[cfe-dev] [llvm-dev] RFC: Replacing the default CRT allocator on Windows

Mitch Phillips via cfe-dev cfe-dev at lists.llvm.org
Tue Jul 7 15:02:40 PDT 2020 

> If I use clang with -fsanitize=address to build my program, and then run
my program, what difference does it make for the execution of my program
whether the compiler itself was instrumented or not

Yes, it doesn't make a difference to your final executable whether the
compiler was built with ASan or not.

> Do you mean that ASAN runtime itself should be instrumented, since your
program loads that at runtime?

Sanitizer runtimes aren't instrumented with sanitizers :).

-------

To be clear, we're talking about replacing the runtime allocator for
clang/LLD/etc., right? We're not talking about replacing the default
allocator for -O0 executables?

In either instance, using the ASan allocator (for either clang or
executables) is possible, but won't provide any of the bug detection
capabilities you describe without also ensuring that clang/your executable
is built with ASan instrumentation (-fsanitize=address implies both
"replace my allocator" and "instrument my code").

On Tue, Jul 7, 2020 at 2:53 PM Zachary Turner <zturner at roblox.com> wrote:

> I hadn't heard this before.  If I use clang with -fsanitize=address to
> build my program, and then run my program, what difference does it make for
> the execution of my program whether the compiler itself was instrumented or
> not?  Do you mean that ASAN runtime itself should be instrumented, since
> your program loads that at runtime?
>
> On Tue, Jul 7, 2020 at 2:04 PM Mitch Phillips <mitchp at google.com> wrote:
>
>> Bearing in mind that the ASan allocator isn't particularly suited to
>> detecting memory corruption unless you compile LLVM/Clang with ASan
>> instrumentation as well. I don't imagine anybody would be proposing making
>> the debug build for Windows be ASan-ified by default.
>>
>> On Tue, Jul 7, 2020 at 1:49 PM Adrian McCarthy via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> Asan and the Debug CRT take different approaches, but the problems they
>>> cover largely overlap.
>>>
>>> Both help with detection of errors like buffer overrun, double free, use
>>> after free, etc.  Asan generally gives you more immediate feedback on
>>> those, but you pay a higher price in performance.  Debug CRT lets you do
>>> some trade off between the performance hit and how soon it detects problems.
>>>
>>> Asan documentation says leak detection is experimental on Windows, while
>>> the Debug CRT leak detection is mature and robust (and can be nearly
>>> automatic in debug builds).  By adding a couple calls, you can do finer
>>> grained leak detection than checking what remains when the program exits.
>>>
>>> Debug CRT lets you hook all of the malloc calls if you want, so you can
>>> extend it for your own types of tracking and bug detection.  But I don't
>>> think that feature is often used.
>>>
>>> Windows's Appverifier is cool and powerful.  I cannot remember for sure,
>>> but I think some of its features might depend on the Debug CRT.  One thing
>>> it can do is simulate allocation failures so you can test your program's
>>> recovery code, but most programs nowadays assume memory allocation never
>>> fails and will just crash if it ever does.
>>>
>>> On Tue, Jul 7, 2020 at 10:25 AM Zachary Turner via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> Note that ASAN support is present on Windows now.  Does the Debug CRT
>>>> provide any features that are not better served by ASAN?
>>>>
>>>> On Tue, Jul 7, 2020 at 9:44 AM Chris Tetreault via llvm-dev <
>>>> llvm-dev at lists.llvm.org> wrote:
>>>>
>>>>> For release builds, I think this is fine. However for debug builds,
>>>>> the Windows allocator provides a lot of built-in functionality for
>>>>> debugging memory issues that I would be very sad to lose. Therefore, I
>>>>> would request that:
>>>>>
>>>>>
>>>>>
>>>>>    1. This be added as a configuration option to either select the
>>>>>    new allocator or the windows allocator
>>>>>    2. The Windows allocator be used by default in debug builds
>>>>>
>>>>>
>>>>>
>>>>> Ideally, since you’re doing this work, you’d implement it in such a
>>>>> way that it’s fairly easy for anybody to use whatever allocator they want
>>>>> when building LLVM (on any platform, not just windows), and it’s not just
>>>>> hardcoded to system allocator vs whatever allocator ends up getting added.
>>>>> However, as long as I can use the windows debug allocator I’m happy.
>>>>>
>>>>>
>>>>>
>>>>> Thanks,
>>>>>
>>>>>    Christopher Tetreault
>>>>>
>>>>>
>>>>>
>>>>> *From:* cfe-dev <cfe-dev-bounces at lists.llvm.org> *On Behalf Of *Alexandre
>>>>> Ganea via cfe-dev
>>>>> *Sent:* Wednesday, July 1, 2020 9:20 PM
>>>>> *To:* cfe-dev at lists.llvm.org; LLVM Dev <llvm-dev at lists.llvm.org>
>>>>> *Subject:* [EXT] [cfe-dev] RFC: Replacing the default CRT allocator
>>>>> on Windows
>>>>>
>>>>>
>>>>>
>>>>> Hello,
>>>>>
>>>>>
>>>>>
>>>>> I was wondering how folks were feeling about replacing the default
>>>>> Windows CRT allocator in Clang, LLD and other LLVM tools possibly.
>>>>>
>>>>>
>>>>>
>>>>> The CRT heap allocator on Windows doesn’t scale well on large core
>>>>> count machines. Any multi-threaded workload in LLVM that allocates often is
>>>>> impacted by this. As a result, link times with ThinLTO are extremely slow
>>>>> on Windows. We’re observing performance inversely proportional to the
>>>>> number of cores. The more cores the machines has, the slower ThinLTO
>>>>> linking gets.
>>>>>
>>>>>
>>>>>
>>>>> We’ve replaced the CRT heap allocator by modern lock-free thread-cache
>>>>> allocators such as rpmalloc (unlicence), mimalloc (MIT licence) or snmalloc
>>>>> (MIT licence). The runtime performance is an order of magnitude faster.
>>>>>
>>>>>
>>>>>
>>>>> Time to link clang.exe with LLD and -flto on 36-core:
>>>>>
>>>>>   Windows CRT heap allocator: 38 min 47 sec
>>>>>
>>>>>   mimalloc: 2 min 22 sec
>>>>>
>>>>>   rpmalloc: 2 min 15 sec
>>>>>
>>>>>   snmalloc: 2 min 19 sec
>>>>>
>>>>>
>>>>>
>>>>> We’re running in production with a downstream fork of LLVM + rpmalloc
>>>>> for more than a year. However when cross-compiling some specific game
>>>>> platforms we’re using other downstream forks of LLVM that we can’t change.
>>>>>
>>>>>
>>>>>
>>>>> Two questions arise:
>>>>>
>>>>>    1. The licencing. Should we embed one of these allocators into the
>>>>>    LLVM tree, or keep them separate out-of-the-tree?
>>>>>    2. If the answer for above question is “yes”, given the tremendous
>>>>>    performance speedup, should we embed one of these allocators into Clang/LLD
>>>>>    builds by default? (on Windows only) Considering that Windows doesn’t have
>>>>>    a LD_PRELOAD mechanism.
>>>>>
>>>>>
>>>>>
>>>>> Please see demo patch here: https://reviews.llvm.org/D71786
>>>>>
>>>>>
>>>>>
>>>>> Thank you in advance for the feedback!
>>>>>
>>>>> Alex.
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> llvm-dev at lists.llvm.org
>>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>
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