On Sat, 7 Apr 2007 18:32:04 -0700 (PDT) Christoph Lameter <[email protected]> wrote:
> On Sat, 7 Apr 2007, Andrew Morton wrote:
>
> > > I just tried the approach that we discussed earlier and it was not
> > > nice either.
> >
> > We've discussed at least three approaches, so we don't know to what you refer.
>
> Thats the approach of checking two flags at the same time. In that case
> the compiler will generate and "and-immediate" and then a
> "compare-immediate" one branch but .... Yuck.
Right.
movl (%ebx), %eax # <variable>.flags, tmp399
andl $48, %eax #, tmp399
cmpl $48, %eax #, tmp399
je .L265 #,
what's "yuck" about that?
With the single page flag:
movl (%ebx), %eax #* page.521, D.21940
testb $32, %al #, D.21940
jne .L265 #,
So you're talking about saving one sole single silly solitary instruction.
> > Because I don't expect there will be much efficiency difference between the
> > above and the use of another page flag.
>
> Then we end up with all these small efficiency differences in all
> the code paths. I'd rather go for optimal performance in a frequently used
> construct like this.
You can save that worrisome single instruction in the common case by putting the
handling of the uncommon compound pages out of line, as I indicated.
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