cmp: merge operators with-exit-label and with-optional-exit-label

Their core difference was that the latter coudl reuse the destination. We make
the exit argument explicit and optional in with-exit-label.

src/cmp/cmpbackend-cxx/cmpc-util.lsp

@@ -161,20 +161,14 @@
 (defun next-label (used-p)
   (make-label :id (incf *last-label*) :denv *unwind-exit* :used-p used-p))
 
-(defun maybe-next-label ()
-  (if (labelp *exit*)
-      *exit*
-      (next-label nil)))
-
-(defmacro with-exit-label ((label) &body body)
-  `(let* ((,label (next-label nil))
-          (*unwind-exit* (cons ,label *unwind-exit*)))
-     ,@body
-     (wt-label ,label)))
-
-(defmacro with-optional-exit-label ((label) &body body)
-  `(let* ((,label (maybe-next-label))
-          (*unwind-exit* (adjoin ,label *unwind-exit*)))
-     ,@body
-     (unless (eq ,label *exit*)
-       (wt-label ,label))))
+;;; This macro binds VAR to a label where forms may exit or jump.
+;;; LABEL may be supplied to reuse a label when it exists.
+(defmacro with-exit-label ((var &optional exit) &body body)
+  (ext:with-gensyms (reuse label)
+    `(let* ((,label ,exit)
+            (,reuse (labelp ,label))
+            (,var (if ,reuse ,label (next-label nil)))
+            (*unwind-exit* (adjoin ,var *unwind-exit*)))
+       ,@body
+       (unless ,reuse
+         (wt-label ,var)))))

src/cmp/cmpbackend-cxx/cmppass2-cont.lsp

@@ -112,14 +112,11 @@
              (c2expr this-form))
             (t
              (let* ((next-form (second l))
-                    (*exit* (if (tag-p next-form)
-                                (tag-jump next-form)
-                                (next-label nil)))
-                    (*unwind-exit* (cons *exit* *unwind-exit*))
-                    (*destination* 'TRASH))
-               (c2expr this-form)
-               (unless (tag-p next-form)
-                 (wt-label *exit*))))))))
+                    (maybe-tag (when (tag-p next-form)
+                                 (tag-jump next-form))))
+               (with-exit-label (*exit* maybe-tag)
+                 (let ((*destination* 'TRASH))
+                   (c2expr this-form)))))))))
 
 (defun c2go (c1form tag nonlocal)
   (declare (ignore c1form))
@@ -133,8 +130,10 @@
   (case (c1form-name tag)
     ((VARIABLE LOCATION) (setq loc (c1form-arg 0 tag)))
     (t (setq loc (make-temp-var))
-     (let ((*destination* loc)) (c2expr* tag))))
-  (let ((*destination* 'VALUEZ)) (c2expr* val))
+     (let ((*destination* loc))
+       (c2expr* tag))))
+  (let ((*destination* 'VALUEZ))
+    (c2expr* val))
   (wt-nl "cl_throw(" loc ");"))
 
 (defun c2catch (c1form tag body)

src/cmp/cmpbackend-cxx/cmppass2-eval.lsp

@@ -51,7 +51,7 @@
               (eq (c1form-name form2) 'LOCATION))
          ;; Optimize (IF condition true-branch) or a situation in which
          ;; the false branch can be discarded.
-         (with-optional-exit-label (false-label)
+         (with-exit-label (false-label *exit*)
            (let ((*destination* `(JUMP-FALSE ,false-label)))
              (c2expr* fmla))
            (c2expr form1)))
@@ -59,7 +59,7 @@
               (eq (c1form-name form1) 'LOCATION))
          ;; Optimize (IF condition useless-value false-branch) when
          ;; the true branch can be discarded.
-         (with-optional-exit-label (true-label)
+         (with-exit-label (true-label *exit*)
            (let ((*destination* `(JUMP-TRUE ,true-label)))
              (c2expr* fmla))
            (c2expr form2)))