On Mon, 19 May 2003, Jan Hudec wrote:
> On Fri, May 16, 2003 at 05:33:58PM -0400, Ed L Cashin wrote:
> > x ? : y
> > .. is the same as
> > x ? x : y
> >
<snip/>
> > is there some better reason than being one character shorter -- like
> > maybe better asm gets produced when the extension is used?
>
> I would bet it's used because it's two characters less to type... and
> perhaps because anyone who wrote it is used to it and it's more readable
> for them.
You got me curious, so I checked. With the kernel optimisations of -O2
then there's no difference. Hence I'd go with the above suggestion that
it's just something that the author felt comfortable with.
(BTW, I'm Australian, so I can spell "optimisation" with an 's' ;-)
Without optimisation then the generated assembler *is* different.
y = x ? x : 2;
When x is on the stack (at 8(%ebp)) then this translates to:
movl 8(%ebp),%eax
cmpl $0,8(%ebp)
jne .L3
movl $2,%eax
.L3:
...and the result is in %eax. Whereas, the expression
y = x ? : 2;
results in:
movl 8(%ebp),%edx
movl %edx,%eax
testl %edx,%edx
jne .L3
movl $2,%eax
.L3:
However, under -O2 then both notations give:
movl 8(%ebp),%eax
testl %eax,%eax
jne .L3
movl $2,%eax
.L3:
Regards,
Paul Gearon
Software Engineer Telephone: +61 7 3876 2188
Plugged In Software Fax: +61 7 3876 4899
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