From: Jason Merrill <jason@redhat.com>
To: Marek Polacek <polacek@redhat.com>,
GCC Patches <gcc-patches@gcc.gnu.org>
Subject: Re: C++ PATCH for c++/91264 - detect modifying const objects in constexpr
Date: Mon, 05 Aug 2019 20:37:00 -0000 [thread overview]
Message-ID: <902366c6-754a-de65-f78e-25834263ac8a@redhat.com> (raw)
In-Reply-To: <20190731192659.GP32749@redhat.com>
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 wonder about setting a flag on the CONSTRUCTOR after
initialization is complete to indicate that the value is now constant.
Jason
next prev parent reply other threads:[~2019-08-05 19:54 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-31 19:39 Marek Polacek
2019-08-05 20:37 ` Jason Merrill [this message]
2019-08-06 19:35 ` Marek Polacek
2019-08-08 15:18 ` Jason Merrill
2019-08-08 19:48 ` Marek Polacek
2019-08-14 19:51 ` Jason Merrill
2019-08-15 22:02 ` Marek Polacek
2019-08-16 0:28 ` Jason Merrill
2019-08-16 12:33 ` Marek Polacek
2019-08-17 0:51 ` Jason Merrill
2019-08-18 16:52 ` Marek Polacek
2019-08-19 1:19 ` Jason Merrill
2019-08-19 1:21 ` Marek Polacek
2019-08-19 2:31 ` Marek Polacek
2019-08-19 8:39 ` Jason Merrill
2019-08-06 20:01 ` Paolo Carlini
2019-08-06 20:04 ` Marek Polacek
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