Messages in this thread | | | Date | Mon, 6 Apr 2015 03:00:25 +0200 | From | Nicholas Mc Guire <> | Subject | Re: [PATCH 2/3] time: allow gcc to fold constants when using msecs_to_jiffies |
| |
On Sun, 05 Apr 2015, Joe Perches wrote:
> On Sun, 2015-04-05 at 09:23 +0200, Nicholas Mc Guire wrote: > > The majority of the msecs_to_jiffies() users in the kernel are passing in > > constants which would allow gcc to do constant folding by checking with > > __builtin_constant_p() in msecs_to_jiffies(). > > > > The original msecs_to_jiffies is renamed to __msecs_to_jiffies and aside > > from the removal of the check for negative values being moved out, is > > unaltered. > > At least for gcc 4.9, this doesn't allow the compiler > to optimize / precalculation msecs_to_jiffies calls > with a constant. > > This does: (on top of your patch x86-64 defconfig) > > $ size vmlinux.o.* > text data bss dec hex filename > 11770523 1505971 1018454 14294948 da1fa4 vmlinux.o.next-b0a12fb5bc8 > 11770530 1505971 1018454 14294955 da1fab vmlinux.o.next-b0a12fb5bc8-inline > 11768734 1505971 1018454 14293159 da18a7 vmlinux.o.next-b0a12fb5bc8-macro > > I think this should still move the if (m) < 0 back into the > original __msecs_to_jiffies function. >
could you check if you can reproduce the results below ? my assumption was that gcc would always optimize out an if(CONST < 0) return CONST; reducing it to the return CONST; only and thus this should not make any difference but Im not that familiar with gcc.
gcc versions here are: for x86 gcc version 4.7.2 (Debian 4.7.2-5) for powerpc it is a gcc version 4.9.2 (crosstool-NG 1.20.0) for arm gcc version 4.9.2 20140904 (prerelease) (crosstool-NG linaro-1.13.1-4.9-2014.09 - Linaro GCC 4.9-2014.09)
Procedure used: root@debian:~/linux-next# make distclean root@debian:~/linux-next# make defconfig root@debian:~/linux-next# make drivers/net/wireless/p54/p54usb.lst root@debian:~/linux-next# make drivers/net/wireless/p54/p54usb.s
same setup in unpatched /usr/src/linux-next/
e.g: root@debian:/usr/src/linux-next# grep msecs_to_jiffies drivers/net/wireless/p54/p54usb.c timeout = jiffies + msecs_to_jiffies(1000); timeout = jiffies + msecs_to_jiffies(1000);
So both calls are constants and should be optimized out if it works as expected.
without the patch applied:
root@debian:/usr/src/linux-next# grep msecs_to_jiffies drivers/net/wireless/p54/p54usb.s call msecs_to_jiffies # call msecs_to_jiffies # root@debian:/usr/src/linux-next# grep msecs_to_jiffies drivers/net/wireless/p54/p54usb.lst timeout = jiffies + msecs_to_jiffies(1000); e19: R_X86_64_PC32 msecs_to_jiffies+0xfffffffffffffffc timeout = jiffies + msecs_to_jiffies(1000); timeout = jiffies + msecs_to_jiffies(1000); fd8: R_X86_64_PC32 msecs_to_jiffies+0xfffffffffffffffc timeout = jiffies + msecs_to_jiffies(1000);
with the patch applied this then gives me:
root@debian:~/linux-next# grep msecs_to_jiffies drivers/net/wireless/p54/p54usb.s root@debian:~/linux-next# grep msecs_to_jiffies drivers/net/wireless/p54/p54usb.lst timeout = jiffies + msecs_to_jiffies(1000); timeout = jiffies + msecs_to_jiffies(1000); timeout = jiffies + msecs_to_jiffies(1000); timeout = jiffies + msecs_to_jiffies(1000);
Conversely in kernel/sched/core.c the msecs_to_jiffies is not a constant and the result is that it calls __msecs_to_jiffies
patched: root@debian:~/linux-next# grep msecs_to_jiffies kernel/sched/core.s call __msecs_to_jiffies #
unpatched: root@debian:/usr/src/linux-next# grep msecs_to_jiffies kernel/sched/core.s call msecs_to_jiffies #
Could you check if you get these results for this test-case ? If this really were compiler dependant that would be very bad.
I did move the < 0 check - but that did not change the situation here. but it well may be that there are some cases where this does make a difference
thx! hofrat
| |