Fixing Xilinx MLIR-AIE Neighboring Tile Buffer Link Errors

Hey folks! Ever been diving deep into Xilinx MLIR-AIE development and hit a snag that just makes you scratch your head? Today, we're tackling a pretty specific, but super important, issue: why initializing buffers in neighboring tiles can cause a dreaded linking error when you're compiling with the CHESS compiler (specifically xchesscc). If you're using mlir-aie to push the boundaries of AI Engine programming, this is one of those quirks you absolutely need to understand. We’ll break down what’s happening, why the CHESS compiler seems to struggle where Peano sails through, and most importantly, how to work around it. Get ready to level up your AIE debugging game!

Decoding the Xilinx MLIR-AIE Linker Error: Why Neighboring Tile Buffers Get Stuck

Alright, let's kick things off by laying out the scenario. Imagine you're building a complex AI Engine application on a Xilinx device. You've got your tiles, your cores, and you're orchestrating data flow like a pro. A common pattern in AIE programming is to have a main tile that needs to access data stored in the local memories of its adjacent, or neighboring, tiles. This is totally normal, right? We're talking about a setup where, say, main_tile at (1,3) needs lut0_buf from west_tile at (0,3), lut1_buf from north_tile at (1,4), and lut2_buf from south_tile at (1,2). Plus, lut3_buf is right there on main_tile itself. Nothing too crazy, theoretically.

Here’s the rub, guys: when you try to initialize these neighboring tile buffers directly within your mlir-aie code using the initial_value parameter – as shown in the snippet below – things can go sideways with the CHESS compiler. Specifically, you'll encounter a linking error. Let's look at the example code that sets up this situation:

main_tile = tile(1,3)
west_tile = tile(0,3)
north_tile = tile(1,4) 
south_tile = tile(1,2)

lut0_buf = buffer(west_tile, lut0_ty, initial_value=np.array(lut0_arr, dtype=np.int16))
lut1_buf = buffer(north_tile, lut1_ty, initial_value=np.array(lut1_arr, dtype=np.int16))
lut2_buf = buffer(south_tile, lut2_ty, initial_value=np.array(lut2_arr, dtype=np.int16))
lut3_buf = buffer(main_tile, lut3_ty, initial_value=np.array(lut3_arr, dtype=np.int16))

@core(main_tile, archvie)
def core_body():
    for _ in range_(sys.maxsize):
        di = self.din.acquire(ObjectFifoPort.Consume, 1)
        func(di, lut0_buf, lut1_buf, lut2_buf, lut3_buf)
        self.din.relase(ObjectFifoPort.Consume, 1)

What happens under the hood is that mlir-aie translates these buffer definitions with initial_value into llvm IR global variables. For instance, lut3_buf might become something like @lut3_buf = global [20064 x i16] [i16 0, i16 0 ...]. The crucial part here is the global keyword, implying static storage with a predefined initial state. The problem arises when the CHESS compiler, specifically xchesscc, attempts to link the generated code. It throws errors like: Error: could not find space for DefSymbol 'lut1_buf' using base address 0 and Error: error copying memory section item. These messages strongly suggest a memory allocation conflict. It seems xchesscc is trying to map all these globally initialized buffers, regardless of their declared tile, into a single, shared memory space – or perhaps, more specifically, into the main_tile's local memory. This is a huge issue because AIE tiles have limited local memory, and trying to cram all those buffers into one spot quickly exhausts the available resources. What’s truly baffling is that Peano, another compiler option for AIE, handles this exact scenario without a hitch. This discrepancy points to a fundamental difference in how these compilers, or their underlying linkers, interpret and allocate memory for globally initialized buffers across different AIE tiles. We need to figure out why CHESS behaves this way and how to guide it properly.

Understanding the AIE Architecture and Buffer Management

To fully grasp why our CHESS compiler is throwing a fit, let's quickly recap some basics of the Xilinx AI Engine (AIE) architecture and how mlir-aie helps us manage memory. The AIE is a powerful, tiled array of processors, each tile typically comprising a vector processor core, local data memory, and a dedicated memory-mapped interface (MMIO) for communication. Each core has its own local memory, which is essential for high-performance operations, as it allows fast access to frequently used data without needing to go off-chip. Communication between tiles is handled by a sophisticated network-on-chip (NoC) or direct memory access (DMA) between adjacent tile memories.

Now, mlir-aie is this fantastic high-level framework that lets us describe our AIE applications without getting bogged down in the nitty-gritty of low-level hardware registers. When we define a buffer in mlir-aie, we specify which tile it belongs to, its type, and optionally, an initial_value. The intent is clear: buffer(west_tile, ...) means this buffer should reside in the west_tile's local memory. When you include initial_value, you're telling the compiler,