code review 108870044: cmd/gc: fix &result escaping into result

src/cmd/gc/esc.c

 // Copyright 2011 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
 // Escape analysis.
 
 #include <u.h>
 #include <libc.h>
 #include "go.h"
 
@@ skipping 655 matching lines @@
 
         case OADDR:
                 n->esc = EscNone;  // until proven otherwise
                 e->noesc = list(e->noesc, n);
                 // current loop depth is an upper bound on actual loop depth
                 // of addressed value.
                 n->escloopdepth = e->loopdepth;
                 // for &x, use loop depth of x if known.
                 // it should always be known, but if not, be conservative
                 // and keep the current loop depth.
-                if(n->left->op == ONAME && (n->left->escloopdepth != 0 || n->left->class == PPARAMOUT)) {
+                if(n->left->op == ONAME) {
                         switch(n->left->class) {
                         case PAUTO:
+                                if(n->left->escloopdepth != 0)
+                                        n->escloopdepth = n->left->escloopdepth;
+                                break;
                         case PPARAM:
                         case PPARAMOUT:
-                                n->escloopdepth = n->left->escloopdepth;
+                                // PPARAM is loop depth 1 always.
+                                // PPARAMOUT is loop depth 0 for writes
+                                // but considered loop depth 1 for address-of,
+                                // so that writing the address of one result
+                                // to another (or the same) result makes the
+                                // first result move to the heap.
+                                n->escloopdepth = 1;
                                 break;
                         }
                 }
                 break;
         }
 
         lineno = lno;
 }
 
 // Assert that expr somehow gets assigned to dst, if non nil.  for
@@ skipping 570 matching lines @@
                                 ll->n->paramfld->note = mktag(ll->n->esc);
                         break;
                 case EscHeap:   // touched by escflood, moved to heap
                 case EscScope:  // touched by escflood, value leaves scope
                         break;
                 }
         }
 
         curfn = savefn;
 }