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.
Segher
