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Re: Hashing of "switch/case" selections
- To: Michael Meissner <meissner at cygnus dot com>
- Subject: Re: Hashing of "switch/case" selections
- From: Andy Walker <jawalker at stl dot quik dot com>
- Date: Sun, 07 Jan 2001 20:39:39 -0600
- CC: Zack Weinberg <zackw at stanford dot edu>, GCC general mailing list <gcc at gcc dot gnu dot org>
- References: <3A4AC4FC.35E70BEE@stl.quik.com> <20010104013910.F12948@wolery.stanford.edu> <20010105135428.B2417@cse.cygnus.com>
Interesting comment. I will take it into account.
Michael Meissner wrote:
> On Thu, Jan 04, 2001 at 01:39:10AM -0800, Zack Weinberg wrote:
> > On Wed, Dec 27, 2000 at 10:43:40PM -0600, Andy Walker wrote:
> > [much interesting stuff]
> > > My experience as a programmer is that programs are almost never written
> > > with switches to more than a few hundred cases. I personally have never
> > > seen one. More than that, and the programmer finds a more efficient,
> > > more readable, more maintainable solution, and he can do so because he
> > > has a deep knowledge of the problem that is unavailable to the
> > > compiler. My seat-of-pants estimate is that 400 cases is a good, maybe
> > > even high, maximum for Hash evaluations. A program containing a switch
> > > statement with more than 400 cases in it probably has enough other
> > > severe design problems that it will never be worth running anyway.
> > This is true in my experience for code written by humans, but not
> > necessarily for machine-generated code. Take a look at insn-attrtab.c
> > in your GCC build directory, or at cp/parse.c in the source tree.
> Some of the simulators in the gdb releases have the ability to build very giant
> switch tables to speed up decoding machine instructions, though we haven't
> built them with the largest tables in quite some time, because the compiler's
> memory was going into the gigabytes.
> At a former job, I recall hearing that they had built a profiler/simulator for
> the machine, and used a completely dense 65k element switch table.
> Michael Meissner, Red Hat, Inc. (GCC group)
> PMB 198, 174 Littleton Road #3, Westford, Massachusetts 01886, USA
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