This patch extends the optimization of VID-sequence BUILD_VECTORs
introduced in D104921 to include simple fractional steps composed of a
separated integer numerator and denominator.
A notable limitation in this sequence detection is that only sequences
with steps N/1 or 1/D are found, meaning that the step between elements
and the frequency with which it changes is consistent across the whole
sequence. Fractional steps such as 2/3 won't be matched as those would
involve more complex tracking of state or some level of backtracking.
As is stands, however, this patch is sufficient to match common
interleave-type shuffle indices, for example matching `<0,0,1,1>` (or
commonly `<0,u,1,u>` or `<u,0,u,1>`) to an index sequence divided by 2.
While the optimization is relatively `undef`-tolerant, due to greedy
pattern-matching there even are some simple patterns which confuse the
sequence detection into identifying either a suboptimal sequence or no
sequence at all.
Currently only fractional-step sequences identified as having a
power-of-two denominator are actually lowered to RVV instructions. This
is to avoid introducing divisions into the generated code.
Reviewed By: craig.topper
Differential Revision: https://reviews.llvm.org/D106533
Often when lowering vector shuffles, we split the shuffle into two
LHS/RHS shuffles which are then blended together. To do so we split the
original indices into two, indexed into each respective vector. These
two index vectors are then separately lowered as BUILD_VECTORs.
This patch forwards on any undef indices to the BUILD_VECTOR, rather
than having the VECTOR_SHUFFLE lowering decide on an optimal concrete
index. The motiviation for ths change is so that we don't duplicate
optimization logic between the two lowering methods and let BUILD_VECTOR
do what it does best.
Propagating undef in this way allows us, for example, to generate
`vid.v` to produce the LHS indices of commonly-used interleave-type
shuffles. I have designs on further optimizing interleave-type and other
common shuffle patterns in the near future.
Reviewed By: craig.topper
Differential Revision: https://reviews.llvm.org/D104789
Fraser Cormack