General

My goal is to create a simple tool that compiles C++ programs to Dart. The emscripten project converts LLVM bitcode to JavaScript. In a similar fashion, this project will convert LLVM IR to Dart.

Restrictions

The conversion between C++ and Dart is made difficult by C++'s pointer semantics. Because of this, several restrictions are in place:

Do not cast a pointer to an integer or vice versa

Doing so makes assumptions about the size of a pointer. Dart has no concept of "pointers" in the numeric sense.

char myData[3];
uint64_t myPtr = (uint64_t)myData;
char * myData2 = (char *)myPtr;

Do not cast a structure to a pointer and use the pointer to make cross-field memory accesses

Structures are not backed by a single buffer. Instead, each field has its own memory store. This is done so function pointers and other data can be stored in the middle of a structure.

struct {
  char c1;
  char c2;
} obj; 
short * s = (short *)&obj;
s[0] = 0x1337;

LLVM IR Patterns

Here are some common examples of LLVM IR patters and my thoughts on how I will represent them.

Field access

%c = alloca %struct.MainClass.0, align 8
%1 = getelementptr inbounds %struct.MainClass.0* %c, i32 0, i32 1

Steps:

• Use GEP to get a pointer to the field
• Dereference the field

Thoughts:

• A structure is allocated on the stack by declaring it as a variable.
• A structure is represented as a _Pointer object.
• The getelementptr call should simply add a number to the _Pointer object.
• Reading a value simply dereferences a pointer.

Static cast

%s = alloca %struct.Sub, align 8
...
%1 = bitcast %struct.Sub* %s to i8*
%2 = getelementptr inbounds i8* %1, i64 16
%3 = bitcast i8* %2 to %struct.SuperB*

Steps:

• Cast structure to uint8_t *
• Offset the pointer by 0x16 or some other offset
• Cast pointer to other structure's type