[PATCH] D119115: [RISCV] Improve insert_vector_elt for fixed mask registers.

[Jianjian Guan via Phabricator via llvm-commits]

jacquesguan added a comment.

In D119115#3790207 <https://reviews.llvm.org/D119115#3790207>, @reames wrote:

> I don't think we should land this patch.  It involves moving a value from vector to scalar and then back again.  The vector to scalar domain crossing is likely to be expensive on at least some real hardware.
>
> We can probably do better by forming the bitmask on the vector side entirely.  Given an index in a scalar register, we can produce a single bit mask by comparing a vid vector against that index.  With that, we can construct a vmerge with a scalar operand to set or clear the desired bit in the mask.

I don't understand your last sentence well, I think that vmerge could not be used for mask type, so how to only merge the desired bit?


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