[cfe-dev] [analyzer] Thoughts on Temporary Destructors

Daniel Connelly dconnelly at google.com
Mon Dec 16 06:57:19 PST 2013

Thanks for the overview. This is all pretty new to me, so I'm going to dig
into the problem for a couple of days and hopefully come up with some
intelligent questions.

On Fri, Dec 13, 2013 at 7:13 PM, Jordan Rose <jordan_rose at apple.com> wrote:

> Background: recently Anna had to revert r189090 "Refactor conditional
> expression evaluating code", which was added in order to handle temporary
> destructors that fire conditionally. The motivation for that commit was
> expressions like "(A() && B()) || (C() && D())", where the destructors for
> B, C, and D only fire under certain conditions. In order to make sure these
> destructors only fire at the right times, Pavel structured the CFG so that
> the various terminator expressions ("A() && ...", "(A() && B()) || ...",
> etc.) would be used twice: first during normal expression evaluation, and
> then again during cleanup. This falls out of a long discussion on PR11645<http://llvm.org/bugs/show_bug.cgi?id=11645>
> .
> The problem was that before his commits, logical operators && and || were
> computed in the analyzer by walking back up the CFG to see how we got to
> this point. When we're deciding whether to run destructors, the CFG blocks
> where the && or || values were computed is long gone. Pavel's approach
> (which both Ted and I reviewed) was to (a) explicitly record whether an
> expression evaluated to true or false when used as a branch, and (b) keep
> the leaf expressions of && and || expression trees alive, so that the
> branch information could be recovered later on.
> Unfortunately, two problems fell out of this: first, that in a loop it was
> possible to accidentally reuse a still-live expr value from the *previous*iteration, and second, that the GNU binary ?: operator uses the left-hand
> side as both a condition and a result. (Coincidentally, both of these bugs
> were reported to us at the same time based on one internal team getting a
> new Clang build.)
> In discussing the first problem, Ted and Anna realized that the liveness
> information for nested && and || expressions was still incorrect: the
> outermost expression may potentially depend on all of the inner
> expressions, but that means that those sub-expressions are *always* considered
> live (since expressions are marked dead, in a reverse analysis, at the
> point of evaluation, and there are paths that don't evaluate a particular
> sub-expression in a complex logical expression). This wasn't an issue
> before because && and || didn't actually use those expressions to compute
> their value, but it's still incorrect.

Is this http://llvm.org/bugs/show_bug.cgi?id=18159 ?

> Ted remarked that if we were more precisely modeling execution, we'd just
> have an extra bit on the side that got set to 1 if we needed to run a
> particular temporary's destructor. We could do something similar in the CFG
> by inserting fake boolean variables and using *those* to decide whether
> or not to run destructors later; we're not bound by the limits of the AST
> (on where variables can be inserted), and we already synthesize fake
> DeclStmts elsewhere (to break up a DeclStmt with multiple decls in it). I
> will admit we've had a handful of problems related to these fake decls in
> the past, but that doesn't automatically invalidate the plan. We just
> haven't explored it much, and I thought it was worth mentioning.

Cool. I'll look into this approach once I better understand what's going on.

> Anyway, clearly we'd like to properly evaluate temporary destructors, but
> just as clearly we don't want to accept regressions in other parts of the
> analyzer, especially in pure C code or C++ code with cfg-temporary-dtors
> still turned off. So that's where we are today.

> Still interested in trying to solve these problems? ;-)

Definitely. I need something interesting to work on :)


Daniel Connelly | Software Engineer – Chrome | dconnelly at google.com |

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