On Sun, 31 May 1998, Simon Kirby wrote:
> On Sun, 31 May 1998, Tigran Aivazian wrote:
>
> > Hello guys,
> >
> > Someone asked me a simple but classical question:
> >
> > What is better:
> >
> > This: if (some_syscall() < 0) {
> > ....
> > }
> >
> > Or this: if (some_syscall() == -1) {
> > ....
> > }
> >
> > For some system calls (e.g. lseek(2)) one does not have the luxury of
> > choice (because negative results aren't necessarily erroneous) but for
> > most others it is only a matter of taste and both are "right"; so I
> > answered "since in the first case cc(1) will generate jge
> > instruction and in the second jne it depends on what is faster: jge or
> > jne".
> >
> > But what *is* faster? I don't know. And Intel's website doesn't
> > immediately face one with something useful, such as a table of timings of
> > their instructions or detailed specs of some future generation
> > processors but instead greets one with tons of useless b.s. :)
>
> Both instructions are equally expensive. ja, jb, je, jne, jg, jl, jge,
> jle take the same number of cycles. It's been this way on every version
> of the x86, I believe.
>
> So, (x < 1), (x <= 1), (x == 1), (x != 1), etc., are all the same. This
> is only on an Intel chip, however. I'm not sure about others...
First, I think I really agree with Monnier that you should never think of
which is _faster_ in this kind of situation. You may use what is the most
obvious.
Compare to any value other than 0 will cause some code like:
cmp ax, -1 (in a MASM form, rather than GNU as'es one)
jle SomeWhere (or ja, jb, jl, jge...)
They use same instruction circle, like what Simon said. However, when
comparing with 0 it should be something like (I'm not sure if GCC is so
wise to do this):
or ax, ax
jle SomeWhere
But the key point is: (according to Intel's 80386 Programmer's Reference)
"The first clock count is for the true condition (branch taken); the
second clock count is for the false condition (branch ont taken)." The
first clock is 7+m in every Jcc instructions other than J(E)CXZ, and the
second is 3. So you may consider in writing:
if (! some condition) {
Something you don't suppose to do;
} else {
Something you really want to do;
} /* Expression A */
However, according to the output of egcs 1.0.2, the result is not what we
supposed. The express of
if (Some condition)
Yes();
else
No(); /* Expression B */
will be translated as
cmpl $0,-4(%ebp)
jne .L4
call Yes
jmp .L5
.align 4
.L4:
call No
.L5:
So that we know Expression B is more efficient, 'cause egcs has reversed
the instructions due to Jcc's (-128 to +127) limit. However the theory is
not proven in i486 chips or newer. It seems the alignation is more
important to newer chips.
.e'osai ko sarji la lojban. ==> Please support the logical language.
co'o mi'e lindjy,min. ==> Goodbye, I'm Lin Zhe Min.
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