Description
It was mentioned earlier #111 that a different LMUL might affect hazard, and the response given by the development team was to utilize stalling when the LMUL is changed (WAIT_STATE in the ara_dispatcher) to avoid this situation.
However, when we verified it using formal verification, we found the following errors.
When the first incoming instruction vslideup.vx,vl=144,EW16,LMUL=2,vs2=v0,vd=6,vm=1,stride=128
Next instruction is vadd vl=16,EW16,LMUL=1/2,vs1=v11, vs2=v7,vd=v31,vadd.vv v31, v7, v11
The original slideup instruction that would have written vd=6 would have written to v7 (because stride and LMUL=2), but the next vadd instruction would have already taken out the data from the source register, v7, and done the math.
This causes the vadd instruction, which is supposed to stall and wait for v7 to be written back before taking out the data, to get the data that has not yet been written back beforehand, causing an error.
The main reason for this failure is that the stall mechanism in ara_dispatcher is not well thought out (only LMUL[2]=0 && LMUL_old > LMUL_new are taken into account).
To complete the above consider that it might be possible to rewrite it as follows:
if ((!vtype_q.vlmul[2] && (vtype_d.vlmul[2:0] < vtype_q.vlmul[2:0])) || (vtype_q.vlmul[2] && (vtype_d.vlmul[2:0] > vtype_q.vlmul[2:0])))
state_d = WAIT_IDLE;
end