RFR: 8286876: NMT.test_unaliged_block_address_vm_assert fails if using clang toolchain

[Thomas Stuefe]

On Thu, 1 Dec 2022 19:47:16 GMT, Johan Sjölen <jsjolen at openjdk.org> wrote:

> This fixes a bug by removing some UB which Clang used in order to make a test fail. Specifically, Clang optimized the `is_aligned` call in ` MallocHeader::assert_block_integrity` to `true`, because an unaligned `this` is  UB. This is fixed by making a static method, essentially. The new code liberally uses `reinterpret_cast<uintptr_t>()`, this is a specifically mentioned as legal use on https://en.cppreference.com/w/cpp/language/reinterpret_cast .
> 
> **The idea is essentially:** Make sure the pointer looks OK before casting it to a `MallocHeader*`.
> 
> I also changed the definition of `malloc_header`:
> 
> ```c++
> // old
>     return (MallocHeader*)((char*)memblock - sizeof(MallocHeader));
> // new
>     return &(((MallocHeader*)memblock)[-1]);
> 
> 
> The previous definition was UB, because you shouldn't cast from a pointer with smaller alignment to one with larger alignment requirements.
> 
> I also made `MallocHeader::print_block_on_error` static, as the `this` argument was always equal to the `bad_address`.

I opened https://bugs.openjdk.org/browse/JDK-8298003 to track the printout error separately and will file a PR for this shortly. I'd like to fix this quickly before JDK 20 code freeze.

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PR: https://git.openjdk.org/jdk/pull/11465