llvm dev - Nov 2012 - [LLVMdev] Do I need to calculate padding by myself to construct a StructType ?

I am using LLVM C++ API to generate IR for some structure data.

For example:

struct mystruct {int a:1; int b:2; unsigned long c} = {{1, 2, 3}};

I read the document and tried the demo cgi, it seems the API requires user
to handle padding and value combination for the initializer by themselves.

Is there any way to create the struct more simply like this:

fields.push_back(Type::getIntNTy(context, 1));
fields.push_back(Type::getIntNTy(context, 2));
fields.push_back(Type::getIntNTy(context, 64));
structTy.setBody(fields, false /* isPacked*/);

It's really tough to do layout work and it's very likely to cause bugs.
Combining the values for the initializer is also not easy.

As my understanding, if I tell llvm API that the struct is not packed, it
should handle padding automatically, right?

Any comments? Any pointer to references is also appreciated.
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On Thu, Nov 8, 2012 at 5:22 PM, Wei Li <limijd at gmail.com>
wrote:
> I am using LLVM C++ API to generate IR for some structure data.
>
> For example:
>
> struct mystruct {int a:1; int b:2; unsigned long c} = {{1, 2, 3}};
>
> I read the document and tried the demo cgi, it seems the API requires user
> to handle padding and value combination for the initializer by themselves.
>
> Is there any way to create the struct more simply like this:
>
> fields.push_back(Type::getIntNTy(context, 1));
> fields.push_back(Type::getIntNTy(context, 2));
> fields.push_back(Type::getIntNTy(context, 64));
> structTy.setBody(fields, false /* isPacked*/);
>
> It's really tough to do layout work and it's very likely to cause
bugs.
> Combining the values for the initializer is also not easy.
>
> As my understanding, if I tell llvm API that the struct is not packed, it
> should handle padding automatically, right?
>
> Any comments? Any pointer to references is also appreciated.
LLVM struct types will handle simple cases, but it doesn't know
anything about bitfield layout.  i1 and i2 each take up a whole byte
no matter how you specify the packing.

-Eli