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Date: Wed, 26 May 2021 08:22:34 -0500
From: Segher Boessenkool <segher@kernel.crashing.org>
To: Richard Biener <rguenther@suse.de>
Cc: Jiufu Guo <guojiufu@linux.ibm.com>, gcc-patches@gcc.gnu.org,
 wschmidt@linux.ibm.com, dje.gcc@gmail.com, jlaw@tachyum.com,
 bin.cheng@linux.alibaba.com
Subject: Re: [PATCH V2] Split loop for NE condition.
Message-ID: <20210526132233.GJ10366@gate.crashing.org>
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On Wed, May 26, 2021 at 11:50:07AM +0200, Richard Biener wrote:
> > We can split the loop into two loops:
> > 
> >   while (++k > n)
> >     a[k] = b[k]  + 1;
> >   while (l++ < n)
> >     a[k] = b[k]  + 1;
> > 
> > then for the second loop, it could be optimized.
> 
> Btw, I think even the first loop should be vectorized.  I see we do
> not handle it in niter analysis:
> 
> Analyzing loop at t.c:3
> t.c:3:14: note:  === analyze_loop_nest ===
> t.c:3:14: note:   === vect_analyze_loop_form ===
> t.c:3:14: note:    === get_loop_niters ===
> t.c:3:14: missed:   not vectorized: number of iterations cannot be 
> computed.
> 
> but the number of iterations should be UINT_MAX - k (unless I'm
> missing sth), may_be_zero would be sth like k < n.  It would be
> nice to not split this into loops that niter analysis cannot handle ...

As long as it doesn't do that for signed loop counters, because that
would be a waste -- ever executing such code is UB, so vectorising it
will only cost extra insns (usually).


Segher