remove unnecessary Box when an argument is used before assigned (#36245)

Author: JeffBezanson

src/ast.scm

@@ -288,7 +288,7 @@
        (symbol? (cadr e))))
 
 (define (lam:args x) (cadr x))
-(define (lam:vars x) (llist-vars (lam:args x)))
+(define (lam:argnames x) (llist-vars (lam:args x)))
 (define (lam:vinfo x) (caddr x))
 (define (lam:body x) (cadddr x))
 (define (lam:sp x) (cadddr (lam:vinfo x)))

src/julia-syntax.scm

@@ -2556,7 +2556,7 @@
           (and (memq var (scope:locals scope))  'local)
           (and (memq var (scope:globals scope))
                (if (and exclude-top-level-globals
-                        (null? (lam:vars (scope:lam scope)))
+                        (null? (lam:args (scope:lam scope)))
                         ;; don't inherit global decls from the outermost scope block
                         ;; in a top-level expression.
                         (or (not (scope:prev scope))
@@ -2636,13 +2636,13 @@
              '(false)
              '(true)))
         ((eq? (car e) 'lambda)
-         (let* ((args (lam:vars e))
+         (let* ((args (lam:argnames e))
                 (body (resolve-scopes- (lam:body e) (make-scope e args '() '() sp '() scope))))
            `(lambda ,(cadr e) ,(caddr e) ,body)))
         ((eq? (car e) 'scope-block)
          (let* ((blok            (cadr e)) ;; body of scope-block expression
                 (lam             (scope:lam scope))
-                (argnames        (lam:vars lam))
+                (argnames        (lam:argnames lam))
                 (toplevel?       (and (null? argnames) (eq? e (lam:body lam))))
                 (current-locals  (caddr lam)) ;; locals created so far in our lambda
                 (globals         (find-global-decls blok))
@@ -2755,7 +2755,7 @@
            ,(resolve-scopes- (cadddr e) scope (method-expr-static-parameters e))))
         (else
          (if (and (eq? (car e) '=) (symbol? (cadr e))
-                  scope (null? (lam:vars (scope:lam scope)))
+                  scope (null? (lam:args (scope:lam scope)))
                   (warn-var?! (cadr e) scope)
                   (= *scopewarn-opt* 1))
              (let* ((v    (cadr e))
@@ -2782,7 +2782,7 @@
 
 ;; names of arguments and local vars
 (define (lambda-all-vars e)
-  (append (lam:vars e) (caddr e)))
+  (append (lam:argnames e) (caddr e)))
 
 ;; compute set of variables referenced in a lambda but not bound by it
 (define (free-vars- e tab)
@@ -3201,6 +3201,7 @@ f(x) = yt(x)
   ;; This does a basic-block-local dominance analysis to find variables that
   ;; are never used undef.
   (let ((vi     (car (lam:vinfo lam)))
+        (args   (lam:argnames lam))
         (unused (table))  ;; variables not (yet) used (read from) in the current block
         (live   (table))  ;; variables that have been set in the current block
         (seen   (table))) ;; all variables we've seen assignments to
@@ -3221,6 +3222,11 @@ f(x) = yt(x)
       (restore (table)))
     (define (mark-used var)
       ;; remove variable from the unused table
+      ;; Note arguments are only "used" for purposes of this analysis when
+      ;; they are captured, since they are never undefined.
+      (if (and (has? unused var) (not (memq var args)))
+          (del! unused var)))
+    (define (mark-captured var)
       (if (has? unused var)
           (del! unused var)))
     (define (assign! var)
@@ -3272,7 +3278,7 @@ f(x) = yt(x)
                                          (get-methods e (lam:body lam))
                                          (list e))))
                    (for-each (lambda (ex)
-                               (for-each mark-used
+                               (for-each mark-captured
                                          (map car (cadr (lam:vinfo (cadddr ex))))))
                              all-methods)
                    (assign! (cadr e))))

test/core.jl

@@ -5178,6 +5178,12 @@ end
 @test let_Box5()() == 46
 @test let_noBox()() == 21
 
+function _assigns_and_captures_arg(a)
+    a = a
+    return ()->a
+end
+@test !any(contains_Box, code_lowered(_assigns_and_captures_arg,(Any,))[1].code)
+
 module TestModuleAssignment
 using Test
 @eval $(GlobalRef(TestModuleAssignment, :x)) = 1