Re: [PATCH v2 1/4] siphash: add cryptographically secure hashtable function

Hannes Frederic Sowa
2016-12-15T20:31:29Z

Hello,

On 15.12.2016 19:50, Jason A. Donenfeld wrote:
> Hi David & Hannes,
>
> This conversation is veering off course.

Why?

> I think this doesn't really
> matter at all. Gcc converts u64 into essentially a pair of u32 on
> 32-bit platforms, so the alignment requirements for 32-bit is at a
> maximum 32 bits. On 64-bit platforms the alignment requirements are
> related at a maximum to the biggest register size, so 64-bit
> alignment. For this reason, no matter the behavior of __aligned(8),
> we're okay. Likewise, even without __aligned(8), if gcc aligns structs
> by their biggest member, then we get 4 byte alignment on 32-bit and 8
> byte alignment on 64-bit, which is fine. There's no 32-bit platform
> that will trap on a 64-bit unaligned access because there's no such
> thing as a 64-bit access there. In short, we're fine.

ARM64 and x86-64 have memory operations that are not vector operations
that operate on 128 bit memory.

How do you know that the compiler for some architecture will not chose a
more optimized instruction to load a 64 bit memory value into two 32 bit
registers if you tell the compiler it is 8 byte aligned but it actually
isn't? I don't know the answer but telling the compiler some data is 8
byte aligned while it isn't really pretty much seems like a call for
trouble.

Why can't a compiler not vectorize this code if it can prove that it
doesn't conflict with other register users?

Bye,
Hannes