Re: [PATCH 2/8] opensm/complib: define "if" statements with branch prediction hints

[Yevgeny Kliteynik <kliteyn-LDSdmyG8hGV8YrgS2mwiifqBs+8SCbDb@public.gmane.org>]

Hi,

Sorry for the delay with a response - just got back from a long vacation.

On 8/15/2012 12:54 AM, Jason Gunthorpe wrote:
> On Tue, Aug 14, 2012 at 09:39:23PM +0000, Hefty, Sean wrote:
>>> +#define if_PF(cond)		if(CL_PREDICT_FALSE(cond))
>>> +#define if_PT(cond)		if(CL_PREDICT_TRUE(cond))
>>
>> If CL_PREDICT_TRUE/FALSE are too long, why not just shorten those,
>> rather than abstract if statements behind a macro?

It's not the size - it's just the readability of the macro.
First macro (CL_PREDICT_*) is close to the GCC's way to define
it. Second macro (if_PF/T) is close to the usual 'if' statement
syntax.

If you think it's a deal breaker, I can lose the first macro and
have everything in one macro.

> If you are typing these so often (ie more than in some datastructure
> kernels) that the length is a problem then you really should be using
> profile guided optimization instead...

I'm trying to optimize all the 'if' conditions in the SM.
As I described in patch 3/8 (next patch in these series),
I don't touch all the non-fatal cases. The goal is not to
help SM to perform better in general (in which case I agree
than PGO would be the right way to go), but to make sure
that the fatal cases will never be predicted wrong, so any
fatal check (existing or a newly added one) won't affect
the SM performance.

Also, the plan is to define a new ASSERT macro that will
use this if_PF thing to make sure that ASSERT statement
doesn't cost much (if any).

-- YK

> Jason
> 

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