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

[James Y Knight via llvm-dev]

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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