Re: [PATCH qemu 1/9] target/riscv: rvv: Add mask agnostic for vv instructions

[Weiwei Li]

在 2022/4/27 上午2:20, eop Chen 写道:

Weiwei Li <liweiwei@iscas.ac.cn> 於 2022年4月26日 下午4:47 寫道：

在 2022/3/17 下午3:26, ~eopxd 写道:

From: Yueh-Ting (eop) Chen <eop.chen@sifive.com>

This is the first commit regarding the mask agnostic behavior.
Added option 'rvv_ma_all_1s' to enable the behavior, the option
is default to false.

Signed-off-by: eop Chen <eop.chen@sifive.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>

Similar to our last discussion,  vext_set_elems_1s_fns array can be simplified to single vext_set_elems_1s,

since the fourth argement can be used as the start offset.

Another question, may be not related to this patchset, in section 3.4.3 of the spec:

"Mask destination tail elements are always treated as tail-agnostic, regardless of the setting of vta."

What does "Mask destination tail elements" mean?

Regards,

Weiwei Li

I have just updated a new version for the tail agnostic patch set, it also includes a bug fix discovered today.

Regarding the question, mask / masked-off are for body elements and respects the mask policy, and tail elements respect the tail policy.

Regards,

eop Chen

I find another descriptions in the spec. For the instructions that write mask register (such as vmadc, vmseq,vmsne,vmfeq...), they all have similar description

(You can search "tail-agnostic" in the spec):

Section 11.4: "Because these instructions produce a mask value, they always operate with a tail-agnostic policy"

Section 11.8/13.13: "Compares write mask registers, and so always operate under a tail-agnostic policy"

Section 15.1: "Mask elements past vl, the tail elements, are always updated with a tail-agnostic policy"

Section 15.4/15.5/15.6: "The tail elements in the destination mask register are updated under a tail-agnostic policy"

So I think "Mask destination tail elements" may means the tail element for instructions that take mask register as destination register.  For these instructions,

maybe the setting of VTA can be ignored. 

Regards,

Weiwei Li