i486 base architecture

[William M. Quarles]

Pekka Pietikainen wrote:
> On Sun, Nov 28, 2004 at 05:54:43PM -0500, Jeff Spaleta wrote:
> 
>>On Sun, 28 Nov 2004 17:47:35 -0500, William M. Quarles 
>>
>>>What kind of pain are we talking about here?
>>
>>just as importantly... what kind of gain do you expect to see?
>>Since the issue raised was gain to pain.... is there really any useful
>>gain in moving to i486 as the base arch?
> 
> Indeed:
> 
> http://www.ee.oulu.fi/~pp/faqentry
> 
> (submitted to the fedora faq some time ago, didn't hear anything back and it's
> potentially a bit too complex for that context).
> 
> For the instruction set bits, 
> Chapter 17 of http://www.intel.com/design/pentiumii/manuals/243192.htm
> has details on the instruction set differences between the different x86
> iterations.
> 
> Just some ballpark figures on how often gcc gets to use these instructions, 
> and this is glibc which might have used these in handcoded assembly:
> (objdump --disassemble /lib/i686/libc.so.6 | grep <instruction> |wc -l )
> 
> cpmxchg:7
> xadd: 8
> bswap: 136
> cmov: 1099 (and this already limits us to non-VIA C3 i686)
> Total lines: 297992
> 
> Doesn't take into account how often this code is called and how much slower 
> the i386 instruction set alternative is in reality. My guess is
> "unmeasurable".
> 
> Someone feel like doing an experiment on some real code, glibc isn't really
> representative of typical code? Just compile some large package
> with different -march= options (keeping mtune at pentium4) and see what
> non-i386 instructions it actually generates. Bonus points for listing 
> the functions and showing whether they are in the oprofile/gprof top #10
> or not.

I would, but are there any free ways of doing benchmarks?  Not to 
mention I'm not really much of a programmer, so I don't know what 
oprofile/gprof are.

BTW, I think that you mean -mcpu, not -mtune, as long as we are talking 
about ix86 processors.

----
Peace,
William