[LLVMdev] Extracting a value from an union
Frank Winter
fwinter at jlab.org
Wed Oct 23 08:22:32 PDT 2013
I cannot figure out the right way to extract values from an
array of unions. Unions are not directly supported and this
seems to complicate things.
Background:
I am call a function returned by the JIT execution machine
and pass it 1 argument, namely the base address of an array
of unions which contain the real arguments.
The data structure is set up from C like:
std::array<union{int,float*}> arguments(5);
The sequence of occurences of 'int' and 'float*' is encoded
in a vector<llvm::Type*>:
i32
i32
float*
float*
float*
Right now I am trying this (this is the jitted function):
define void @main([8 x i8]* %arg_ptr) {
entrypoint:
%0 = getelementptr [8 x i8]* %arg_ptr, i32 0
%1 = getelementptr [8 x i8]* %arg_ptr, i32 1
%2 = getelementptr [8 x i8]* %arg_ptr, i32 2
%3 = getelementptr [8 x i8]* %arg_ptr, i32 3
%4 = getelementptr [8 x i8]* %arg_ptr, i32 4
}
First of all, is the functions' signature correct (assuming
pointers are 8 bytes)?
How do I get the first 'i32' out of the [8 x i8] stored in %0?
Do I need to cast the array [8 x i8] first to a pointer i32*,
then create another GEP to its first element?
Frank
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