Date: Tue, 21 Apr 2015 00:52:35 +1000 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Bruce Evans <brde@optusnet.com.au> Cc: Konstantin Belousov <kostikbel@gmail.com>, Jung-uk Kim <jkim@freebsd.org>, Alan Cox <alc@rice.edu>, John Baldwin <jhb@freebsd.org>, src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r280279 - head/sys/sys Message-ID: <20150421003316.M10305@besplex.bde.org> In-Reply-To: <20150420220347.B9956@besplex.bde.org> References: <201503201027.t2KAR6Ze053047@svn.freebsd.org> <550DA656.5060004@FreeBSD.org> <20150322080015.O955@besplex.bde.org> <17035816.lxyzYKiOWV@ralph.baldwin.cx> <552BFEB2.8040407@rice.edu> <552C215D.8020107@FreeBSD.org> <20150420115351.GD2390@kib.kiev.ua> <20150420220347.B9956@besplex.bde.org>
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On Tue, 21 Apr 2015, Bruce Evans wrote:
> On Mon, 20 Apr 2015, Konstantin Belousov wrote:
>
>> On Mon, Apr 13, 2015 at 04:04:45PM -0400, Jung-uk Kim wrote:
>>> Please try the attached patch.
>>> ...
>>> - __asm __volatile("xorl %k0,%k0;popcntq %1,%0"
>>> - : "=&r" (result) : "rm" (elem));
>>> ...
>>> + __asm __volatile("xorl %k0, %k0; popcntq %1, %0"
>>> + : "=r" (count) : "m" (pc_map[field]));
>> ...
>> Yes, this worked for me the same way as for you, the argument is taken
>> directly from memory, without temporary spill. Is this due to silly
>> inliner ? Whatever the reason is, I think a comment should be added
>> noting the subtlety.
>>
>> Otherwise, looks fine.
>
> Erm, this looks silly. It apparently works by making things too complicated
> for the compiler to "optimize" (where one of the optimizations actually
> gives pessimal spills). Its main changes are:
> ...
> It works better to change the constraint to "r":
It's even sillier than that. The problem is not limited to this function.
clang seems to prefer memory whenever you use the "rm" constraint. The
silliest case is when you have a chain of simple asm functions. Say the
original popcntq (without the xorl):
return (popcntq(popcntq(popcntq(popcntq(popcntq(x))))));
gcc compiles this to 5 sequential popcntq instructions, but clang
spills the results of the first 4.
This is an old bug. clang does this on FreeBSD[9-11]. cc does this
on FreeBSD[10-11] (not on FreeBSD-9 since cc = gcc there.
Asms should always use "rm" if "m" works. Ones in cpufunc.h always
do except for lidt(), lldt() and ltr(). These 3 are fixed in my version.
So cpufunc.h almost always asks for the pessimization.
Bruce
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