Re: [ECOS] arm-elf-gcc question

[Gary Thomas <gthomas at redhat dot com>]

This is definitely a C compiler issue.   I'd suggest that you
take it to them (gcc@gcc.gnu.org) if you want to pursue it.

On 10-Nov-2000 Grant Edwards wrote:
> 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