llvm dev - Apr 2021 - Eliminating global memory roots (or not) to help leak checkers

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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Hello,
If its about runtime leak checking rather than compile time we might be
prying the already open door here. Aka i might have something important to
say.

As of code sample at what url can i see the code that is or isnt a leak or
can somebody tell where its allocated where its freed and as i understand
its stored in global var ptr,  right?

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.
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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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.
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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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.
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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