[LLVMdev] Alignment of pointee

[Frank Winter]

On 03/25/2014 10:08 AM, Krzysztof Parzyszek wrote:
> On 3/25/2014 8:53 AM, Frank Winter wrote:
>>
>> One can see that if the initial alignment of the pointee of %arg0 was 32
>> bytes and since the vectorizer operates on a loop with a fixed trip
>> count of 4 and the size of double is 8 bytes, the vector loads and
>> stores could be ideally aligned with 32 bytes (which on my target
>> architecture would result in vector loads without additional 
>> permutations.
>>
>> Is it somehow possible to achieve this? I am generating the IR with the
>> builder, i.e. I am not coming from C or clang.
>
> If you are generating the loads and stores, you could just set the 
> alignment to whatever you want, i.e. 32 bytes in your case.
I can't. Take a look again at the first piece of code. The loads occur 
in the 'inner' loop. Only for the first iteration the alignment of 32 
bytes is true, not for iteration 2, 3 and 4. So, the alignment 
information cannot enter at the point of loading. Thus, the idea of 
attaching the information right at the pointer's definition, i.e. as the 
argument.
>
> I have wondered about it in a general case, when you simply want to 
> have an alignment information on the pointer, and not on 
> loads/stores.  My idea was to invent a builtin, something like 
> "assert_aligned", that does nothing, other than manifest the alignment 
> by the fact of its existence. For example:
>   %argp = call i8* llvm.assert.aligned(%arg0, 32)
> would state that the pointer %argp is aligned to 32 bytes, and the 
> value of it is the same as %arg0 at the place of the "call".
>
> That was a while ago and maybe there are other ways of doing it now.
Should be doable this way. Although I am not sure whether a assertion or 
an annotation would be cleaner.

There should already be a solution.
>
> -Krzysztof
>