Mill Computing, Inc. Forums The Mill Architecture Control flow divergence and The Belt Reply To: Control flow divergence and The Belt

Ivan Godard
Post count: 689

Yes, each side has three variable-sized blocks, and within each block the element sizes are statically fixed (though not necessarily the same). In the exu side each block has a different op set, with the sets partitioned by phase, and each operation is complete in its set.

The flow side is different: only the flowBlock has operation elements like on the exu side, while in the con block the elements are bytes and in the ext block the elements are morsels big enough to hold a belt position number. Those flow ops that take an argument list (besides the branches there are the calls and various special ops including rescue, retire, and con) encode as much as they can in the flow block entry and put the rest of the list in the con and ext blocks. Each flow block entry has two bits to indicate how much has been put in the other blocks: 0/1/2/4 bytes and 0/1/2/3 morsels.

If the list is long enough that it doesn’t fit in an entry plus 4 bytes and 3 morsels then the op is ganged with a flowArgs pseudo op in the next slot, which provides another 4 bytes and three morsels. Rinse repeat. The maximal sizes of the blocks (for each family member) are set so that it is possible to encode a belt list with entries for every position, or a con with enough bits to express the largest possible operand. The constraints of the ext and con blocks are recognized by the specializer, so that (for example) it may encode two calls with few arguments in a single instruction, but only one call with a long argument list even though there are two slots supporting call.