[Bug tree-optimization/113441] [14 Regression] Fail to fold the last element with multiple loop

["juzhe.zhong at rivai dot ai" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113441

--- Comment #24 from JuzheZhong <juzhe.zhong at rivai dot ai> ---
(In reply to Richard Biener from comment #19)
> (In reply to Richard Biener from comment #18)
> > (In reply to Tamar Christina from comment #17)
> > > Ok, bisected to
> > > 
> > > g:2efe3a7de0107618397264017fb045f237764cc7 is the first bad commit
> > > commit 2efe3a7de0107618397264017fb045f237764cc7
> > > Author: Hao Liu <hliu@os.amperecomputing.com>
> > > Date:   Wed Dec 6 14:52:19 2023 +0800
> > > 
> > >     tree-optimization/112774: extend the SCEV CHREC tree with a nonwrapping
> > > flag
> > > 
> > > Before this commit we were unable to analyse the stride of the access.
> > > After this niters seems to estimate the loop trip count at 4 and after that
> > > the logs diverge enormously.
> > 
> > Hum, but that's backward and would match to what I said in comment#2 - we
> > should get better code with that.
> > 
> > Juzhe - when you revert the above ontop of trunk does the generated code
> > look better for Risc-V?
> 
> It doesn't revert but you can do
> 
> diff --git a/gcc/tree-scalar-evolution.cc b/gcc/tree-scalar-evolution.cc
> index 25e3130e2f1..7870c8d76fb 100644
> --- a/gcc/tree-scalar-evolution.cc
> +++ b/gcc/tree-scalar-evolution.cc
> @@ -2054,7 +2054,7 @@ analyze_scalar_evolution (class loop *loop, tree var)
>  
>  void record_nonwrapping_chrec (tree chrec)
>  {
> -  CHREC_NOWRAP(chrec) = 1;
> +  CHREC_NOWRAP(chrec) = 0;
>  
>    if (dump_file && (dump_flags & TDF_SCEV))
>      {

Hmmm. With experiments. The codegen looks slightly better but still didn't
recover back to GCC-12.


Btw, I compare ARM SVE codegen, even with cost model:

https://godbolt.org/z/cKc1PG3dv

I think GCC 13.2 codegen is better than GCC trunk with cost model.