On Fri, Dec 11, 2020 at 09:45:15AM +0000, David Laight wrote:
> From: Arvind Sankar
> > Sent: 10 December 2020 18:14
> ...
> > I wasn't aware of str_has_prefix() at the time. It does seem useful to
> > eliminate the duplication of the string literal, I like the
> > skip_prefix() API suggestion, maybe even
> >
> > bool str_skip_prefix(const char **s, const char *pfx)
> > {
> > size_t len = str_has_prefix(*s, pfx);
> > *s += len;
> > return !!len;
> > }
> > ...
> > if (str_skip_prefix(&option, prefix)) { ... }
> >
> > to avoid the intermediate variable.
>
> That'll generate horrid code - the 'option' variable has to be
> repeatedly reloaded from memory (unless it is all inlined).
>
> Perhaps the #define
>
> #define str_skip_prefix(str, prefix) \
> {( \
> size_t _pfx_len = strlen(prefix)); \
> memcmp(str, pfx, _pfx_len) ? 0 : ((str) += _pfx_len, 1); \
> )}
>
> There's probably something that'll let you use sizeof() instead
> of strlen() for quoted strings (if only sizeof pfx != sizeof (char *)).
>
> David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
Avoiding the intermediate varable is for readability at the call-site,
not performance. The performance of this function shouldn't matter --
you shouldn't be parsing strings in a hotpath anyway, and nobody cares
if the EFI stub takes a few nanoseconds longer if that makes the code
more robust and/or readable.
That said,
- I'd be surprised if the overhead of an L1D cache access was measurable
over the strncmp()/strlen() calls.
- You can't replace strncmp() with memcmp().
- Regarding sizeof() vs strlen(), you can simply use __builtin_strlen()
to optimize string literals, there's no need for hacks using the
preprocessor.
