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, sa...@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.guent...@gmail.com] > Sent: Friday, August 14, 2015 09:28 > To: sa...@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, sa...@hederstierna.com > <fred...@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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