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Re: arm-elf-gcc question
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