llvm dev - May 2017 - How exactly is datatype alignment determined?

On Mon, 22 May 2017, Krzysztof Parzyszek via llvm-dev wrote:
> Probably from LargeArrayMinWidth/LargeArrayAlign settings in Targets.cpp
(in
> clang).
Wait what? In clang? But my input is already LLVM IR. 
MF->getDataLayout().getPrefTypeAlignment(Ty) must be basing its answer on 
either something in the IR file, or in the target implementation, but 
clang is not really in the picture.

On Mon, 22 May 2017, Dr. ERDI Gergo wrote:
> On Mon, 22 May 2017, Krzysztof Parzyszek via llvm-dev wrote:
>
>> Probably from LargeArrayMinWidth/LargeArrayAlign settings in
Targets.cpp
>> (in clang).
>
> Wait what? In clang? But my input is already LLVM IR. 
> MF->getDataLayout().getPrefTypeAlignment(Ty) must be basing its answer
on
> either something in the IR file, or in the target implementation, but clang
> is not really in the picture.
>
Actually, tracking down the sequence of function calls, it turns out that 
'8' is ultimately coming from the following call in 
DataLayout::getAlignment:

getAlignmentInfo(AGGREGATE_ALIGN, 0, abi_or_pref, Ty);

this seems to return 8 with the following datalayout string:

e-S8:p:16:8-i8:8-i16:8-i32:8-i64:8-f32:8-f64:8-n8-a:8

I think my problem is that 'a:8' probably doesn't mean what I think
it
should mean. What is the difference between 'a:8' and 'a:0'?

On Mon, 22 May 2017, Dr. ERDI Gergo wrote:
> Actually, tracking down the sequence of function calls, it turns out that
'8'
> is ultimately coming from the following call in DataLayout::getAlignment:
>
> getAlignmentInfo(AGGREGATE_ALIGN, 0, abi_or_pref, Ty);
>
> this seems to return 8 with the following datalayout string:
>
> e-S8:p:16:8-i8:8-i16:8-i32:8-i64:8-f32:8-f64:8-n8-a:8
>
> I think my problem is that 'a:8' probably doesn't mean what I
think it should
> mean. What is the difference between 'a:8' and 'a:0'?
OK I just now tried with 'a:0' for identical result. I am now quite sure
I
am fully confused about what these alignment settings mean.

Does 'a:8' mean single byte alignment (i.e. *no* alignment)? If yes,
does
that mean getAlignment for a struct type should return 1? In fact, when 
getAlignment returns 8, does that mean 8 bits (no alignment) or 8 bytes 
(64 bits) alignment? Or does that mean the lowest 8 bits of the address 
needs to be 0, i.e. 256-byte alignment?