cvs commit: src/sys/sparc64/include in_cksum.h

[Christoph Mallon]

Marius Strobl wrote:
> On Sat, Jun 28, 2008 at 02:08:10PM +0200, Christoph Mallon wrote:
>> Marius Strobl wrote:
>>>> On a related note: Is inline assembler really necessary here? For 
>>>> example couldn't in_addword() be written as
>>>> static __inline u_short
>>>> in_addword(u_short const sum, u_short const b)
>>>> {
>>>>    u_int const t = sum + b;
>>>>    return t + (t >> 16);
>>>> } ?
>>>> This should at least produce equally good code and because the compiler 
>>>> has more knowledge about it than an assembler block, it potentially 
>>>> leads to better code. I have no SPARC compiler at hand, though.
>>> With GCC 4.2.1 at -O2 the code generated for the above C version
>>> takes on more instruction than the inline assembler so if one 
>> On SPARC?  What code does it produce? I have not SPARC compiler at hand.
>> Even if it is one more instruction, I think the reduced register 
>> pressure makes more than up for it.
> 
> Correct, it only uses two registers:
> 
> 0000000000000000 <in_addword>:
>    0:   92 02 00 09     add  %o0, %o1, %o1
>    4:   91 32 60 10     srl  %o1, 0x10, %o0
>    8:   90 02 00 09     add  %o0, %o1, %o0
>    c:   91 2a 20 10     sll  %o0, 0x10, %o0
>   10:   91 32 20 10     srl  %o0, 0x10, %o0
>   14:   81 c3 e0 08     retl 
>   18:   91 3a 20 00     sra  %o0, 0, %o0
>   1c:   01 00 00 00     nop 

One more instruction? That's five instructions for the actual 
calculation afaict, just like the inline assembler version. The sra in 
the delay slot should be present in the inline assembler version, too.

>> This should work fine and only use two registers (though the compiler 
>> can choose to use three, if it deems it beneficial):
>>
>> static __inline u_short
>> in_addword(u_short const sum, u_short const b)
>> {
>>   u_long const sum16 = sum << 16;
>>   u_long const b16   = b   << 16;
>>   u_long       ret;
>>
>>   __asm(
>>     "addcc %1, %2, %0\n\t"
>>     "srl   %0, 16, %0\n\t"
>>     "addc  %0,  0, %0\n"
>>     : "=r" (ret) : "r" (sum16), "r" (b16) : "cc");
>>
>>   return (ret);
>> }
> 
> This is ten instructions with two registers. Where is the
> break even regarding instructions vs. registers for sparc64? :)

I still have no SPARC compiler. Ten instructions? All I did was write 
the two shifts in C and adjust the register constraints. It should 
produce identical code.

>> But I still prefer the C version.
>>
> 
> And I prefer to not re-write otherwise working code for
> micro-optimizations, there are enough unfixed real bugs

Obviously the inline assembler magic did not work and is/was a real bug.

> to deal with. Similarly we should not waste time discussing
> how to possibly optimize MD versions even more but rather
> spend the time improving the MI version so it's good enough
> that using MD versions isn't worth the effort.

The C alternative is MI and in length on par with the inline assembler 
version, isn't it?


Regards
	Christoph