Re: GNAT (GCC) Profile Guided Compilation

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Fri, 29 Jun 2012 03:01:30 -0700 (PDT), Keean Schupke wrote:

> On Friday, 29 June 2012 10:34:19 UTC+1, Dmitry A. Kazakov  wrote:
>> On Fri, 29 Jun 2012 02:17:19 -0700 (PDT), Keean Schupke wrote:
>> 
>>> Anyone have any ideas why profile guided compilation performs so poorly
>>> with Ada compared to C++?
>> 
>> Probably because you programmed it using low level C-ish stuff like
>> addresses instead of accessing data directly. In general, if you write a C
>> program in Ada (or any other language), you cannot expect it do better than
>> in C. To beat C, the precondition is to design it in Ada way.

> In general I find the choice of algorithm dominates performance. In this
> case most of the performance is dominated by a disjoint-set (union-find)
> algorithm operating on an array of data. The data nominally represents a
> 2D space (although I am using a 1D array to avoid the cost of
> multiplication on every access).

That still does not justify machine addresses or pointers to limited
targets.

I would not be so sure that access to 2D array is slower when done by the
compiler. That depends on too many factors.

I also do not know what effect has profiling on inlining and optimizations
of generic instances in presence of numerous very short subprograms. I
guess that should severely distort the picture, maybe beyond recognition. I
must admit it, I never used gcc profiling any seriously, instead, I always
did direct time measures.

If performance is OK without profiling, why do you care?

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de