4.4 Instruction Dispersal and Issue

A traditional VLIW with fixed width MultiOps has no need to disperse operations. However, when using a compressed format like that of the Defoe, there is a need to expand the operations, and insert NOPs for function units to which no operation is to be issued. To make the dispersal task easy we make the following assumptions:

• A few bits in the opcode specify the type of function unit (i.e. load/store, simple arithmetic, complex arithmetic or branch) the operation needs.
• The compiler ensures that the instructions that comprise a MultiOp are sorted in the same order as the function units in the processor. This reduces the circuit complexity of the instruction dispersal stage. For example, if a MultiOp consists of a load, 32-bit divide and a branch, then the ordering (load, multiply, branch) is legal, but the ordering (load, branch, multiply) is not legal.
• The compiler ensures that all the operations in the same MultiOp are independent.
• The compiler ensures that the function units are not over subscribed. For example, it is legal to have two loads in a MultiOp, but it is not legal to have three loads.
• It is illegal to not have a stop bit in a sequence of more than 6 instructions.
• Basic blocks are aligned at 32-byte boundaries.

Apart from reducing wastage of memory, another reason to prefer a compressed format VLIW over an uncompressed one is that the former provides better I-Cache utilization. To improve performance, we use a predecode buffer that can hold up to 8 uncompressed MultiOps. The dispersal network can use a wide interface (say 512 bits) to the I-cache to uncompress up to 2 MultiOps every cycle and save them in the predecode buffer. Small loops of up to 8 MultiOps (maximum 48 operations) will experience repeated hits in the predecode buffer. It may also help lower the power consumption of a low-power VLIW processor. Defoe supports in-order issue and out of order completion. Further, all the operations in a MultiOp are issued simultaneously. If even one operation cannot be issued, issue of the whole MultiOp stalls.