We're out of tree codes; now what?

Basile STARYNKEVITCH basile@starynkevitch.net
Mon Mar 19 22:29:00 GMT 2007


Le Mon, Mar 19, 2007 at 05:41:08PM -0500, Gabriel Dos Reis écrivait/wrote:
> Nicholas Nethercote <njn@csse.unimelb.edu.au> writes:
> 
> | On Mon, 19 Mar 2007, Doug Gregor wrote:
> | 
> | >> > It's going to have a big performance impact. To extract a 9-bit value,
> | >> > the compiler will need to do a lot of masking every time it accesses
> | >> > the TREE_CODE.
> | >>
> | > So, about 16% slower with --enable-checking, 5% slower with
> | > --disable-checking.
> | >
> | > Subcodes might still be the way to go, but I'm feeling less bad about
> | > the 9-bit tree code option.
> | 
> | As an interested outsider:  GCC's compile-time speed has been
> | gradually decreasing for a while now.  It seems to be acknowledged as
> | an undesirable thing, but not much has happened to change it.  AIUI,
> | this is largely because it's very difficult.  Nonetheless, seeing a 5%
> | slow-down caused by fixing a data structure design bogon is
> | disappointing.
> 
> yeah, the trouble is that we don't seem to agree on what is good for
> long-term, or if and when we agree whether we would find and allocate
> resources to implement the solution.  So, we end up accumulating
> small% regressions over small% regressions.  Then we get busy with
> short-term fixes, because that is what gets the releases out the door.
> Until the next trouble. 

I'm sorry to ask such a (probably naive) question, but do someone have a
precise idea of the performance strengths and weaknesses of GCC and of other
opensource compilers (like LLVM, Open64, TinyCC, ...) or even compiler for
different languages (SBCL, Haskell, SML, Java, ...). I don't have the big
picture of where is GCC strong - speaking about performance - (w.r.t. to
others compilers) and where is GCC weak.

Maybe a Wiki page on this might help, but I don't have any ideas on this.

Just thinking loud (maybe wrongly). And perhaps is it acceptable to lose
temporarily some performance, if we could hope to recover it later....


Regards.
-- 
Basile STARYNKEVITCH         http://starynkevitch.net/Basile/ 
email: basile<at>starynkevitch<dot>net mobile: +33 6 8501 2359 
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