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

Fri Apr 23 07:50:47 PDT 2021

For runtime leak checker rather than compile time, my solution doesnt give
false positives thus fewer supressions needed for complete well written
albeit possibly leaky app. We might need them for testsuite incomplete code
if its scanned with it but that would feel silly.

Hint: i saw it somewhere and liked it. We might think of version with bit
ugly syntax or pretty one but needing some compiler support for adding
lineno and srcfilename parameters to alloc fun call.

If its compile time checker, apologies and sorry for intrusion.

Best regards,
Pawel Kunio

pt., 23.04.2021, 16:19 użytkownik James Y Knight via llvm-dev <
llvm-dev at lists.llvm.org> napisał:

>
>
> 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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