Re: [PATCH] tree-optimization/108724 - vectorized code getting piecewise expanded

[Jeff Law <jeffreyalaw@gmail.com>]

On 2/13/23 07:51, Richard Biener wrote:
> On Mon, 13 Feb 2023, Jeff Law wrote:
> 
>>
>>
>> On 2/10/23 04:02, Richard Biener via Gcc-patches wrote:
>>> This fixes an oversight to when removing the hard limits on using
>>> generic vectors for the vectorizer to enable both SLP and BB
>>> vectorization to use those.  The vectorizer relies on vector lowering
>>> to expand plus, minus and negate to bit operations but vector
>>> lowering has a hard limit on the minimum number of elements per
>>> work item.  Vectorizer costs for the testcase at hand work out
>>> to vectorize a loop with just two work items per vector and that
>>> causes element wise expansion and spilling.
>>>
>>> The fix for now is to re-instantiate the hard limit, matching what
>>> vector lowering does.  For the future the way to go is to emit the
>>> lowered sequence directly from the vectorizer instead.
>>>
>>> Bootstrapped and tested on x86_64-unknown-linux-gnu, OK?
>>>
>>> Thanks,
>>> Richard.
>>>
>>>   PR tree-optimization/108724
>>>   * tree-vect-stmts.cc (vectorizable_operation): Avoid
>>>   using word_mode vectors when vector lowering will
>>>   decompose them to elementwise operations.
>>>
>>>   * gcc.target/i386/pr108724.c: New testcase.
>> OK.  Though can't this be a problem with logicals too?  Or is there something
>> special about +- going on here?
> 
> Logical ops do not cross lanes even when using scalar operations on GPRs.
> For +- you have to compute the MSB separately to avoid spilling over to
> the next vector lane.
Oh, yes, makes perfect sense.

jeff