Re: [PATCH] vect: Check that vector factor is a compile-time constant

[Michael Collison <collison@rivosinc.com>]

Okay there seems to be consensus on using constant_lower_bound (vf), but 
I don't understand how that is a replacement for "vf.is_constant ()"? In 
one case we are checking if "vf" is a constant, on the other we are 
asking for the lower bound. For the crash in question 
"constant_lower_bound (vf) " returns the integer value of two.

On 2/27/23 09:51, Richard Sandiford wrote:
> FWIW, this patch looks good to me.  I'd argue it's a regression fix
> of kinds, in that the current code was correct before variable VF and
> became incorrect after variable VF.  It might be possible to trigger
> the problem on SVE too, with a sufficiently convoluted test case.
> (Haven't tried though.)
>
> Richard Biener <richard.guenther@gmail.com> writes:
>> On Wed, Feb 22, 2023 at 12:03 AM Michael Collison <collison@rivosinc.com> wrote:
>>> While working on autovectorizing for the RISCV port I encountered an
>>> issue where vect_do_peeling assumes that the vectorization factor is a
>>> compile-time constant. The vectorization is not a compile-time constant
>>> on RISCV.
>>>
>>> Tested on RISCV and x86_64-linux-gnu. Okay?
>> I wonder how you arrive at prologue peeling with a non-constant VF?
> Not sure about the RVV case, but I think it makes sense in principle.
> E.g. if some ISA takes the LOAD_LEN rather than fully-predicated
> approach, it can't easily use the first iteration of the vector loop
> to do peeling for alignment.  (At least, the IV steps would then
> no longer match VF for all iterations.)  I guess it could use a
> *different* vector loop, but we don't support that yet.
>
> There are also some corner cases for which we still don't support
> predicated loops and instead fall back on an unpredicated VLA loop
> followed by a scalar epilogue.  Peeling for alignment would then
> require a scalar prologue too.
>
>> In any case it would probably be better to use constant_lower_bound (vf)
>> here?  Also it looks wrong to apply this limit in case we are using
>> a fully masked main vector loop.  But as said, the specific case of
>> non-constant VF and prologue peeling probably wasn't supposed to happen,
>> instead the prologue usually is applied via an offset to a fully masked loop?
> Hmm, yeah, agree constant_lower_bound should work too.
>
> Thanks,
> Richard
>
>> Richard?
>>
>> Thanks,
>> Richard.
>>
>>> Michael
>>>
>>> gcc/
>>>
>>>       * tree-vect-loop-manip.cc (vect_do_peeling): Verify
>>>       that vectorization factor is a compile-time constant.
>>>
>>> ---
>>>    gcc/tree-vect-loop-manip.cc | 2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/gcc/tree-vect-loop-manip.cc b/gcc/tree-vect-loop-manip.cc
>>> index 6aa3d2ed0bf..1ad1961c788 100644
>>> --- a/gcc/tree-vect-loop-manip.cc
>>> +++ b/gcc/tree-vect-loop-manip.cc
>>> @@ -2930,7 +2930,7 @@ vect_do_peeling (loop_vec_info loop_vinfo, tree
>>> niters, tree nitersm1,
>>>          niters = vect_build_loop_niters (loop_vinfo, &new_var_p);
>>>          /* It's guaranteed that vector loop bound before vectorization is at
>>>         least VF, so set range information for newly generated var. */
>>> -      if (new_var_p)
>>> +      if (new_var_p && vf.is_constant ())
>>>        {
>>>          value_range vr (type,
>>>                  wi::to_wide (build_int_cst (type, vf)),
>>> --
>>> 2.34.1
>>>