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Date: Wed, 23 Oct 2019 16:37:00 -0000
From: Alexander Monakov <amonakov@ispras.ru>
To: Eduard-Mihai Burtescu <eddyb@lyken.rs>
cc: Ian Lance Taylor <iant@google.com>, gcc-patches <gcc-patches@gcc.gnu.org>,     Ian Lance Taylor <ian@airs.com>
Subject: Re: [PATCH] Refactor rust-demangle to be independent of C++ demangling.
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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.

Alexander