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

[Philip Reames via llvm-dev]

On 4/23/21 7:18 AM, James Y Knight via llvm-dev wrote:
>
>
> On Thu, Apr 22, 2021, 7:28 PM Chris Lattner <clattner at nondot.org 
> <mailto:clattner at nondot.org>> wrote:
>
>
>
>     > On Apr 19, 2021, at 5:24 AM, James Y Knight <jyknight at google.com
>     <mailto: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.

I want to object here.  :)

A program with dynamic allocation which has not been reclaimed by 
program termination does have a leak.  It simply happens to be a leak 
that we've chosen by convention to not treat as interesting. This is a 
reasonable convention because standard process tear mechanisms will 
deallocate it for most classes of memory.  The program could be 
converted to one which actually doesn't leak by using static destructors 
to free the pointed to object.

Just to be clear, this objection is purely on terminology.  I think it's 
important to distinguish between programs which leak (e.g. don't reclaim 
all memory), and programs which simply aren't interesting from a leak 
detection standpoint (because the memory is about to be reclaimed anyways.)

Separately from the terminology point above, I'll share my own weakly 
held opinion from reading along with this thread.

I have generally found the arguments against optimizing away globals to 
avoid leak reports unconvincing.  The results on the optimization 
benefit are clearly worthwhile.  If forced to chose at this moment, I'd 
trade the optimization impact for the leak detection usage complexity.  
To me, it is critical to note that there are multiple source level 
changes possible to address the (true) leaks reported, several of which 
have already been suggested in this thread.  It's also important to note 
that we have other optimizations already in tree which require the same 
type of source change.

I would suggest that if the advocates for leak suppression in the 
compiler continue to want to argue this point that the burden of work 
needs to shift.  In particular, I would really like to see some 
proactive efforts to either a) assess the optimization potential 
tradeoff of an SROA-ish approach, or b) proposals for making the desired 
preservation well defined in IR.  (i.e. a set of rules which describe 
which optimizations are legal - the current code does not do this!)



>
>     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 <mailto: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 <mailto: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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