When the Zbkb extension is enabled an arbitrary 64bit constant can be loaded in a maximum of 5 instructions.  A lui+addi for the upper and lower 32 bits and a pack to merge them.   High performance uarchs will execute this in 2c and even a simplistic uarch would probably take a maximum of 5c.   Contrast to the what we do now where we push complex constants into the constant pool.  That's probably 3 instructions and 5c on most uarchs.  Naturally we would like to see constant synthesis improved when Zbkb is enabled.  The basic support for Zbkb is development-complete.  So the effort here is really just to use it in constant synthesis.

There are also cases that could be improved for designs without Zbkb, but which do have Zbs.  Consider a constant with just 4 bits set.  Say two non-consecutive bits in the high 32bit part of a 64bit word, then two bits down in the low 12 bits.   Such constants will tend to end up in the constant pool.  But this could be implemented with two bsets+addi.  That is going to be the same size and almost certainly faster than a constant pool reference. 

Stakeholders/Partners

RISE:

Ventana: Jeff Law – general oversight / guidance.

External:

Dependencies

Status

Updates

02 Apr 2024 

• Basic Zbkb patterns written.  Havend started on synthesis changes though.
• Noted as a 1H2024 work item.