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
