Re: Fixed point cairo.. or no cairo?

[Michael Meeks <michael meeks novell com>]

On Sun, 2006-08-20 at 00:12 +0000, Aivars Kalvans wrote:
> That would be an improvement on ARM, but on x86 branching costs more
> than doing the math and simple inlining will perform better:
> 
> Athlon XP
> aivarsk benq ~/dev/poo $ ./cycles
> branching:              24 cycles
> function call:          698 cycles
> inline multiply:        15 cycles

	Out of interest, where are you getting these numbers from ? they
certainly don't tally with my understanding of processors, assembler,
branch prediction. etc.

	__builtin_expect of course, can be used to straighten all the common
paths so there is no icache hit on the fast paths, if we know unit
matricees are the common case.

	Also - * 698 * cycles per function call seems amazingly high; of course
- if you include all the 1-off lazy linking overhead and you run the
function only a handful of times, I guess you could get that number ;-)
but ...

> P.S. Does anyone know if it's possible to inline __adddf3() and
> __muldf3() when compiling with -msoft-float ? It might be that function
> call overhead is the bottleneck and most problems can be solved by
> compiler flags.

	Just read that code: cf. gcc/gcc/gcc/config/arm/ieee754-df.S,
interestingly it has a nice check for:

	@ Convert mantissa to unsigned integer.
	@ If power of two, branch to a separate path.

	some interesting assembler for sure; if you inline it you may find you
bloat the code substantially, and make it slower.

	Either way - IMHO you're confused wrt. the cost of a function call:
that is unless you're passing a ton of huge in-line arguments on the
stack or something :-) When you the generated assembler, about the only
really odd inefficiency to be seen in PIC code is the ebx fixup:

	8b 1c 24                mov    (%esp),%ebx
	c3                      ret

	necessary to access any functions / variables [ and not present on the
x86_84 ].

	HTH,

		Michael.

-- 
 michael meeks novell com  <><, Pseudo Engineer, itinerant idiot