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
