[gcc r14-4021] fortran: Remove reference count update [PR108957]

[gcc r14-4021] fortran: Remove reference count update [PR108957]

[Mikael Morin]

https://gcc.gnu.org/g:e6dba708c8627080f4ca776590a191683b38336c

commit r14-4021-ge6dba708c8627080f4ca776590a191683b38336c
Author: Mikael Morin <mikael@gcc.gnu.org>
Date:   Fri Sep 15 13:40:08 2023 +0200

    fortran: Remove reference count update [PR108957]
    
    Remove one reference count incrementation following the assignment of a
    symbol pointer to a local variable.  Most symbol pointers are "weak" pointer
    and don't need any reference count update when they are assigned, and it is
    especially the case of local variables.
    
    This fixes a memory leak with the testcase from the PR (not included).
    
            PR fortran/108957
    
    gcc/fortran/ChangeLog:
    
            * gfortran.h (gfc_symbol): Add comment documenting reference counting.
            * parse.cc (parse_interface): Remove reference count incrementation.

Diff:
---
 gcc/fortran/gfortran.h | 20 ++++++++++++++++++++
 gcc/fortran/parse.cc   |  3 ---
 2 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/gcc/fortran/gfortran.h b/gcc/fortran/gfortran.h
index f4a1c106cea..6caf7765ac6 100644
--- a/gcc/fortran/gfortran.h
+++ b/gcc/fortran/gfortran.h
@@ -1944,7 +1944,27 @@ typedef struct gfc_symbol
      according to the Fortran standard.  */
   unsigned pass_as_value:1;
 
+  /* Reference counter, used for memory management.
+
+     Some symbols may be present in more than one namespace, for example
+     function and subroutine symbols are present both in the outer namespace and
+     the procedure body namespace.  Freeing symbols with the namespaces they are
+     in would result in double free for those symbols.  This field counts
+     references and is used to delay the memory release until the last reference
+     to the symbol is removed.
+
+     Not every symbol pointer is accounted for reference counting.  Fields
+     gfc_symtree::n::sym are, and gfc_finalizer::proc_sym as well.  But most of
+     them (dummy arguments, generic list elements, etc) are "weak" pointers;
+     the reference count isn't updated when they are assigned, and they are
+     ignored when the surrounding structure memory is released.  This is not a
+     problem because there is always a namespace as surrounding context and
+     symbols have a name they can be referred with in that context, so the
+     namespace keeps the symbol from being freed, keeping the pointer valid.
+     When the namespace ceases to exist, and the symbols with it, the other
+     structures referencing symbols cease to exist as well.  */
   int refs;
+
   struct gfc_namespace *ns;	/* namespace containing this symbol */
 
   tree backend_decl;
diff --git a/gcc/fortran/parse.cc b/gcc/fortran/parse.cc
index 8f09ddf753c..58386805ffe 100644
--- a/gcc/fortran/parse.cc
+++ b/gcc/fortran/parse.cc
@@ -4064,9 +4064,6 @@ loop:
   accept_statement (st);
   prog_unit = gfc_new_block;
   prog_unit->formal_ns = gfc_current_ns;
-  if (prog_unit == prog_unit->formal_ns->proc_name
-      && prog_unit->ns != prog_unit->formal_ns)
-    prog_unit->refs++;
 
 decl:
   /* Read data declaration statements.  */