Re: [ECOS] arm-elf-gcc question

[Grant Edwards <grante@visi.com>]

On Fri, Nov 10, 2000 at 03:47:28PM -0700, Gary Thomas wrote:

> On 10-Nov-2000 Grant Edwards wrote:
> > This is really a gcc question, but I figure this is the list
> > with the most people using the same version I am.  ;)
> > 
> > Do other people with the arm-elf-gcc 2.95.2 with ecos patches
> > get this sort of incredibly odd-looking code, or is mine
> > broken?
> > 
> > [I've only written two compilers in my life, neither of which
> > was anything to brag about, but...  yikes!]
> 
> This basically a jump table representing your switch statement.
> The compiler makes choices about how to implement such a statement
> and in this case, it was decided that a table of addresses indexed
> by the "case" selector (i.e. a jump table) was the fastest/cheapest
> way to go.
> 
> What did you want/expect instead?

I dunno.  Something more like what you get if you write it as
an equivalent if/else.  I thought it was pretty standard for
compilers to figure out whether a switch() was better
represented by sequential tests or by a jump table.  For a
sparsely populated "case space" compilers I've used in the past
have generally swtiched to sequential compares to save space.
Even with size optimization turned on (-Os), it generates the
jumptable version which is 5X larger than sequential compares.

Memory is cheap, but it's never cheap enough.  ;)

If the size of the case space is increased slightly (from 0x20
to 0x28) gcc does switch to sequential compares.

The threshold could probably be lower -- especially on the ARM.
The ARM better at comparing for multiple values than many other
CPUs.  You can test for any of 8 const values in 8 instructions
(best case):

        cmp    r3, #1
        cmpne  r3, #2
        [...]
        cmpne  r3, #8
        
While on other CPUs it takes roughly twice as many instructions.        

-- 
Grant Edwards
grante@visi.com