[cfe-commits] r171262 - /cfe/trunk/lib/AST/CommentLexer.cpp

[Matthieu Monrocq]

On Sun, Dec 30, 2012 at 8:45 PM, Dmitri Gribenko <gribozavr at gmail.com>wrote:

> Author: gribozavr
> Date: Sun Dec 30 13:45:46 2012
> New Revision: 171262
>
> URL: http://llvm.org/viewvc/llvm-project?rev=171262&view=rev
> Log:
> Comment lexing: replace manual comparison with StringRef::find_first_of
>
> This gives an about 1.8% improvement on Clang bootstrap with
> -Wdocumentation
>
> Modified:
>     cfe/trunk/lib/AST/CommentLexer.cpp
>
> Modified: cfe/trunk/lib/AST/CommentLexer.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/AST/CommentLexer.cpp?rev=171262&r1=171261&r2=171262&view=diff
>
> ==============================================================================
> --- cfe/trunk/lib/AST/CommentLexer.cpp (original)
> +++ cfe/trunk/lib/AST/CommentLexer.cpp Sun Dec 30 13:45:46 2012
> @@ -415,15 +415,12 @@
>          return;
>
>        default: {
> -        while (true) {
> -          TokenPtr++;
> -          if (TokenPtr == CommentEnd)
> -            break;
> -          const char C = *TokenPtr;
> -          if(C == '\n' || C == '\r' ||
> -             C == '\\' || C == '@' || C == '&' || C == '<')
> -            break;
> -        }
> +        size_t End = StringRef(TokenPtr, CommentEnd - TokenPtr).
> +                         find_first_of("\n\r\\@&<");
> +        if (End != StringRef::npos)
> +          TokenPtr += End;
> +        else
> +          TokenPtr = CommentEnd;
>          formTextToken(T, TokenPtr);
>          return;
>        }
>
>
Interesting! I just had a look at the implementation of "find_first_of" and
realized it built an array of bits (thanks to bitset) so that it had a O(nb
chars + len string) complexity.

This table implementation seems very much akin to what a compiler would
generate for a switch though, and thus I wonder whether the compiler could
optimize a manual for loop with a switch even better; notably because the
set of characters to look for is known at compile time.

Did you tested this approach ?

-- Matthieu
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