llvm dev - Feb 2013 - [LLVMdev] arm compiler benchmarks

I haven't tried using -Os/z on any ARM device for the last 3 years, and
back then, -Os would break many things.

People normally care about code size on Cortex-R/M and ARM9 or older, and
in there, not many LLVM users.

--renato


On 27 February 2013 00:38, Jim Grosbach <grosbach at apple.com> wrote:
> I've not run any on non-iOS devices, and haven't looked at GCC
since
> v4.2.1 due to licensing issues, so I don't have a good feel for
comparative
> benchmarking.
>
> -Jim
>
>
> On Feb 26, 2013, at 4:20 PM, Reed Kotler <rkotler at mips.com> wrote:
>
> What about benchmarks on other Arm devices?
>
> On 02/26/2013 02:52 PM, Jim Grosbach wrote:
>
> Cortex-M0 is a Thumb1 only device. There hasn't been any concerted
> effort to tune LLVM's Thumb1 output in quite a long time. Even back
then
> (2008 or so), the effort was mainly to get it to work acceptably, with
> the real performance tuning work being done for Thumb2. I'm not
> surprised that an embedded market focussed compiler like IAR is better
> tuned for a chip like that.
>
> -Jim
>
> On Feb 26, 2013, at 2:11 PM, matic at nimp.co.uk <mailto:matic at
nimp.co.uk<matic at nimp.co.uk>
> >
> wrote:
>
> Hi,
>
>
> I didn't do any serious benchmarking but so far I never found a case
> where LLVM does better than IAR on CortexM0, but I saw a lot of cases
> where IAR output is better than LLVM...
>
> That said I did not use -Os or -Oz, I just used -O3.
>
> A recurrent situation is that LLVM push/pop one or two extra registers
> compared to IAR, I guess it does so in order to comply with a standard
> ABI or something but in the world of deeply embedded system, we don't
> care of such things 99.99% of the time...
>
> If you want to see some good code output, take a look at IAR, there is
> a free version limited in code size online.
>
> Cheers,
>
> Sebastien
>
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On 27 Feb 2013, at 00:45, Renato Golin <renato.golin at linaro.org> wrote:
> People normally care about code size on Cortex-R/M and ARM9 or older, and
in there, not many LLVM users.
There are a lot of A8 devices around with 256KB (or less) of L2 cache (32KB of
L1 i-cache), and so code density, if not code size, matters a lot for these. 
Cache sizes in mobile chips tend to be as small as possible, as it's very
hard to turn off those transistors (there are several projects at ARM and
elsewhere that I'm aware of in this direction, but I don't know of any
in shipping products yet).

David

On 27 February 2013 08:54, David Chisnall <David.Chisnall at
cl.cam.ac.uk>wrote:
> > People normally care about code size on Cortex-R/M and ARM9 or older,
> and in there, not many LLVM users.
>
> There are a lot of A8 devices around with 256KB (or less) of L2 cache
> (32KB of L1 i-cache), and so code density, if not code size, matters a lot
> for these.  Cache sizes in mobile chips tend to be as small as possible, as
> it's very hard to turn off those transistors (there are several
projects at
> ARM and elsewhere that I'm aware of in this direction, but I don't
know of
> any in shipping products yet).
>
I didn't mean to say that code size is not important, especially for ARM,
just that the average LLVM user (even ARM users) will not care yet that
much.

Most benchmarks I've seen around are all platform software, comparing
performance with Atom and not a single mention on code size. When I did
some benchmarks a few years back, I noticed that LLVM's code was at least
twice as big as GCC with the same specs, some times more than 4x bigger,
would be good to know how it compares now to see how much people really
care about code size.

Reed,

Will you be able to share your results? I think you'll find that LLVM has
indeed progressed in that area, as I've seen some commits going through
during the last few years on that area. I'd like to know if we're
getting
close to GCC.

cheers,
--renato
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