Hi.
I've a situation where I need to build and execute a function on the
fly (with LLVM of course :-). The function will need to take a pointer
to a C struct, which is defined by the host. That is, the built
function will be something like:
void func(struct XYZ *p)
{
/* need to access members of p here */
}
How will I educate LLVM IR about 'struct XYZ' so that the code inside
can access it's members properly? How can I be sure that the alignment
and padding of the members of the existing struct layout and the LLVM
struct layout matches?
In the worst case I can construct the function as taking individual
struct members as individual parameters, but that would be inelegant..
Thanks,
-Mahadevan.
> I've a situation where I need to build and execute a function on the > fly (with LLVM of course :-). The function will need to take a pointer > to a C struct, which is defined by the host. That is, the built > function will be something like: > > void func(struct XYZ *p) > { > /* need to access members of p here */ > } > > How will I educate LLVM IR about 'struct XYZ' so that the code inside > can access it's members properly? How can I be sure that the alignment > and padding of the members of the existing struct layout and the LLVM > struct layout matches?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? Ciao, D.
> > 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.
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)? Regards, -Mahadevan.