Glas List: Re: [glas] summary: auto-resizing vectors in assignment

From: Erik Charlebois (erikc_at_[hidden])
Date: 2005-02-04 13:15:51

A compiler should be able to recognize the hot path (perhaps using profile 
run data) and reorder the code
so that the branch prediction for the size check chooses the hot path. The 
penalty for the check in the majority
of cases will therefore only be 1 cycle.

Erik Charlebois
Compiler Development, IBM Toronto Lab
Email: erikc_at_[hidden]

Toon Knapen <toon.knapen_at_[hidden]> 
Sent by: glas-bounces_at_[hidden]
02/04/2005 07:44 AM
Please respond to
Generic Linear Algorithm Software

To
Generic Linear Algorithm Software <glas_at_[hidden]>
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Subject
[glas] summary: auto-resizing vectors in assignment

The discussion on auto-resizing (or not) vectors in an assignment are 
very intense so I tried to summarize the discussion on following page:

http://www.femtown.org/glas/doc/auto_resizing_vector_on_assignment.html

Please let me know if there are some points that I did not cover (we had 
some discussions in circles and the same argument was given in different 
forms and thus finally the total list of pro's for one or the other 
approach are still limited). I will add additional arguments to this 
page (so that you don't have to go through the archives the whole time).

Additionally, performance was mentioned a few times and David 
(correctly) argumented that the effect on performance was not proven 
yet. So I tried a to verify the performance-hit using a very simple test 
which copies a large number of vectors. Once the test first verifies the 
size before copying the vector, once it directly copies the vector 
without verifying the sizes. And the test repeats this a few times.
Although the if test costs a few instructions, it is not certain that 
this has an effect on the execution _time_ on intelligent processors 
(like they all are now). However I think my test showed that there is 
indeed a measureable (albeit small) difference. But actually such things 
are exactly why I also invited compiler- and machine-vendors to this 
list because they will certainly be able to clarify this issue (using 
the knowledge on how their machine/cpu works internally and how the 
compiler can optimise the scheduling). Vendors?

toon
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