Re: [PATCH 2/3] time: allow gcc to fold constants when using msecs_to_jiffies

[Nicholas Mc Guire <der.herr@hofr.at>]

On Sun, 05 Apr 2015, Joe Perches wrote:

> On Mon, 2015-04-06 at 03:00 +0200, Nicholas Mc Guire wrote:
> > 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.
> 
> Hi Nicholas.
> 
> Your inline version has not worked with any of
> x86-64 gcc 4.4, 4.6, 4.7, or 4.9
> 
> I suggest you add some lines to
> lib/test_module.c/test_module_init like:
> 
> 	unsigned int m;
> 
> 	for (m = 10; m < 200; m += 10)
> 		pr_info("msecs_to_jiffies(%u) is %lu\n",
> 			m, msecs_to_jiffies(m));
> 
> 	pr_info("msecs_to_jiffies(%u) is %lu\n",
> 		10, msecs_to_jiffies(10));
> 	pr_info("msecs_to_jiffies(%u) is %lu\n",
> 		100, msecs_to_jiffies(100));
> 	pr_info("msecs_to_jiffies(%u) is %lu\n",
> 		1000, msecs_to_jiffies(1000));
> 
> Then it's pretty easy to look at the assembly/.lst file
> 
> Your inline function doesn't allow gcc to precompute
> the msecs_to_jiffies value.  The macro one does for all
> those gcc versions.
> 
> Try it and look at the generated .lst files with and
> without the patch I sent.
>
will do that - thanks !

Managed to fool my self - the difference in the .lst/.s files is
simply due to chaning msecs_to_jiffies being inline
(which it was not) and thus it "vanished" - kind of stupid - sorry
back to gcc manual first - need to understand __builtin_constant_p
better and the constraints - from all that I understood it should
be doable both as macro and inline.

thx!
hofrat