Re: [Grep-devel] proposed new function for dfa

[Paul Eggert]

On 11/22/2016 11:56 PM, address@hidden wrote:
> I did some profiling and in this case
> dfasyntax was using a huge percentage of CPU time. I could try to
> reproduce it if I really need to convince you.

Sounds like too much work for too little benefit. Unfortunately I'm 
still confused. If Awk needs to reuse a DFA's syntax, presumably the use 
case would be like this:

   struct dfa *d1 = dfalloc (), *d2 = dfaalloc ();
   dfasyntax (d1, ...);
   for (...)
     {
       dfacopysyntax (d2, d1);
       dfacomp (..., d2, ...);
       dfaexec (d2, ...);
     }
   dfafree (d1); dfafree (d2); free (d1); free (d2)

I don't see how this could work if dfacopysyntax doesn't clear the 
non-syntax part of the struct dfa, and yet you indicated that it's OK to 
remove that memset.

Also, I'm not following how the copied syntax can start with the same 
value for canychar as the original. Aren't these indexes into uncopied data?

More generally, I'm worried that the copied and the original objects 
might share pointers, and that freeing one might harm the other. Is that 
not possible?


Another thought. The intent is that there be a fast way to do what you 
want, and clearly the current dfa.c doesn't do that. How about the 
following idea instead?

* Add an opaque type 'struct dfasyntax' to dfa.h.

* dfacomp accepts a new 'struct dfasyntax *' arg, specifying the syntax.

* dfasyntax no longer accepts a 'struct dfa *' arg.  Instead, it returns 
a pointer of type 'struct dfasyntax *' to a newly allocated opaque 
object O that contains all the info that dfasyntax currently computes.  
O can be freed via 'free'; this shouldn't happen as long as anyone is 
using a struct dfa object for which dfacomp has specified O.

The goal is to give Awk a way to efficiently compute the syntax just 
once, while better handling the abovementioned storage-allocation and 
canychar issues. The use case would be like this:

   struct dfa *d = dfalloc ();
   struct dfasyntax *s = dfasyntax (...);
   for (...)
     {
       dfacomp (..., d, s, ...);
       dfaexec (d, ...);
     }
   dfafree (d); free (d); free (s);