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 :/. Marek
