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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/62171] restrict pointer to struct with restrict pointers parm doesn't prevent aliases
Date: Tue, 22 Sep 2015 08:01:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62171

--- Comment #7 from Richard Biener <rguenth at gcc dot gnu.org> ---
I was thinking about sth like

struct X { int i; int * __restrict__ q; };

int foo (X& __restrict__ x, X *p)
{
  *x.q = 1;
  p->i = 0;
  return *x.q;
}

int main()
{
  X x;
  x.q = &x.i;
  return foo (x, &x);
}

but I see we're still conservatively requiring decls in visit_loadstore:

      tree ptr = TREE_OPERAND (base, 0);
      if (TREE_CODE (ptr) == SSA_NAME)
        {
          /* ???  We need to make sure 'ptr' doesn't include any of
             the restrict tags in its points-to set.  */
          return false;

Of course this kind of testcase would also break if X were passed by
value (and changed by the frontend to DECL_BY_REFERENCE).  That is,
using the 2nd level restrict sounds bogus iff there can be a testcase
that can be created by the user (thus is not under strict control by
the compiler).


That said, generally the tree-ssa-structalias.c:visit_loadstore needs
some more sophistication as well.