Re: wishlist: support for shorter pointers

["Richard Earnshaw (lists)" <Richard.Earnshaw@arm.com>]

On 03/07/2023 17:42, Rafał Pietrak via Gcc wrote:
> Hi Ian,
> 
> W dniu 3.07.2023 o 17:07, Ian Lance Taylor pisze:
>> On Wed, Jun 28, 2023 at 11:21 PM Rafał Pietrak via Gcc 
>> <gcc@gcc.gnu.org> wrote:
> [--------]
>>> I was thinking about that, and it doesn't look as requiring that deep
>>> rewrites. ABI spec, that  could accomodate the functionality could be as
>>> little as one additional attribute to linker segments.
>>
>> If I understand correctly, you are looking for something like the x32
>> mode that was available for a while on x86_64 processors:
>> https://en.wikipedia.org/wiki/X32_ABI .  That was a substantial amount
>> of work including changes to the compiler, assembler, linker, standard
>> library, and kernel.  And at least to me it's never seemed
>> particularly popular.
> 
> Yes.
> 
> And WiKi reporting up to 40% performance improvements in some corner 
> cases is impressive and encouraging. I believe, that the reported 
> average of 5-8% improvement would be significantly better within MCU 
> tiny resources environment. In MCU world, such improvement could mean 
> fit-nofit of a project into a particular device.
> 
> -R

I think you need to be very careful when reading benchmarketing (sic) 
numbers like this.  Firstly, this is a 32-bit vs 64-bit measurement; 
secondly, the benchmark (spec 2000) is very old now and IIRC was not 
fully optimized for 64-bit processors (it predates the 64-bit version of 
the x86 instruction set); thirdly, there are benchmarks in SPEC which 
are very sensitive to cache size and the 32-bit ABI just happened to 
allow them to fit enough data in the caches to make the numbers leap.

R.