[LLVMdev] [RFC] Adding functions for unaligned load/store to Support for JIT/RuntimeDyld

[Lang Hames]

Hi Alexey,

That sounds good to me. RuntimeDyldMachO already has writeBytesUnaligned
for this reason, though it's use is somewhat patchy. We should also make
better use of the facilities provided in llvm/Support/Endian.h - it has
alignment and endianness aware definitions for common primitive types.

Cheers,
Lang.


On Tue, Aug 19, 2014 at 4:40 PM, Alexey Samsonov <vonosmas at gmail.com> wrote:

> Hi,
>
> I've noticed that certain parts of LLVM (RuntimeDyld, JIT) use unaligned
> memory accesses to load/store pointers or just 32- or 64-bit integers.
> Technically, this is undefined behavior, and UBSan reports errors when this
> happens. Even if we believe we're running some x86-specific code (e.g.
> RuntimeDyldELF::resolveX86_64Relocation) what matters for unaligned acceses
> in source code is the host architecture, not the target one.
>
> What do you think of adding
>
>   T unaligned_load(const void *addr)
>   void unaligned_store(void *addr, T value);
>
> to support headers? We can then provide the default memcpy implementations
> of these methods and, optionally, straightforward implementations for x86
> hosts (and disable UBSan alignment checks for the latter).
>
> --
> Alexey Samsonov
> vonosmas at gmail.com
>
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