Historically, one could uniquely decode any RISC-V instruction based on
the instruciton to decode, plus a MATCH and MASK pair.
The scalar crypto extension adds instructions for accelerating the AES
algorithm which work very differently on RV32 and RV64. However, they
overlap in terms of opcodes. The instructions are always mutually
exclusive, and so it makes sense to overlap them this way to save
encoding space.
This exposed a problem, where previously Spike assumed the decoder
function was something like:
> decode(instr_word, MATCH, MASK)
Now it needed to be
> decode(instr_word, MATCH, MASK, current_xlen)
To get around this in the initial implementation, the instructions which
shared opcodes were implemented in the same *.h file - e.g. aesds.h
contained an implementation of aes32dsi, and aes64ds. We detected
xlen in the file, and executed the appropriate instruction logic.
This worked fine for our limited set of benchmarks.
After more extensive testing, we found that Spike has an optimisation
which changes the order in which it tries to decode instructions based
on past instructions.
Running more extensive tests exposed the fact that the decoding logic
could still not unambiguously decode the instructions. Hence, more
substantial changes were needed to tell spike that an instruction is
RV32 or RV64 only.
These changes have been implemented as part of
- riscv/encoding.h
- disasm/disasm.cc
- riscv/processor.cc/h
Basically, every instr_desc_t has an extra field which marks which
base architecture the instruction can be exectuted on. This bitfield
can be altered for particular instructions.
The changes to riscv/insns/* simply split out the previously combined
instructions into a separate header files.
On branch scalar-crypto-fix
Changes to be committed:
modified: disasm/disasm.cc
modified: riscv/encoding.h
new file: riscv/insns/aes32dsi.h
new file: riscv/insns/aes32dsmi.h
new file: riscv/insns/aes32esi.h
new file: riscv/insns/aes32esmi.h
new file: riscv/insns/aes64ds.h
new file: riscv/insns/aes64dsm.h
new file: riscv/insns/aes64es.h
new file: riscv/insns/aes64esm.h
deleted: riscv/insns/aesds.h
deleted: riscv/insns/aesdsm.h
deleted: riscv/insns/aeses.h
deleted: riscv/insns/aesesm.h
modified: riscv/processor.cc
modified: riscv/processor.h
modified: riscv/riscv.mk.in
Chih-Min Chao
Ben Marshall
Chih-Min Chao
Andrew Waterman
Abhinay Kayastha