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Re: PATCH RFC: Patches to speed up the demangler



On Dec 28, 2003, at 3:56 PM, Daniel Berlin wrote:



On Dec 28, 2003, at 2:47 PM, Daniel Jacobowitz wrote:


On Sun, Dec 28, 2003 at 10:56:43AM -0800, Mark Mitchell wrote:
On Sat, 2003-12-20 at 12:16, Ian Lance Taylor wrote:
I spent some time investigating how to speed up the new demangler,
because it turns out to affect gdb startup time on C++ executables.

I don't have anything bad to say about your patch, but I do think that
the behavior you mention here is a bug in GDB. I noticed that ages ago,
but have never done anything about it. GDB should lazily demangle
names; that would be much more scalable. In fact, I would keep an LRU
cache of a few thousand such names; in a very big program, the memory
required to demangle all of the names may well be prohibitive.

The issue seems to be a little more complex than that. Given "break
foo", we need to find all the methods whose base name is foo - the most
apparent way to handle the existing symbol table does not lend itself
to a query of that sort without either demangling, or substantial hacks
for each supported mangling, to guarantee that _Z3foo1a and _Z3foo1b
hash to nearby locations.

Uh, this is one of the things search trees excel at. libiberty has an implementation of ternary search tree. Try it.


You can simply insert all names, and map them to a linked list of symbols with that name, and then use that list to see which ones are approriate for the given context.

I forgot to add the important part for mangled names:
In addition, you can simply do a pattern search. Ternary search trees lend themselves to "a*b" type searches as well. So for "foo::bar" you just search for "*foo*bar" in the mangled name TST (or is that *bar*foo").


I never got around to implementing the pattern searches for the TST, but i guess i could if it's useful.

If you want it even faster, you could simply build a search tree based on an understanding of our mangling format so that we index all symbols containing given names on that name.

IE when we see 3foo, we insert that symbol (regardless of the rest of the name) in the list attached to the tree for "foo".

Then for foo::bar, you can simply do a search on foo, a search on bar, take the intersection, and go from there.

This should trim the number of things you have to demangle.


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