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Postings from April 2000
This is an update on my earlier bug report. After reading the "This Week on
P5P" output by Mark-Jason Dominus I felt sheepish for offering complaints
and no patches. So here is a full response to everything that appears in:
http://www.perl.com/pub/2000/04/p5pdigest/THISWEEK-20000423.html#Pseudohash_Field_Names_Hash_Performance_and_map_Performance
So an update on my complaints from last week and a patch.
First of all I was exactly right about what was going wrong with map() and I
am attaching a patch at the end of this emaill. My patch is not ideal since
how much you resize the stack should depend upon how many times you have had
to resize it already. But it is a significant improvement and perfectly
solves the common array2hash 2-for-1 map.
Secondly there is a patch in the Changes for 5.6 that solves the missing
field-names in pseudo-hash warnings. If there is ever a 5.005_04 I really
want that change in there. (As well as the map() change that I am
including.)
Thirdly I had an off-hand hash comment about making the hashing algorithm
fail gracefully if the hashing function was not working well. Well I did a
lot of looking at Perl's hashing algorithm in hv.c and hv.h, and I am not
sure it is feasible. That is, enough of the API is exposed in the HE and
HEK structs that there would be a huge overhead in having a bucket really
populated by a linked list. Oh, you could do it. For instance if
(he)->hent_hek is NULL then follow (he)->hent_next to a node in a balanced
binary tree. But it would be a lot of overhead, and I for one don't feel
comfortable suggesting a change like that. (Does everyone consistently uses
the convenience macros?)
BTW something random I noticed. Wouldn't the hash value be better in HE
than HEK? Too late to change it now, but since compilers like to allocate
things in powers of 2 that should save a couple of pointers per hash
entry..?
Cheers,
Ben
PS The patch:
--- pp_ctl.c.bak Tue Mar 21 00:42:26 2000
+++ pp_ctl.c Tue Apr 25 04:03:36 2000
@@ -736,6 +736,8 @@
if (diff > PL_markstack_ptr[-1] - PL_markstack_ptr[-2]) {
shift = diff - (PL_markstack_ptr[-1] - PL_markstack_ptr[-2]);
count = (SP - PL_stack_base) - PL_markstack_ptr[-1] + 2;
+ if (shift < count)
+ shift = count; /* Avoid shifting too often */
EXTEND(SP,shift);
src = SP;
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