linux-kernel - Re: [PATCH 5/5] lib: Add error string support to printks

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Date:	Fri, 20 Sep 2013 00:21:25 -0500
From:	Daniel Santos <danielfsantos@....net>
To:	Joe Perches <joe@...ches.com>
CC:	Daniel Santos <daniel.santos@...ox.com>,
	David Howells <dhowells@...hat.com>,
	linux-kbuild <linux-kbuild@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Michal Marek <mmarek@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	George Spelvin <linux@...izon.com>
Subject: Re: [PATCH 5/5] lib: Add error string support to printks


On 09/19/2013 08:07 PM, Joe Perches wrote:
> On Wed, 2013-09-18 at 20:27 -0500, Daniel Santos wrote:
>> if I use ERR_PTR() on a signed int on a x86_64 where pointer
>> is 64 bits and int is 32, wouldn't that mean a signed conversion
>> instruction where the sign bit has to be moved from bit 31 to 63?
> No.  It's cast to long
>
> static inline void * __must_check ERR_PTR(long error)
> {
> 	return (void *) error;
> }

Yes, but it is that cast from int to long that costs us a signed extend 
instruction on platforms where sizeof(int) != sizeof(long).  This 
example should demonstrate the issue:

         extern void funca(void *ptr);

         static inline void * ERR_PTR(long error)
         {
                 return (void *) error;
         }

         void funcb(int i)
         {
                 funca(ERR_PTR(i));
         }

         void funcc(long l)
         {
                 funca(ERR_PTR(l));
         }

And here is the generated code on x86_64 with -O2:

         0000000000000000 <funcb>:
                 return (void *) error;
         }

         void funcb(int i)
         {
                 funca(ERR_PTR(i));
            0:   48 63 ff                movslq %edi,%rdi
            3:   e9 00 00 00 00          jmpq   8 <funcb+0x8>
                                 4: R_X86_64_PC32 funca-0x4
            8:   0f 1f 84 00 00 00 00    nopl 0x0(%rax,%rax,1)
            f:   00

         0000000000000010 <funcc>:
         }

         void funcc(long l)
         {
                 funca(ERR_PTR(l));
           10:   e9 00 00 00 00          jmpq   15 <funcc+0x5>
                                 11: R_X86_64_PC32 funca-0x4


So on x86_64 this movslq is 3 bytes of text, plus register pollution for 
each time you convert an int to a pointer. I don't know the precise 
number of cases where error numbers are passed to printk as ints, but I 
presume it is enough that this could add several kilobytes of text.  
Either way, for a popular function like vsnprintf, it's better to take a 
moderate bloat in the function than a little bloat at many call sites, 
especially when it's not performance critical.

>> Either way, %pE does seem to make a lot of sense for conditions where we
>> already have a pointer that we would otherwise use PTR_ERR() to convert,
>> but it just seems klunky to use it on an int, to have it treated like a
>> pointer and then re-interpreted as an int.  Maybe we can add %pE as well
>> as %dE and leave [ioxXu] out of it?
> I think having just one way to format is better.
>
Yeah, I do agree, I just don't see how to do it without introducing 
unnecessary bloat.

Daniel
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