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Re: [rfc] struct dictionary


On Thu, 24 Apr 2003 22:17:23 -0400, Andrew Cagney <ac131313 at redhat dot com> said:


Blocks currently store symbols in one of three different ways:
using a hash table, using an unsorted list, or using a sorted list.
Most blocks are built by buildsym.c, which use only the former two
mechanisms.  Sorted list blocks are only being produced by
mdebugread.c.  And, to make matters worse, jv-lang.c produces one
unsorted list block for which the predicate BLOCK_SHOULD_SORT
matches; the chain of events by which GDB actually treats that
block correctly is very tenuous.


Um, didn't BLOCK_SHOULD_SORT get deleted?


No.  I tried in last fall, but Jim didn't like it because it probably
would have made performance for mdebugread be awful.  Daniel recently
proposed hashing sorted blocks on the fly, but he withdrew that patch
quickly when he found a way to work around whatever problem caused him
to be particularly annoyed with sorted blocks.

Hmm, the lack of activity towards mdebugread suggests it's either dieing or dead. Perhaphs by dropping that sort we (as in the developer community) can encourage it in the direction we (as developers) would likely prefer ....


Dig dig: http://sources.redhat.com/ml/gdb-patches/2002-09/msg00564.html
My reading is that there was simply a request for more tangable numbers on how bad mdebug read was going to become. So ..., what was the performance gain for stabs when the hash table was added? Given that a binary sort's [average] performance is not as good as a hashtable, that number should give you an upper bound on the consequences.


Dig dig: http://sources.redhat.com/ml/gdb-patches/2001-05/msg00508.html
suggests that things could potentially be twice as slow ``Oops''. Looking at `break captured_main; run' I suspect it is typically less.


Assuming, now that there is a number, is this ok? I'm willing to be the fall-guy (my job) and be the one adding something to the NEWS file.

Basically, either somebody needs to buildsym-ify mdebugread.c or else
symbol lookup has to be made a bit more abstract.  These would
actually both be good ideas for reasons other than getting rid of
BLOCK_SHOULD_SORT, so it's certainly not an either-or choice.

Ok, humor me ...
http://sources.redhat.com/ml/gdb/2003-04/msg00017.html
why even build these data structures during symbol reading? It takes time and space, yet is probably never used. Why not on-demand build this dictionary specialized for the block? If the user for some reason accesses a block then, in all probablility, they will make later accesses to that same block - getting benefit.


David Carlton
carlton at math dot stanford dot edu

Andrew




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