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
