Function LLVMConstRealGetDouble 

pub unsafe extern "C" fn LLVMConstRealGetDouble(
    ConstantVal: LLVMValueRef,
    losesInfo: *mut LLVMBool,
) -> f64

Obtain the double value for an floating point constant value. losesInfo indicates if some precision was lost in the conversion.

@see llvm::ConstantFP::getDoubleValue