[BUG] Out-of-Order Not Working

Created by: sifferman

Is there an existing CVA6 bug for this?

• I have searched the existing bug issues

Bug Description

Bug Explanation

I noticed the docs say that Out-of-Order can occur between the multiplier and the ALU, however this is not the case according to the RTL.

Docs:

https://github.com/openhwgroup/cva6/blob/a29b231070f2dccca53eae2681120116633a4ffc/docs/03_cva6_design/issue_stage.md?plain=1#L42-L54

RTL:

https://github.com/openhwgroup/cva6/blob/adf99b53042d2b14cbbb36689babf8cd4c03cf81/core/issue_read_operands.sv#L135-L152

For example, I have this code snippet:

        div     t2, t0, t1
        fsqrt.s f1, f1
        lw      t5, 0(s1)
        add     t6, t0, t1

And this is it's execute stage:

As you can see, the add instruction isn't run until after the div finishes.

Proposed Fix

Either:

• The RTL could be changed to separate the ALU and multiplier into two separate busy signals.
• The example in the docs could be changed to something else, such as div and lsu.