About loop unrolling and optimize for size

[Simon Schubert]

I just tried both -Os -fpeel-loops and -Os -frename-registers for a code
I'm trying to compact, and both settings increased code size compared to
-Os.

cheers
  simon


On 08/14/2015 11:53 AM, sarah@hederstierna.com wrote:
> I think I found explanation, the -fpeel-loops trigger some extra flags:
>
> from "toplev.c":
>
>   /* web and rename-registers help when run after loop unrolling.  */
>   if (flag_web == AUTODETECT_VALUE)
>     flag_web = flag_unroll_loops || flag_peel_loops;
>
>   if (flag_rename_registers == AUTODETECT_VALUE)
>     flag_rename_registers = flag_unroll_loops || flag_peel_loops;
>
> actually its -frename-registers that causes the code size to decrease.
> This flags seems to be set when enable -fpeel-loops.
>
> Maybe this flag could be enabled in -Os, shouldn't have any downside besides makes possibly debugging harder?
>
> Thanks/Fredrik
>
>
> ________________________________________
> From: Richard Biener [richard.guenther@gmail.com]
> Sent: Friday, August 14, 2015 09:28
> To: sarah@hederstierna.com
> Cc: gcc@gcc.gnu.org
> Subject: Re: About loop unrolling and optimize for size
>
> On Thu, Aug 13, 2015 at 6:26 PM, sarah@hederstierna.com
> <fredrik@hederstierna.com> wrote:
>> Hi
>> I'm using an ARM thumb cross compiler for embedded systems and always do optimize for small size with -Os.
>>
>> Though I've experimented with optimization flags, and loop unrolling.
>>
>> Normally loop unrolling is always bad for size, code is duplicated and size increases.
>>
>> Though I discovered that in some special cases where the number of iteration is very small, eg a loop of 2-3 times,
>> in this case an unrolling could make code size smaller - eg. losen up registers used for index in loops etc.
>>
>> Example when I use the flag "-fpeel-loops" together with -Os I will 99% of the cases get smaller code size for ARM thumb target.
>>
>> Some my question is how unrolling works with -Os, is it always totally disabled,
>> or are there some cases when it could be tested, eg. with small number iterations, so loop can be eliminated?
>>
>> Could eg. "-fpeel-loops" be enabled by default for -Os perhaps? Now its only enabled for -O2 and above I think.
> Complete peeling is already enabled with -Os, it is just restricted to
> those cases where GCCs cost modeling of the
> unrolling operation determines the code size shrinks.  If you enable
> -fpeel-loops then the cost model allows the
> code size to grow - sth not (always) intended with -Os.
>
> The solution is of course to improve the cost modeling and GCCs idea
> of followup optimization opportunities.
> I do have some incomplete patches to improve that and hope to get back
> to it for GCC 6.
>
> If you have (small) testcases that show code size improvements with
> -Os -fpeel-loops over -Os and you are
> confident they are caused by unrolling please open a bugzilla containing them.
>
> Thanks,
> Richard.
>
>> Thanks and Best Regards
>> Fredrik


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