Re: [PATCH] VECT: Add WHILE_LEN pattern for decrement IV support for auto-vectorization

["Kewen.Lin" <linkw@linux.ibm.com>]

Hi Juzhe,

on 2023/4/14 11:39, juzhe.zhong@rivai.ai wrote:
> And also I already decided to make remove WHILE_LEN pattern since it seems to be unnecessary.
> And as Richard said, it's just a simple airthmetic and it's not worthwhile to do that.
> > So, I plan to replace WHILE_LEN into MIN_EXPR and make everything RVV specific done in RISC-V port.

Yeah, MIN_EXPR is enough for IBM ports, but with seeing the special semantic of vsetvli on
"vl = ceil(AVL / 2) for VLMAX < AVL < 2*VLMAX", I'm not sure if it's a good idea for RV, it seems
to put the burden to RV backend.  For one case that the iteration count is known, on the environment
with the above capability, using the vector setting as [1], assuming the given iterations is 10,
fully unrolled, when using MIN_EXPR, the lengths for two iterations would be folded into 8/2, while
using WHILE_LEN artificial folding can make the lengths be 5/5.  I assumed that on the environment
with the above capability 5/5 is optimal than 8/2? that means if we use MIN then RV backend has to
try to make 8/2 to 5/5.  Or it's trivial since RV backend already supports and plans to support this
kind of vsetvli load re-balancing?

[1] https://gcc.gnu.org/pipermail/gcc-patches/2023-April/615634.html

> I think it's more reasonable for IBM use and more target use in the future.
> 

If RV needs WHILE_LEN, IMHO they can co-exist, like: for ports defining len_{load,store} but no
while_len, use MIN; for ports defining while_len, then use WHILE_LEN.

> So, this patch will need to changed as "introduce a new flow to do vectorization loop control" which is a new loop control flow
> with saturating subtracting n down to zero, and add a target hook for it so that we can switch to this flow ?

Yes, if you don't need WHILE_LEN, this proposal is more like to enhance the current partial
vectorization with length (mainly on length preparation and loop control).  But why would we need
a new target hook?  You want to keep the existing length handlings in vect_set_loop_controls_directly
unchanged? it seems not necessary.  IIUC, not requiring WHILE_LEN also means that this patch
doesn't necessarily block the other RV backend patches on vector with length exploitation since
the existing vector with length support already works well on functionality.

BR,
Kewen