arm: Only load TLS values when needed

[Jonathan Austin]

Hi André,

On 16/07/13 20:27, André Hentschel wrote:
> Hi Jonathan, First, thank you for your review.
>
> Am 16.07.2013 19:31, schrieb Jonathan Austin:
>> Hi André,
>>
>> On 15/07/13 18:14, André Hentschel wrote:
>>> From: André Hentschel <nerv at dawncrow.de>
>>>
>>> This patch intents to reduce loading instructions when the
>>> resulting value is not used. It's a follow up on
>>> a4780adeefd042482f624f5e0d577bf9cdcbb760
>>>
>>
>> Have you done any benchmarking to see that this has any real
>> impact? Or tested on a !Vv6k system? It looks possible that the
>> only case where this will perform better is where we're using
>> switch_tls_none or switch_tls_software (both rare cases, I think)
>> and there's some change it will make things worse in other cases?
>
> I have to admit that i only tested it on v6k and did no benchmark.
>
Do you have access to anything v6-NOT-k-ish? If not I can try and test 
this on something appropriate. How does your test-case access tpidrurw? 
If it uses inline asm then it won't work on v6-not-k, as those 
instructions aren't defined...

>> One of the reasons for Russell's suggestion of placing the ldrd
>> (which became the two ldr instructions you've removed from
>> __switch_to, in order to maintain building for older cores) where
>> it is was in order to reduce the chance of pipeline stalls.
>>
>> As I've pointed out below, there is some risk that changing that
>> has implications for the v6 only case below (and the v6k case is
>> now more prone to stalls with !CONFIG_CPU_USE_DOMAINS, but newer
>> cores should have more advanced scheduling to avoid such issues
>> anyway...)
>
> I'm not sure how this could make things worse on v6k, could you
> elaborate please? Besides of the ldr and str being too close to each
> other

Yea, that's the only issue, and in the !CONFIG_CPU_USE_DOMAINS case 
things are slightly worse than they were before

> i thought this patch is a good idea, because it removes two ldr
> which are always executed. (Continuing below...)

Indeed, as long as it doesn't cause pipeline stalls then that's a gain 
for some cases :)

[...]
>> Now we've only got one instruction between the store and the load
>> and risk stalling the pipeline...
>>
>> Dave M cautiously says "The ancient advice was that one instruction
>> was enough" but this is very core dependent... I wonder if anyone
>> has a good idea about whether this is an issue here...?
>
> We could use a ldrd at the top, that'd be nearly what we have right
> now, don't we?

Yea, that'd be good - as far as I can see from an 1136 TRM, the ldrd 
*may* be two cycles (depending on alignment of the words) but the ldr 
and ldrne will always be two cycles. Ahhh, the joys of modifying the 
fast path ;)

Jonny
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