Proposal: Add a configuration option for RTL-specific difftest

[tqxu23]

Hello!

I am using NEMU as a reference to align the rvv instruction implementation in GEM5. I noticed that the implementation of fredusum in NEMU is aligned with the RTL implementation. Specifically, in NEMU/src/isa/riscv64/instr/rvv/vcompute.h, the implementation of fredusum is as follows:

	def_EHelper(vfredusum) {
	#ifdef CONFIG_DIFFTEST
	  FREDUCTION(FREDUSUM)    // use ordered reduction
	#else
	  float_reduction_computing(s);   // when NEMU is ref, use unordered reduction which is same as XiangShan
	#endif
	}

Currently, using NEMU as the reference causes GEM5's difftest to fail (we need to remove the #ifdef CONFIG_DIFFTEST lines in order to execute FREDUCTION(FREDUSUM)).

Since the RTL implementation might change again for implementation-specific reasons, I’d like to propose adding a configuration option (e.g., in nemuconfig) to distinguish whether the behavior should align specifically with RTL. This would help maintain flexibility and improve compatibility for different use cases.

[gevvuF-mipty4-tyfnet]

j

[cebarobot]

Hi. I think you are right. We hmay have a option for this. However, NEMU is designed to align with XiangShan's RTL, so we may just leave it as it.