Re: C++ PATCH for c++/91264 - detect modifying const objects in constexpr

[Marek Polacek <polacek@redhat.com>]

On Wed, Aug 14, 2019 at 02:50:13PM -0400, Jason Merrill wrote:
> On Thu, Aug 8, 2019 at 3:25 PM Marek Polacek <polacek@redhat.com> wrote:
> >
> > On Thu, Aug 08, 2019 at 11:06:17AM -0400, Jason Merrill wrote:
> > > On 8/6/19 3:20 PM, Marek Polacek wrote:
> > > > On Mon, Aug 05, 2019 at 03:54:19PM -0400, Jason Merrill wrote:
> > > > > On 7/31/19 3:26 PM, Marek Polacek wrote:
> > > > > > One of the features of constexpr is that it doesn't allow UB; and such UB must
> > > > > > be detected at compile-time.  So running your code in a context that requires
> > > > > > a constant expression should ensure that the code in question is free of UB.
> > > > > > In effect, constexpr can serve as a sanitizer.  E.g. this article describes in
> > > > > > in more detail:
> > > > > > <https://shafik.github.io/c++/undefined%20behavior/2019/05/11/explporing_undefined_behavior_using_constexpr.html>
> > > > > >
> > > > > > [dcl.type.cv]p4 says "Any attempt to modify a const object during its lifetime
> > > > > > results in undefined behavior." However, as the article above points out, we
> > > > > > aren't detecting that case in constexpr evaluation.
> > > > > >
> > > > > > This patch fixes that.  It's not that easy, though, because we have to keep in
> > > > > > mind [class.ctor]p5:
> > > > > > "A constructor can be invoked for a const, volatile or const volatile object.
> > > > > > const and volatile semantics are not applied on an object under construction.
> > > > > > They come into effect when the constructor for the most derived object ends."
> > > > > >
> > > > > > I handled this by keeping a hash set which tracks objects under construction.
> > > > > > I considered other options, such as going up call_stack, but that wouldn't
> > > > > > work with trivial constructor/op=.  It was also interesting to find out that
> > > > > > the definition of TREE_HAS_CONSTRUCTOR says "When appearing in a FIELD_DECL,
> > > > > > it means that this field has been duly initialized in its constructor" though
> > > > > > nowhere in the codebase do we set TREE_HAS_CONSTRUCTOR on a FIELD_DECL as far
> > > > > > as I can see.  Unfortunately, using this bit proved useless for my needs here.
> > > > >
> > > > > > Also, be mindful of mutable subobjects.
> > > > > >
> > > > > > Does this approach look like an appropriate strategy for tracking objects'
> > > > > > construction?
> > > > >
> > > > > For scalar objects, we should be able to rely on INIT_EXPR vs. MODIFY_EXPR
> > > > > to distinguish between initialization and modification; for class objects, I
> > > >
> > > > This is already true: only class object go into the hash set.
> > > >
> > > > > wonder about setting a flag on the CONSTRUCTOR after initialization is
> > > > > complete to indicate that the value is now constant.
> > > >
> > > > But here we're not dealing with CONSTRUCTORs in the gcc sense (i.e. exprs with
> > > > TREE_CODE == CONSTRUCTOR).  We have a CALL_EXPR like Y::Y ((struct Y *) &y),
> > > > which initializes the object "y".  Setting a flag on the CALL_EXPR or its underlying
> > > > function decl wouldn't help.
> > > >
> > > > Am I missing something?
> > >
> > > I was thinking that where in your current patch you call
> > > remove_object_under_construction, we could instead mark the object's value
> > > CONSTRUCTOR as immutable.
> >
> > Ah, what you meant was to look at DECL_INITIAL of the object we're
> > constructing, which could be a CONSTRUCTOR.  Unfortunately, this
> > DECL_INITIAL is null (in all the new tests when doing
> > remove_object_under_construction), so there's nothing to mark as TREE_READONLY :/.
> 
> There's a value in ctx->values, isn't there?

Doesn't seem to be the case for e.g.

struct A {
  int n;
  constexpr A() : n(1) { n = 2; }
};

struct B {
  const A a;
  constexpr B(bool b) {
    if (b)
      const_cast<A &>(a).n = 3; // { dg-error "modifying a const object" }
    }
};

constexpr B b(false);
static_assert(b.a.n == 2, "");

Here we're constructing "b", its ctx->values->get(new_obj) is initially
"{}".  In the middle of constructing "b", we construct "b.a", but that
has nothing in ctx->values.

Marek