OK
On Sat, Aug 4, 2018 at 2:18 AM, Jakub Jelinek <ja...@redhat.com> wrote:
> Hi!
>
> Some time ago, I've moved the poor man's offsetof recognizing hack to
> cp_fold. On the following testcase that means we don't fold it early during
> parsing. Now, if we try to evaluate those inside of constexpr contexts
> with !ctx->quiet, it is diagnosed as invalid (but *non_constant_p is not
> set). Worse, with ctx->quiet, we pretend it is a constant expression but
> don't actually fold it, and then we have e.g. in array index evaluation
> VERIFY_CONSTANT (index);
> ...
> VERIFY_CONSTANT (nelts);
> if ((lval
> ? !tree_int_cst_le (index, nelts)
> : !tree_int_cst_lt (index, nelts))
> || tree_int_cst_sgn (index) < 0)
> {
> diag_array_subscript (ctx, ary, index);
> *non_constant_p = true;
> return t;
> }
> where VERIFY_CONSTANT is happy about it, even when index is not an
> INTEGER_CST, but large complex TREE_CONSTANT expression. The above though
> assumes INTEGER_CST. Perhaps we should check for INTEGER_CST somewhere (and
> in other similar code too), but it isn't clear to me what exactly we should
> do if those trees aren't INTEGER_CSTs, especially with !ctx->quiet.
>
> This patch changes a different thing, the usual case (including other spots
> for NULL pointer dereferences or arith) in constexpr.c is
> if (some condition)
> {
> if (!ctx->quiet)
> error* (...);
> *non_constant_p = true;
> return t;
> }
> but the following two spots were different and that caused the array
> handling to see those complex unsimplified constant expressions.
> With this, it is not treated as constant for maybe_constant_value etc.
> purposes, though following cp_fold can still fold it into a constant.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux (including
> check-c++-all testing on both), ok for trunk and 8.3 after a while?
>
> 2018-08-03 Jakub Jelinek <ja...@redhat.com>
>
> PR c++/86738
> * constexpr.c (cxx_eval_binary_expression): For arithmetics involving
> NULL pointer set *non_constant_p to true.
> (cxx_eval_component_reference): For dereferencing of a NULL pointer,
> set *non_constant_p to true and return t.
>
> * g++.dg/opt/pr86738.C: New test.
>
> --- gcc/cp/constexpr.c.jj 2018-07-31 23:57:24.193432388 +0200
> +++ gcc/cp/constexpr.c 2018-08-03 14:54:13.302817282 +0200
> @@ -2082,6 +2082,7 @@ cxx_eval_binary_expression (const conste
> {
> if (!ctx->quiet)
> error ("arithmetic involving a null pointer in %qE", lhs);
> + *non_constant_p = true;
> return t;
> }
> else if (code == POINTER_PLUS_EXPR)
> @@ -2522,9 +2523,13 @@ cxx_eval_component_reference (const cons
> lval,
> non_constant_p, overflow_p);
> if (INDIRECT_REF_P (whole)
> - && integer_zerop (TREE_OPERAND (whole, 0))
> - && !ctx->quiet)
> - error ("dereferencing a null pointer in %qE", orig_whole);
> + && integer_zerop (TREE_OPERAND (whole, 0)))
> + {
> + if (!ctx->quiet)
> + error ("dereferencing a null pointer in %qE", orig_whole);
> + *non_constant_p = true;
> + return t;
> + }
>
> if (TREE_CODE (whole) == PTRMEM_CST)
> whole = cplus_expand_constant (whole);
> --- gcc/testsuite/g++.dg/opt/pr86738.C.jj 2018-08-03 15:03:51.477358712
> +0200
> +++ gcc/testsuite/g++.dg/opt/pr86738.C 2018-08-03 15:02:51.940201694 +0200
> @@ -0,0 +1,12 @@
> +// PR c++/86738
> +// { dg-do compile }
> +
> +struct S { int s; };
> +unsigned char a[20];
> +unsigned char *p = &a[(__UINTPTR_TYPE__) &((S *) 0)->s];
> +
> +void
> +foo ()
> +{
> + __builtin_memcpy (&a[15], &a[(__UINTPTR_TYPE__) &((S *) 0)->s], 2);
> +}
>
> Jakub
