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riscv / riscv-v-spec

Working draft of the proposed RISC-V V vector extension
https://jira.riscv.org/browse/RVG-122
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Are EEW values larger than SEW supported? #856

Open az-sc opened 1 year ago

az-sc commented 1 year ago

Hello! The specification does not clearly state whether it is acceptable to use EEWs larger or smaller than the allowed SEWs (EEW = 128 for widening SEW = 64 for example). Chapter 18 doesn't have any standards with EEW > 64, but would like more clarity in the description.

aitesam961 commented 8 months ago

EEW CAN be larger or shorter than SEW. You are right that the spec does not mention the above mentioned case explicitly but I found the explanation in Section 5.2. "For example, most widening arithmetic instructions have a source group with EEW=SEW and EMUL=LMUL but have a destination group with EEW=2SEW and EMUL=2LMUL."

So if SEW = 64b, the destination element width (EEW) would definitely be 128 bit.