[llvm-dev] Eliminating global memory roots (or not) to help leak checkers

[James Y Knight via llvm-dev]

On Thu, Apr 22, 2021, 7:28 PM Chris Lattner <clattner at nondot.org> wrote:

>
>
> > On Apr 19, 2021, at 5:24 AM, James Y Knight <jyknight at google.com> wrote:
> >
> > There is no problem (no leaks) in the code that users wrote, so adding
> code annotations (sanitizer suppression file, or attributes) is not a good
> solution to this issue. The problem is that this optimization introduces a
> "leak" (from the point of view of the leak checker), which wasn't there
> before. And in practice, this seems to cause a large number of false
> positives.
>
> I think that “from the point of view of the leak checker” is the key thing
> there.
>
> Code that this triggers *is* leaking memory


No, you've got it exactly backwards. "From the point of view of the leak
checker", there is a leak, but in actually, there is not.

I'm afraid you're still arguing from mistaken assumptions. As I've already
mentioned, reachable memory at program exit is not a leak. That's the
definition of "leak" which is always used by leak checkers. (This is not
anything new, it's been how leak checkers work for decades, and how they
must work.)

Therefore, C++ code that allocates memory and assigns it to a global is not
a leak, and it's _still_ not a leak even if it so happens in some
instantiation of the program that all of the users of the global have been
removed by the optimizer.

The code is correct and it's not leaking memory, but with this change, the
leak checker is unable to determine that.

It was just silenced because the leak was spuriously reachable from a
> global.  Global variables aren’t a preferred way to silence leak detectors,
> they have other ways to do so :)


Memory reachable from a global is not a spurious reachability, it is actual
reachability. And, the purpose of assigning a value to a global variable in
the source code isn't to silence the leak checker, it is to make the object
accessible to other code. (People writing code normally aren't and
shouldn't be thinking about leak checking.)


> On Apr 19, 2021, at 4:52 PM, Sterling Augustine <saugustine at google.com>
> wrote:
> >
> > There may be other ways to disable leak checkers, but they put a burden
> on users that was not there before. Further, users who try leak checking
> with and without optimization will get different answers. The bug report
> will read: "clang at -O2 makes my program leak". And, as James notes,
> whether or not you need to suppress the leak depends on whether or not the
> optimizer does away with the variable. Subtle changes to the code that have
> nothing to do with memory allocation will appear to add or fix leaks. That
> is not something I would care to explain or document.
>
> I can see that concern, but this isn’t a battle that we can win:
> optimizations in general can expose leaks.
>

The word "expose" is invalid here -- that implies that the code is buggy
but that the leak checker was previously unable to detect the bug, and now
does. But that is not the case at hand. You maybe could say, instead "I can
see that concern, but this isn’t a battle that we can win: optimizations in
general can cause random false positives in the leak checker." (But, in
practice it was pretty much "won" for the last 9 years.)

IMO, If someone doesn’t want the global to be removed, they should mark it
> volatile.  If they do want it removable, then they can use leak detector
> features to silence the warning.


> On Apr 20, 2021, at 9:12 AM, Sterling Augustine <saugustine at google.com>
> wrote:
> >
> > In order to understand how much benefit this change gives to code size,
> I built clang and related files with and without the patch, both with
> CMAKE_BUILD_TYPE=Release.
> >
> > clang itself gets about 0.4% smaller (from 145217124 to 144631078)
> > lld gets about 1.85% smaller (from 101129181 to 99243810 bytes)
> >
> > Spot checking a few other random targets, in general, it seems that the
> benefits depend heavily on the coding style, but roughly, bigger the
> binary, the less benefit this brings.
> >
> > I suspect that a more aggressive pass as described by Philip Reames
> could capture a significant part of the benefit without sacrificing
> functionality. But that would require a more detailed study to be sure.
>
> A ~2% reduction in code size is a huge win.  I agree with your comment
> about it being different with different coding styles.  I suspect that this
> is the sort of thing that will pay particularly for high abstraction code
> bases.
>
> I don’t see why we would punish general code to make “code that is leaking
> where formerly not detected, and where users don’t want to mark the root as
> volatile”.  This seems really unprincipled to me, and a slippery slope we
> can’t go down.
>

In the way you have restated the issue here, there is no benefit to the
current behavior, but that's only because of the mistaken assumptions. You
have redefined "leak", and are assuming that the problem is buggy software,
whose users are upset that valid bugs are found which were not previously
found. But that's simply not the case we're dealing with. The code is
correct (non-leaking), and it's a regression in leak checker functionality
if we start forcing users to add manual annotations as a workaround.

I don't know what the right thing to do here is. But I'm quite sure we
cannot arrive at a good decision until everyone can at least get on the
same page about what the purpose of a leak checker is. I would hope that
there's a path that makes everyone satisfied, but if not, the disagreement
needs to be based on relative priority of use cases and engineering
trade-offs, not whether the problem EXISTS.
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