Re: I have some questions about the VMADC/VMSBC instructions, thank you for your valuable comments.


Andrew Waterman
 



On Thu, Jun 16, 2022 at 2:43 AM <lilei2@...> wrote:
1. Question for tail bits of mask-producing instructions.
In the case of mask-producing instructions, tail elements are the bits with (vl <= bit index < VLEN).So according to riscv-v-spec-1.0, page 46, VMADC/VMSBC instructions operate with tail-agnostic policy, which means the tail bits can be written with 1 or unchanged. 
While according to page 13, we have a more relax constraint: except for mask load instructions, any element in the tail of a mask result can also be written with the value the mask-producing operation would have calculated with vl=VLMAX. Which means we can overwritten all remaining bits that past vl with 1s or with the value the mask-producing operation would have calculated.
For example, if VLEN=128, LMUL=1, SEW=32, there are only 4 body bits for VMADC instruction. If current vl=2, we can write the calculation results to bit[1:0], and write all other 126 bits with 1s.

It is legal to fill bits vl..VLEN-1 with 1s because of the clause that these instructions are always tail-agnostic.

Or we can write 4 bits calculation results to bit[3:0] in which only bit[1:0] are body bits, and write all other 124 bits with 1s.

It is also legal to compute bits 0..VLMAX-1 (as a function of elements 0..VLMAX-1), then fill bits VLMAX..VLEN-1 with 1s, because of the clause that mask-producing instructions are permitted to write the result they would have written if vl had been set to VLMAX.

But I'm not sure that the remark "in which only bit[1:0] are body bits" matters.  In this style of implementation, the behavior is the same as if the body contained elements 0..VLMAX-1.

Whether either of these implementations is legal?
 
2. Question for inactive body bits of mask-producing instructions.
When vtype.vma=0, which means mask-undisturbed policy, the inactive body bits should retain its value. 
For example, when vma=0, the VMSBF_M and VMSEQ with vm=0, should not change the inactive body bits.
Is my understand correct?thanks.

Right.

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