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

[Joe Perches]

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.

cheers, Joe

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