Re: [PATCH] rs6000/test: Adjust p9-vec-length-7 sensitive to unroll [PR103196]

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

Hi!

on 2022/4/8 3:29 AM, Segher Boessenkool wrote:
> Hi!
> 
> On Thu, Apr 07, 2022 at 09:19:51AM -0500, will schmidt wrote:
>> On Mon, 2022-02-28 at 13:37 +0800, Kewen.Lin via Gcc-patches wrote:
>>> As PR103196 shows, p9-vec-length-full-7.c needs to be adjusted as the
>>> complete unrolling can happen on some of its loops.  This patch is to
>>> use pragma "GCC unroll 0" to disable all possible loop unrollings.
>>> Hope it can help the case not that fragile.
>>
>> ok
>>
>> Is the lack of effectiveness of "-fno-unroll-loops" otherwise
>> understood, or is there further issue behind that option? 
> 
> There is -fpeel-loops as well, and cunroll is independent of all of
> these as well?
> 

Yes, kind of.  As below code, unroll-loops and peel-loops only affect
whether it's acceptable to grow size.

  /* Allow cunroll to grow size accordingly.  */
  if (!opts_set->x_flag_cunroll_grow_size)
    opts->x_flag_cunroll_grow_size
      = (opts->x_flag_unroll_loops
         || opts->x_flag_peel_loops
         || opts->x_optimize >= 3);

This flag_cunroll_grow_size doesn't gate the pass, it's only for
"may_increase_size".

  unsigned int val = tree_unroll_loops_completely (flag_cunroll_grow_size, 
						   true);

The cunroll still can take effect if its transformation doesn't grow
size even if we specify -fno-unroll-loops and -fno-peel-loops.

>> I would
>> expect the effect of the option, versus the pragma, two to roughly
>> equivalent.   Obviously it is not.  :-)
> 
> Yes, me too.  And I do not see what makes the difference, if it isn't
> the peel thing :-(
> 
> Ke Wen, can you try with -fno-peel-loops please?
> 

I had a try and it didn't help as the cunroll pass doesn't grow size
for this case.  By the way, -fdisable-tree-cunroll does work.  But I
thought using pragma looks better as it's not an internal thing like
the disabling option and I also expect other future unrolling related
changes would respect it.

Do you also prefer pragma, or want that disabling option?

BR,
Kewen