Re: [PATCH] unshare expressions in attribute arguments

[Marek Polacek <polacek@redhat.com>]

On Fri, Nov 20, 2020 at 12:00:58PM -0700, Martin Sebor via Gcc-patches wrote:
> To detect a subset of VLA misuses, the C front associates the bounds
> of VLAs in function argument lists with the corresponding variables
> by implicitly adding an instance of attribute access to each function
> declared to take VLAs with the bound expressions chained on the list
> of attribute arguments.
> 
> Some of these expressions end up modified by the middle end, which
> results in references to nonlocal variables (and perhaps other nodes)
> used in these expression getting garbage collected.  A simple example
> of this is described in pr97172.
> 
> By unsharing the bound expressions the patch below prevents this from
> happening (it's not a fix for pr97172).
> 
> My understanding of the details of node sharing and garbage collection
> in GCC is very limited (I didn't expect a tree to be garbage-collected
> if it's still referenced by something).  Is this the right approach
> to solving this problem?

ISTM that a more natural thing would be to use build_distinct_type_copy
to copy the type you're about to modify.

> diff --git a/gcc/c/c-decl.c b/gcc/c/c-decl.c
> index d348e39c27a..4aea4dcafb9 100644
> --- a/gcc/c/c-decl.c
> +++ b/gcc/c/c-decl.c
> @@ -58,7 +58,7 @@ along with GCC; see the file COPYING3.  If not see
>  #include "c-family/name-hint.h"
>  #include "c-family/known-headers.h"
>  #include "c-family/c-spellcheck.h"
> -
> +#include "gimplify.h"
>  #include "tree-pretty-print.h"
> 
>  /* In grokdeclarator, distinguish syntactic contexts of declarators.  */
> @@ -5780,6 +5780,7 @@ get_parm_array_spec (const struct c_parm *parm, tree
> attrs)
>                   /* Each variable VLA bound is represented by the dollar
>                      sign.  */
>                   spec += "$";
> +                 nelts = unshare_expr (nelts);
>                   tpbnds = tree_cons (NULL_TREE, nelts, tpbnds);
>                 }
>             }
> @@ -5834,6 +5835,7 @@ get_parm_array_spec (const struct c_parm *parm, tree
> attrs)
> 
>        /* Each variable VLA bound is represented by a dollar sign.  */
>        spec += "$";
> +      nelts = unshare_expr (nelts);
>        vbchain = tree_cons (NULL_TREE, nelts, vbchain);
>      }
> 

Marek