llvm dev - Nov 2010 - [LLVMdev] StructType member offset

Hi,

how can i correctly calculate the size of a member of a struct 
(including alignment etc)?
This doesn't work :

         const StructType *STy = cast<StructType>(Ty);
         for (StructType::element_iterator I = STy->element_begin(),
                E = STy->element_end(); I != E; ++I)
         {
             usigned size =I->get()->getScalarSizeInBits(); //often 
returns 0, not what I need
         }

Basically I want to add up the sizes of the different members to 
calculate the offset (which I want to compare to
DIType::getOffsetInBits()). Or if there is a direct way to get the 
offset of a struct member without adding their sizes that'd be even better.

Thx,
Alex

Hi Alex,

I think you are looking for getStructLayout from TargetData (the
StructLayout type is also defined in TargetData.h).

Ciao,

Duncan.
> how can i correctly calculate the size of a member of a struct
> (including alignment etc)?
> This doesn't work :
>
>           const StructType *STy = cast<StructType>(Ty);
>           for (StructType::element_iterator I = STy->element_begin(),
>                  E = STy->element_end(); I != E; ++I)
>           {
>               usigned size =I->get()->getScalarSizeInBits(); //often
> returns 0, not what I need
>           }
>
> Basically I want to add up the sizes of the different members to
> calculate the offset (which I want to compare to
> DIType::getOffsetInBits()). Or if there is a direct way to get the
> offset of a struct member without adding their sizes that'd be even
better.
>
> Thx,
> Alex
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

On 02/11/10 22:48, Mark Leone wrote:
> You can also use getelementptr to get the offset of a struct member:
>
> http://nondot.org/sabre/LLVMNotes/SizeOf-OffsetOf-VariableSizedStructs.txt
Yup, except that this returns a constant expression in general, rather than
a number.  There's even a helper for doing this: ConstantExpr:getOffsetOf

Ciao,

Duncan.
>
> - Mark
>
> On Wed, Oct 27, 2010 at 2:25 AM, Duncan Sands <baldrick at free.fr
> <mailto:baldrick at free.fr>> wrote:
>
>     Hi Alex,
>
>     I think you are looking for getStructLayout from TargetData (the
>     StructLayout type is also defined in TargetData.h).
>
>     Ciao,
>
>     Duncan.
>
>      > how can i correctly calculate the size of a member of a struct
>      > (including alignment etc)?
>      > This doesn't work :
>      >
>      >           const StructType *STy = cast<StructType>(Ty);
>      >           for (StructType::element_iterator I =
STy->element_begin(),
>      >                  E = STy->element_end(); I != E; ++I)
>      >           {
>      >               usigned size
=I->get()->getScalarSizeInBits(); //often
>      > returns 0, not what I need
>      >           }
>      >
>      > Basically I want to add up the sizes of the different members to
>      > calculate the offset (which I want to compare to
>      > DIType::getOffsetInBits()). Or if there is a direct way to get
the
>      > offset of a struct member without adding their sizes that'd
be even better.
>      >
>      > Thx,
>      > Alex
>      > _______________________________________________
>      > LLVM Developers mailing list
>      > LLVMdev at cs.uiuc.edu <mailto:LLVMdev at cs.uiuc.edu>
http://llvm.cs.uiuc.edu
>      > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>     _______________________________________________
>     LLVM Developers mailing list
>     LLVMdev at cs.uiuc.edu <mailto:LLVMdev at cs.uiuc.edu>
http://llvm.cs.uiuc.edu
>     http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>