[TCLCORE] Hintermayer's string allocation patch

[Eric M. <er...@aj...>]

(apologies for the verbosity of this message)

Gerhard Hintermayer sent me a patch to adjust the way that the Tcl core
allocates memory for strings when growing them (via an append, for
example).  Formerly, the growth algorithm was:

	newsize = 2 * (oldsize + appendsize)

Now, the growth algorithm is:

	if (oldsize + appendsize >= TCL_GROWTH_LARGE_STRING)
	    newsize = oldsize + (2 * appendsize) +  TCL_GROWTH_MIN_ALLOC
	else 
	    newsize = 2 * (oldsize + appendsize)
	endif

TCL_GROWTH_LARGE_STRING defaults to 1 megabyte; TCL_GROWTH_MIN_ALLOC
defaults to 1 kilobyte, and is included to efficiently handle repeated
small appends to a large string.

The primary goal of this change was to increase the amount of memory that
Tcl could allocate.  With the original algorithm, if you had a string that
was about 1/2 of available memory, you could not add to it anymore,
because it would try to allocate more than all of the available memory.
With the new algorithm, you can effectively allocate all of available
memory (depending on how you are growing the string -- once you get close
to the maximum, you will have to append progressively smaller amounts, but
you should be able to expand to fill all memory, if neccessary).

However, a nice side benefit of the new algorithm is that it boosts core
performance, across the board.  You can run the Tcl benchmark suite to see
all the improvements; I have included just the [append] benchmarks here as
these show the improvements most dramatically, I believe:

000 VERSIONS:                    1:8.4a2 2:8.4a1
001 STR append                        49      65
002 STR append (1KB + 1KB)            42      45
003 STR append (10KB + 1KB)          130     132
004 STR append (1MB + 2b * 1000)   13163   21765
005 STR append (1MB + 1KB)          7977   16320
006 STR append (1MB + 1KB * 20)     8258   16497
007 STR append (1MB + 1MB * 3)     33898   40935
008 STR append (1MB + 1MB * 5)     41330   81164
END VERSIONS:                    1:8.4a2 2:8.4a1

Here, 8.4a2 includes the patch; 8.4a1 has the original behavior.  Nice
work, Gerhard!

   Eric Melski                            The Other Tcl Guy
   ericm at ajubasolutions.com            Ajuba Solutions

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