This batch of intrinsics covers two sets of immediate shift
instructions, which have in common that they only overwrite part of
their output register and so they need an extra input giving its
previous value.
The VSLI and VSRI instructions shift each lane of the input vector
left or right just as if they were normal immediate VSHL/VSHR, but
then they only overwrite the output bits that correspond to actual
shifted bits of the input. So VSLI will leave the low n bits of each
output lane unchanged, and VSRI the same with the top n bits.
The V[Q][R]SHR[U]N family are all narrowing shifts: they take an input
vector of 2n-bit integers, shift each lane right by a constant, and
then narrowing the shifted result to only n bits. So they only
overwrite half of the n-bit lanes in the output register, and the B/T
suffix indicates whether it's the bottom or top half of each 2n-bit
lane.
I've implemented the whole of the latter family using a single IR
intrinsic `vshrn`, which takes a lot of i32 parameters indicating
which instruction it expands to (by specifying signedness of the input
and output types, whether it saturates and/or rounds, etc).
Reviewers: dmgreen, MarkMurrayARM, miyuki, ostannard
Reviewed By: dmgreen
Subscribers: kristof.beyls, hiraditya, cfe-commits, llvm-commits
Tags: #clang, #llvm
Differential Revision: https://reviews.llvm.org/D72328
Simon Tatham