llvm dev - May 2008 - [LLVMdev] Matching struct layouts.

> > If this is an arbitrary struct, it can be quite tricky.  Do you have
> > to worry about bitfields and variable sized fields (for example a
> > variable length array at the end)?  What about unions?
> 
> There are no bitfields, but it does have a variable length array at the
end.
> There are no unions.
You could use llvm-gcc to convert the struct to an LLVM type.
Otherwise, it wouldn't be too hard to convert the struct by
hand as long as it doesn't have any exotic types in it.  A
VLA at the end (with element type T) is currently represented
by placing a field of type T at the end (i.e. just one element).

Ciao,

Duncan.

PS: Please reply to the list as well as to me - that way others can
comment too.

"Mahadevan R" <mdevan.foobar at gmail.com> writes:
>> You could use llvm-gcc to convert the struct to an LLVM type.
>> Otherwise, it wouldn't be too hard to convert the struct by
>> hand as long as it doesn't have any exotic types in it.  A
>> VLA at the end (with element type T) is currently represented
>> by placing a field of type T at the end (i.e. just one element).
>
> But can I assume that both gcc and LLVM will layout the structure
> in the same way (member alignment, padding)?
llvm-gcc and gcc follows the same platform C ABI. So the answer to your
question is "yes".

-- 
Oscar

On May 16, 2008, at 1:40 AM, Mahadevan R wrote:
> On Fri, May 16, 2008 at 12:31 PM, Duncan Sands <baldrick at free.fr>
> wrote:
>>>> If this is an arbitrary struct, it can be quite tricky.  Do you
>>>> have
>>>> to worry about bitfields and variable sized fields (for example
a
>>>> variable length array at the end)?  What about unions?
>>>
>>> There are no bitfields, but it does have a variable length array  
>>> at the end.
>>> There are no unions.
>>
>> You could use llvm-gcc to convert the struct to an LLVM type.
>> Otherwise, it wouldn't be too hard to convert the struct by
>> hand as long as it doesn't have any exotic types in it.  A
>> VLA at the end (with element type T) is currently represented
>> by placing a field of type T at the end (i.e. just one element).
>
> But can I assume that both gcc and LLVM will layout the structure
> in the same way (member alignment, padding)?
On LLVM side, llvm-gcc is responsible to create LLVM IR with  
appropriate struct layout.  llvm-gcc interprets GCC's struct layout  
while creating LLVM IR. GCC's struct layout at this point is target  
specific. So any difference between gcc and LLVM struct layout is a  
bug or oversight in interpreting GCC's struct layout. If there is a  
bug in GCC's struct layout then llvm-gcc will create bug-to-bug  
compatible struct layout :)

-
Devang