llvm dev - May 2009 - [LLVMdev] Set alignment of a structure?

Hello,

Is it possible to set the alignment of a StructType in llvm?
Especially in the case that it may contain OpaqueTypes?

Thanks,
-- 
Nick Johnson

On Thu, May 7, 2009 at 8:53 AM, Nick Johnson <npjohnso at
cs.princeton.edu> wrote:
> Is it possible to set the alignment of a StructType in llvm?
Not directly; structs are either packed or not packed.  You can model
arbitrary alignment, though, by using a packed struct and making sure
the alignment on allocations and loads is set explicitly.
> Especially in the case that it may contain OpaqueTypes?
IIRC, a struct can't directly contain an OpaqueType.

-Eli

On May 7, 2009, at 8:53 AM, Nick Johnson wrote:
> Hello,
>
> Is it possible to set the alignment of a StructType in llvm?
Nope.  If this is for a global variable (For example), you build the  
ConstantStruct with a type that ensures that the elements get the  
right offsets, then you put the alignment on the GlobalVariable itself.

-Chris

>> Is it possible to set the alignment of a StructType in llvm?
>
> Nope.  If this is for a global variable (For example), you build the
> ConstantStruct with a type that ensures that the elements get the
> right offsets, then you put the alignment on the GlobalVariable itself.
That is unfortunate for a few reasons,

(1) I am generating multi-threaded code, and I want to use alignment
to ensure that the memory used by each thread is in separate cache
lines.

(2) The code I generate links against external libraries which contain
complicated data types---also featuring alignment.  Because llvm
doesn't support alignment, I must represent these as opaque types.
However, that means that I cannot even manually pad the size of the
structure with a char array, since there is no way to determine the
size of the opaque type.

I think the best solution to this problem is simply to set a structure
alignment.  If I'm not mistaken, nothing in llvm really needs to know
the exact structure alignment until GEPs have been lowered to native.

-- 
Nick Johnson