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

Mon Apr 19 16:52:51 PDT 2021

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.

Although the code could definitely be improved, the comments that it is
brittle or fragile seems overstated. Google uses this code without issue
against against many millions of lines of code every day, in
tens-of-thousands of targets, and has since 2012. It may be ugly, but it
does work. We are open to improving it, if it is only the ugliness that is
of concern.

I am working on quantifying the benefit the change gives, but I won't have
results until tomorrow.

On Mon, 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.
>
> This can apparently happen simply by having the code which reads from a
> global variable happen to be unused or optimized away in the current
> compilation. Users would be effectively randomly annotating variables to
> work around the compiler, not for any reason apparent in the code they
> wrote.
>
> I don't know what the right answer should be -- I've not looked into it at
> all. But I don't think it's correct to just dismiss this as not-a-problem,
> even if the current solution is not a good solution.
>
>
> On Mon, Apr 19, 2021, 12:16 AM Chris Lattner via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi Sterling,
>>
>> I agree with the others: therIe are better (and more robust ways) to
>> disable leak checkers.  This only addresses one narrow case.  The code is
>> also super verbose and fragile looking.  This is also eliminating an
>> optimization.
>>
>> I’d recommend dropping the code.
>>
>> -Chris
>>
>> On Apr 14, 2021, at 9:38 AM, Sterling Augustine via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>> [Continuing discussion from https://reviews.llvm.org/D69428]
>>
>> Llvm is fairly conservative when eliminating global variables (or fields
>> of such) that may point to dynamically allocated memory. This behavior is
>> entirely to help leak checking tools such as Valgrind, Google's
>> HeapLeakChecker, and LSAN, all of which treat memory that is reachable at
>> exit as "not leaked", even though it will never be freed. Without these
>> global variables to hold the pointer, the leak checkers can't determine
>> that it is actually reachable, and will report a leak. Global variables
>> that dynamically allocate memory but don't clean themselves up are fairly
>> common in the wild, and various leak checkers have long not reported errors.
>>
>> This behavior was added all the way back in 2012 in
>> https://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20120625/145646.html
>> .
>>
>> https://reviews.llvm.org/D69428 removed this behavior, and I
>> subsequently reverted it when many internal Google tests started failing,
>> but I believe many other users who use leak checking will encounter errors
>> when this hits more mainstream releases.
>>
>> So: What to do?
>>
>> Preventing a valid transformation (the global variables are never read
>> and can be eliminated) to help the leak checkers leaves some performance
>> and code size on the table. Just how much is unclear.
>>
>> On the other hand, having leak checkers suddenly start reporting failures
>> where they didn't before also seems suboptimal. Cleaning this somewhat
>> common scenario up is surprisingly difficult at the user level.
>>
>> Some possibilities:
>>
>> 1. Only do this at high optimization levels, say -O3. This would give
>> aggressive users all the performance we can, but also make leak checkers
>> report leaks sometimes, but not others.
>>
>> 2. Hide it behind a flag or configurable option. Users who care can set
>> it as they prefer. Creates more confusing options, different testing
>> matrices and such, but everyone can get the behaviour that they want.
>>
>> 3. Do it all the time, and users who encounter issues can clean up their
>> code. Users get the most performance they possibly can, but have to clean
>> up code or drop leak checking. Seems a little user hostile.
>>
>> Other possibilities?:
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>>
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>
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