Re: [Xen-devel] [PATCH] Cleanup use of strlen() to check for empty string

[Jacob Gorm Hansen <jacobg@xxxxxxx>]

Oops sorry I take that back, gcc3 just produces different output...

Never mind then. Must get coffee.

Jacob

On 10/5/05, Jacob Gorm Hansen <jacobg@xxxxxxx> wrote:
> On 10/5/05, Petersson, Mats <mats.petersson@xxxxxxx> wrote:
>
> > I created a function:
> >
> > int foo(char *s)
> > {
> >         if (strlen(s)) return 1; else return 0;
> > }
> >
> > Compiled with gcc 4.0.0 as:
> > gcc -O3 -s x.c (or gcc -O2 ...)
> > Gives the following code:
> >
> > foo:
> >         xorl  %eax, %eax
> >         cmpb  $(0), (%rdi)
> >         sete  %al
> >         ret
>
> Cool, I am surprised gcc4 is able to fix this. gcc 3.3.4 which I am
> using is not, even with -O3. How does gcc4 behave without -O3?
>
> foo:
> push   %ebp
> mov    %esp,%ebp
> mov    0x8(%ebp),%edx
> pop    %ebp
> cmpb   $0x0,(%edx)
> setne  %dl
> movzbl %dl,%eax
> ret
>
> I guess I only reacted to this because I have seen worse examples
> resulting from the "strlen is constant-time" assumption (people
> iterating over a string in a while(strlen(s)) loop), and because I was
> bored with trying to get block devices running with xenbus. :-)
>
> Jacob
>


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