[cfe-dev] Preprocessor::LookNext

Chris Lattner clattner at apple.com
Mon Jul 7 23:51:24 PDT 2008

On Jul 3, 2008, at 6:14 PM, Argiris Kirtzidis wrote:
> The "nested-name-specifier '::' " syntax occurs at enough places  
> that it would really simplify the code if an efficient  
> Parser::GetLookAheadToken(1) could be used.
> I attached  a Preprocessor::LookNext implementation for this (pp- 
> looknext.patch).
> In the "parse-labeled" patch I use LookNext to determine a label and  
> use a separate Parser::ParseLabeledStatement.

Hi Argiris,

Is there a specific problem with calling LookAhead(0)?  To me, this is  
a performance optimization tweak... I'd rather do this sort of thing  
when more of the C++ front-end is in place.  This would give us better  
grounds for profiling and determine whether it is actually a win in  
the end.

Adding a "LookNext()" method to the preprocessor as a helper for  
LookAhead(0) would be ok if you think it would be significantly useful  
though.  Also, LookAhead could obviously be improved :).

Finally, are you sure it would be completely unreasonable to refactor  
the C++ grammar to avoid the look-ahead in the common case?  While  
always having peektok available is nice, it adds significant cost to  
the hotest part of the C preprocessor.  I would expect a significant  
slowdown from the patch on large .i files with -Eonly for example  
(which spends almost all time in this code).


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