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