[llvm-dev] Sanitizers + New Pass Manager

[Johannes Doerfert via llvm-dev]

On 5/13/20 3:31 PM, David Blaikie via llvm-dev wrote:
> I believe it's meant to run after /some/ optimizations to make it a bit
> more efficient, while not so optimized that it misses opportunities to
> detect bugs - but I could be wrong there. I'll leave it up to other folks
> to chime in.

I think that is right. The more transformations you run the more UB you 
can also "loose" as it is defined to something by the transformation.

Lifetime markers are an example. Once removed, which is generally legal 
in IR, you cannot argue accesses after the end are UB.


> On Wed, May 13, 2020 at 1:04 PM Arthur Eubanks <aeubanks at google.com> wrote:
>
>> Just tested it out, that test does indeed fail under the old PM at -O3 and
>> even at -O2.
>>
>> If the ASan pass runs after optimizations and is designed to detect
>> undefined behavior at runtime, I don't see how it can be super reliable at
>> higher optimization levels.
>>
>> On Wed, May 13, 2020 at 12:39 PM David Blaikie <dblaikie at gmail.com> wrote:
>>
>>> +some sanitizer/new pass manager folks
>>>
>>>
>>>
>>> On Wed, May 13, 2020 at 12:22 PM Arthur Eubanks via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> Hi,
>>>>
>>>> I've been trying to burn down the remaining sanitizer failures under the
>>>> new pass manager. Right now I'm stuck on some ASan tests.
>>>>
>>>> Some ASan tests run under -O1. There are a couple differences between
>>>> the old and new pass managers under -O1, e.g. the old PM doesn't inline
>>>> whereas the new PM does. The differences seem to cause some lifetime
>>>> intrinsics to get stripped out (e.g. via SROA, InstCombine). It might be
>>>> due to ASan specifically testing undefined behavior, and different
>>>> optimizations run means different behavior. For a specific example,
>>>> use-after-scope-dtor-order.cpp runs under -O1 and fails under the new PM
>>>> because SROA strips out the lifetime intrinsics and by the time the ASan
>>>> pass runs it doesn't find the lifetime intrinsics to add its own
>>>> instrumentation.
>>>>
>>> That, to me, sounds like a real bug in the optimizations/asan
>>> implementation if this choice fo optimizations makes the diagnosis go away.
>>> (is ASan able to diagnose the problem at -O3 (where I guess SROA and other
>>> things run) with the legacy pass manager?)
>>>
>>>
>>>> What's the proper way to resolve this? Run the tests under -O0? Change
>>>> the passes pipeline under the new PM when ASan (and maybe other sanitizers)
>>>> is detected?
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>>>>
>
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