Re: [Xen-devel] [PATCH v2 for-next 2/2] x86/string: Use compiler __builtin_str*() where possible

[Julien Grall <julien.grall@xxxxxxx>]

Hi Andrew,

On 12/05/17 16:30, Andrew Cooper wrote:
> On 12/05/17 15:56, Jan Beulich wrote:
> > > > > On 12.05.17 at 16:34, <andrew.cooper3@xxxxxxxxxx> wrote:
> > > --- a/xen/include/asm-x86/string.h
> > > +++ b/xen/include/asm-x86/string.h
> > > @@ -10,4 +10,12 @@
> > >  #define __HAVE_ARCH_MEMSET
> > >  #define memset(s, c, n)       __builtin_memset(s, c, n)
> > >
> > > +#define strcmp(s1, s2)        __builtin_strcmp(s1, s2)
> > > +#define strncmp(s1, s2, n)    __builtin_strncmp(s1, s2, n)
> > > +#define strcasecmp(s1, s2)    __builtin_strcasecmp(s1, s2)
> > > +#define strchr(s1, c)         __builtin_strchr(s1, c)
> > > +#define strrchr(s1, c)        __builtin_strrchr(s1, c)
> > > +#define strstr(s1, s2)        __builtin_strstr(s1, s2)
> > > +#define strlen(s1)            __builtin_strlen(s1)
> > If the lack of __HAVE_ARCH_* additions is intentional here,
> Yes - it is deliberate.
>
> > why do you keep them for mem*()?
> We have x86-specific implementation of mem*(), while we use the common
> implementation of str*().
>
> Defining __HAVE_ARCH_STR* causes the common implementation to be
> omitted, resulting in a link failure.
>
>
> Given that all supported compilers have these builtins, I think it might
> be better to make this adjustment in common code.  The arguments for
> using them in x86 are the same as ARM.
>
> Julien/Stefano: Thoughts?
Using our own implementation rather than the built-in version gives us 
the liberty to do optimization based on the processor using alternative.
I know that we already use built-in for arch_fetch_and_add, but I am 
planning to revert that as we want to take advantage of new atomics 
instruction in ARMv8.1.
Furthemore, someone mentioned a potential legal issue to linked against 
the built-in. Anyone heard about that?
Cheers,

--
Julien Grall

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