On Wed, Oct 23, 2019 at 06:46:14PM +0200, Jakub Jelinek wrote:
> On Wed, Oct 23, 2019 at 11:37:26AM -0500, Segher Boessenkool wrote:
> > On Wed, Oct 23, 2019 at 07:22:47PM +0300, Alexander Monakov wrote:
> > > On Wed, 23 Oct 2019, Eduard-Mihai Burtescu wrote:
> > > > @@ -384,6 +384,14 @@ rust_demangle_callback (const char *mangled, int
> > > > options,
> > > > return 0;
> > > > rdm.sym_len--;
> > > >
> > > > + /* Legacy Rust symbols also always end with a path segment
> > > > + that encodes a 16 hex digit hash, i.e. '17h[a-f0-9]{16}'.
> > > > + This early check, before any parse_ident calls, should
> > > > + quickly filter out most C++ symbols unrelated to Rust. */
> > > > + if (!(rdm.sym_len > 19
> > > > + && !strncmp (&rdm.sym[rdm.sym_len - 19], "17h", 3)))
> > >
> > > This can be further optimized by using memcmp in place of strncmp, since
> > > from
> > > the length check you know that you won't see the null terminator among
> > > the three
> > > chars you're checking.
> > >
> > > The compiler can expand memcmp(buf, "abc", 3) inline as two comparisons
> > > against
> > > a 16-bit immediate and an 8-bit immediate. It can't do the same for
> > > strncmp.
> >
> > The compiler does not currently do that, but it *could*. Or why not? The
> > compiler is always allowed to load 3 characters here, whether some string
> > has a NUL character earlier or not.
>
> It is valid to call strncmp (mmap(...)+page_size-1, "abc", 3), the reading
> of the string should stop when 0 is seen.
Where does it say that, though? I don't see where it prohibits reading
more characters (up to 3 here), and you can get much better code using
that.
I of course know that for e.g. strcmp or strlen we need to be careful of
page crossings; but this is strncmp, which has a size argument saying the
size of the array objects of its arguments!
Segher
