[llvm-bugs] [Bug 52177] New: Undefined behaviour passing sanitizers and not reproducible in other compilers

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https://bugs.llvm.org/show_bug.cgi?id=52177

            Bug ID: 52177
           Summary: Undefined behaviour passing sanitizers and not
                    reproducible in other compilers
           Product: clang
           Version: 12.0
          Hardware: Other
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: Golovanov12345 at gmail.com
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

Hello. I have the following code: https://pastebin.com/nxsF75JV. Basically it
computes sines and cosines for all multiples of pi/32, most of the time
multiplying the previous complex representation by the same constant, but every
8-th time calling expensive functions cos/sin to avoid precision-related
issues. It is clear that the code should report that the cosine of the
outputted angle is zero both times, which happens in ideone/godbolt/wherever,
except my compiler in termux for Android. I compile using the following line:
`g++ -std=c++17 qwe.cpp -o qwe`. When I add `-O0`, everything works properly.
When I add `-O2`, I receive the following output:

```
1
 angles: 1.57080 1.57080
cosines: -0.70711 0.00000
```

When I also pass `-fsanitize=undefined` or `-fsanitize=address`, nothing
changes. My g++ version is clang 12.0.1, but a friend of mine confirmed the
same effect on his clang 11.1.0, also in termux. Please investigate or tell me
that I am wrong.

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