[llvm-dev] Alignment of the StoreInst

[Tim Northover via llvm-dev]

Hi Hongbin,

On 16 December 2016 at 12:22, Hongbin Zheng <etherzhhb at gmail.com> wrote:
> Thanks for the explanation. The above code snippets is actually extracted by
> bugpoint from MultiSource/Applications/ALAC/encode/alacconvert-encode.test.
> I get a different result if I cast the pointer to int and mask away the
> lower two bits and cast the int back to pointer. While casting pointer to
> int and back to pointer do not cause any problem.

It's quite possible there's some undefined behaviour in ALAC that's
only recently become a problem. On almost all targets I'm aware of
32-bit integers should be aligned to 32-bits. There's a bit more
variation for 64-bit ints.

Programs that work on buffers of bytes often cast pointers into those
buffers inappropriately. E.g.

    char *Buf = ...;
    [...]
    return doSomethingWithInts((int32_t *)&Buf[Idx]);

For example I'd be very suspicious of ALACEncoder.cpp lines 340 and
352. I expect there's more, those are just the first two I found by
grepping for "(int32_t *)".

Everything seems to work fine until the compiler becomes clever enough
to realise it can use a slightly faster store since the pointer is
aligned (or one of any number of other optimizations), and then things
blow up.

Generally you need to memcpy part of the buffer into a properly
aligned variable and pass that, or use a carefully under-aligned type.
In Clang you can create a specifically under-aligned integer type, and
as long as all *accesses* go through that type it should work. So
something like this:

    typedef int32_t __attribute__((aligned(1))) unaligned_int32_t;
    [...]
    int32_t doSomethingWithInts(unaligned_int32_t *Ints) {
      return Ints[0] + Ints[1];
    }

But that's obviously a non-standard extension (GCC supports it, and I
believe MSVC has a similar but incompatible concept). It's also really
easy to mess that up, the attribute gets dropped in some really
surprising situations you (or at least I) wouldn't expect.

Cheers.

Tim.