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

Nuno Lopes via llvm-dev llvm-dev at lists.llvm.org
Wed Apr 14 11:03:03 PDT 2021

Most (all?) leak checkers support suppression files. Isn’t that sufficient for your use case?

Marking your leak roots with __attribute((used))__ is also an alternative.


I understand that leaking memory on purpose happens because it’s expensive to clean it up. But reachable memory may well be a true leak. So flagging it as such is useful. None of us has data about the % of reachable memory that is a true leak, so it’s not possible to argue what’s user friendly/hostile.


Programs that leak memory on purpose are often sophisticated. And sophisticated devs can handle a little bit of extra effort to hide those smarts I think.




P.S.: The original patch went in almost a decade ago when the ecosystem was a bit less developed. It was always meant to be temporary.



From: Sterling Augustine
Sent: 14 April 2021 17:39
To: llvm-dev <llvm-dev at lists.llvm.org>
Subject: [llvm-dev] Eliminating global memory roots (or not) to help leak checkers


[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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