llvm dev - Oct 2010 - [LLVMdev] Structure memory layout

Hi Renato,

First, I appreciate your answer again. :)
>> %Char = type { c3, c4, c3, c2 }
>> %Short = type { s3, s4, s3, s2 }
>> %Int = type { i3, i4, i3, i2 }
> See, i is not for int (the C data type) but for every integer type on
> any language/platform combination. Normally, booleans are i1 and char
> i8, in ARM short is 16 and int is 32, and all of them use the 'i'
> notation.
I think you misunderstood our type notation.
By using your suggetion, "s4" means "i4 align short" in our
notation.
In "i4 align 2" for "short a:4", I think that "align
2" is already target dependent
because short can be 2 or 4 bytes according to each target.
so I wonder how do you think about "i4 align short" notation.
(Is it impossible to have c specifier in align in order to be consistent with
the rest of LLVM?)

Your answer is so helpful to me. :)

Thanks,
Jin-Gu Kang

On 21 October 2010 08:26, Jin Gu Kang <jaykang10 at imrc.kist.re.kr>
wrote:
> I think you misunderstood our type notation.
> By using your suggetion, "s4" means "i4 align short" in
our notation.
> In "i4 align 2" for "short a:4", I think that
"align 2" is already target dependent
> because short can be 2 or 4 bytes according to each target.
I see, but that still doesn't change the fact that "short" means
many
things, but 2 means only 2.

Saying that a variable aligns to something is already going towards
platform-specific choices, I agree, but at least with explicit sizes,
the back-end can ponder about its consequences in an organized way.

But if you say "short" compiling to one architecture X, the
architecture Y has no way of knowing what did you really mean, so it
can't determine if it's going to change the alignment of all short or
leave it.

> so I wonder how do you think about "i4 align short" notation.
> (Is it impossible to have c specifier in align in order to be consistent
with the rest of LLVM?)
The symbol 'i' is canonical in LLVM to mean any type of integer, I
doubt that's going to change. Besides, there is already an explicit
way of expressing the alignment, via the 'align' keyword.

Besides, it's already a heavy change just to make sure type and/or
variable alignment won't break all kinds of codegen/optimizations for
all platforms you have access to.

cheers,
--renato

I am pleased to discuss probleams with you. :)
(Your answer is really helpful to me).
If you don't mind I would often like to ask you a question.

Thanks,
Jin-Gu Kang