Re: Suggested addition to documentation

[Brad Lucier <lucier at math dot purdue dot edu>]

Joe Buch writes:
> 
> Brad Lucier writes:
> 
> > I couldn't find the best place in the documentation to put this note.
> > 
> > This is true for 3.0; I believe there have been three questions
> > about this performance issue since last summer posted to the
> > gcc mail list.
> > 
> > Note: If you write a program using computed gotos, a GCC extension,
> > and you compile the program with optimization level @samp{-O2} or higher,
> > you may get better runtime performance if you disable the global common
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > subexpression elmination (gcse) pass by adding the @samp{-fno-gcse} option'
> > to the command line.
> 
> Do we have enough data to be confident that this is true in general?
> Clearly it is true for your applications, and thanks for all the benchmark
> studies you ran.  But will it be true for (almost) anyone who uses
> computed gotos?
> 

In my note I suggested saying "you may get better runtime performance",
which means also that "you may not get better runtime performance".
I believe that the statement is true as I presented it, i.e., this is
something to try if you have a performance problem.

I suppose that people who don't have a problem don't write to the
gcc mail list :-), but there have been several people writing
codes using computed gotos have noticed this performance problem
with 3.0 versus 2.95.*.  See, e.g.:

http://gcc.gnu.org/ml/gcc/2001-03/msg00296.html

and other messages with this subject line in March,

http://gcc.gnu.org/ml/gcc/2001-01/msg00990.html

and other messages with this subject line in January; and, of
course, there are all of my codes ;-).

Brad