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

[Marek Polacek]

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.  (Also, all 6 TREE_LANG_FLAGs for a CONSTRUCTOR
are used.)

Am I missing something?

Marek