Re: [PATCH 2/3] ARM: convert to generated system call tables

[Robert Jarzmik <robert.jarzmik-GANU6spQydw@public.gmane.org>]

Geert Uytterhoeven <geert-Td1EMuHUCqxL1ZNQvxDV9g@public.gmane.org> writes:

> On Sat, Oct 22, 2016 at 10:23 PM, Robert Jarzmik <robert.jarzmik-GANU6spQydw@public.gmane.org> wrote:
>> Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org> writes:
>>
>>> On Friday, October 21, 2016 4:48:56 PM CEST Russell King - ARM Linux wrote:
>>>> What's the point of the x32 mode?
>>>
>>> On x86, the motivation is faster code for most use cases that
>>> don't need a lot of memory, as the 64-bit opcodes have 16 registers
>>> rather than 8 in 32-bit mode but 32-bit pointers have lower
>>> cache footprint than 64-bit pointers.
>>
>> For completness, the second point of x32 AFAIU is the IP-relative addressing
>> which is not available in standard 32 bit mode, which improves PIC code. For
>> simple not algorithmic code (think Android HAL for example) with many shared
>> libraries, it's better in the Hardware Abstraction Layer Libraries, instead of
>> the push-to-stack and pop register.
>
> But that's not an advantage compared to full am64 mode, right?
Nope, the amd64 (that's what you have in mind with am64, not aarch64 right ?)
mode has the IP-relative has well, so x32 has no advantage over amd64 in this
area.

x32 over amd64 advantage is cache and memory footprint AFAICT, I don't see other
benefits. There doesn't seem to be any ISA differences, or I didn't really
notice in my daily system performance analysis job.

There was a nice presentation made by Peter Anvin at a Plumber conference, here
: http://linuxplumbersconf.org/2011/ocw/sessions/531

Cheers.

-- 
Robert