fence instruction throwing unexpected illegal in RV32E congfiguration

[tzwaenn]

Observed Behavior

In a configuration with RV32E, I see an illegal exception being thrown for a fence instruction. While this instruction has fields rs1 and rd, the spec clarifies
"The unused fields in the FENCE instructions—rs1 and rd—are reserved for finer-grain fences in future extensions. For forward compatibility, base implementations shall ignore these fields, and standard software shall zero these fields."
So it is clearly not intended that a fence with rs1[4]==1 or rd[4]==1 should throw an illegal exception on RV32E - it is not accessing any integer registers.

EDA tool and version:
The issue was found with the Siemens Processor Verification App.

Version of the Ibex source code:
033abfc with RTL parameter RV32E=1

[GregAC]

Thanks for the report @tzwaenn. If you have a fix feel free to submit a PR otherwise we will take a look though this isn't a priority for us right now (current development focus is on the opentitan configuration which has E=0).

[tzwaenn]

Understood. I think removing line 1145 in ibex_decoder.sv
alu_op_a_mux_sel_o = OP_A_REG_A;
would do the trick (fence does not use the source register), though I also do not have the bandwidth right now to run experiments.