[patch][rfa] --gprof Performance Improvement

[Dave Brolley]

Hi,

I'd like to commit the attached patch which improves the performance of 
the --gprof option by an order of magnitude. One benchmark I tried with

  sid --board=<board> --final-insn-count --gprof=<file>,cycles=1

improved from 6963 seconds to 640 seconds. This is on top of the 
improvement I obtained with my previous patch. The culprit was the 
sw-profile-gprof component's use of an attribute interface to obtain the 
pc from each sample. It turns out that this alone completely dominates 
all other aspects of the simulation.

The patch does two things:

1) Make use of a local reference whenever 'this->stats[current_stats]' 
is used more than once in a method of gprof_component. 'this->stats' is 
a vector and this change gave me about a 3% improvement

2) Use a pin interface to provide the pc for each sample to the gprof 
component. This was the big win.

Rather than have the gprof component obtain and parse the value of an 
attribute of the cpu for each sample, instead the cpu now drives the pc 
value on two pins (to handle 64 bit pc's) before driving its 
sample-gprof pin.

Since this represents an interface change, I didn't want to commit it 
with some review/approval, however, as far as I can tell, the gprof 
interface is not used by any existing port.

Comments? OK to commit?

Dave
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