheme_Object *first = NULL, *body, *last_body = NULL;
+  Scheme_Object *first = NULL, *body, *last_body = NULL, *last_seq = NULL;
   Scheme_Letrec *letrec;
   mzshort *skips, skips_fast[5];
   char *flonums, flonums_fast[5];
@@ -4215,6 +4232,33 @@ scheme_resolve_lets(Scheme_Object *form, Resolve_Info *info)
         int j;
         Scheme_Object *one_lifted;
 
+        if (!clv->count)
+          expr = drop_zero_value_return(clv->value);
+        else
+          expr = NULL;
+
+        if (expr) {
+          /* changing a [() (begin expr (values))] clause,
+             which can be generated by internal-definition expansion,
+             into a `begin' */
+          expr = scheme_resolve_expr(expr, val_linfo);
+          expr = scheme_make_sequence_compilation(scheme_make_pair(expr,
+                                                                   scheme_make_pair(scheme_false,
+                                                                                    scheme_null)),
+                                                  0);
+          
+          if (last)
+            last->body = expr;
+          else if (last_body)
+            SCHEME_CDR(last_body) = expr;
+          else if (last_seq)
+            ((Scheme_Sequence *)last_seq)->array[1] = expr;
+          else
+            first = expr;
+          last = NULL;
+          last_body = NULL;
+          last_seq = expr;
+        } else {
           expr = scheme_resolve_expr(clv->value, val_linfo);
 
           lv = MALLOC_ONE_TAGGED(Scheme_Let_Value);
@@ -4222,10 +4266,13 @@ scheme_resolve_lets(Scheme_Object *form, Resolve_Info *info)
             last->body = (Scheme_Object *)lv;
           else if (last_body)
             SCHEME_CDR(last_body) = (Scheme_Object *)lv;
+          else if (last_seq)
+            ((Scheme_Sequence *)last_seq)->array[1] = (Scheme_Object *)lv;
           else
             first = (Scheme_Object *)lv;
           last = lv;
           last_body = NULL;
+          last_seq = NULL;
       
           lv->iso.so.type = scheme_let_value_type;
           lv->value = expr;
@@ -4250,10 +4297,13 @@ scheme_resolve_lets(Scheme_Object *form, Resolve_Info *info)
                 boxenv = scheme_make_syntax_resolved(BOXENV_EXPD, pr);
                 if (last)
                   last->body = boxenv;
+                else if (last_seq)
+                  ((Scheme_Sequence *)last_seq)->array[1] = boxenv;
                 else
                   SCHEME_CDR(last_body) = boxenv;
                 last = NULL;
                 last_body = pr;
+                last_seq = NULL;
               } else {
                 /* For regular let, delay the boxing until all RHSs are
                    evaluated. */
@@ -4263,6 +4313,7 @@ scheme_resolve_lets(Scheme_Object *form, Resolve_Info *info)
           }
         }
       }
+    }
     opos += clv->count;
   }
 
@@ -4283,12 +4334,16 @@ scheme_resolve_lets(Scheme_Object *form, Resolve_Info *info)
       last->body = (Scheme_Object *)letrec;
     else if (last_body)
       SCHEME_CDR(last_body) = (Scheme_Object *)letrec;
+    else if (last_seq)
+      ((Scheme_Sequence *)last_seq)->array[1] = (Scheme_Object *)letrec;
     else
       first = (Scheme_Object *)letrec;
   } else if (last)
     last->body = body;
   else if (last_body)
     SCHEME_CDR(last_body) = body;
+  else if (last_seq)
+    ((Scheme_Sequence *)last_seq)->array[1] = (Scheme_Object *)body;
   else
     first = body;