[LLVMdev] About Implementation of Pettis-Hansen's / Gloy's Code Layout Transformation in LLVM

[Arnamoy Bhattacharyya]

Hi;

I was planning to implement a profile guided optimization technique in
LLVM.  In the open source projects list of the LLVM site; I saw "code
layout" is a transformation that can be worth looking at as it will use of
profiles (possibly path profiles).

So I was thinking of implementing either Pettis-Hansen's (Profile guided
code positioning, Pettis & Hansen) or Gloy's (Procedure Placement Using
Temporal-Ordering Information, Gloy, Smith) procedure placement algorithm
 in LLVM because first of all I have not seen any implementation of it in
LLVM till** and second, these are the classic procedure placement
algorithms.

So I would like to get some advice from the seniors -

1.  Has there been any attempt to implement these before that I don't know
about.  And what was the success/ failure of that implementation?

2.  Gloy's algorithm aims at reducing the I-Cache misses.  But how wise
would it be to aim at that to optimize performance while the LLVM already
does something to reduce I-Cache misses?  To rephrase my question, is there
really any scope of improvement for I-Cache misses?  (I know the answer
depends on the kind of application we are trying to compile, but let's say
we are using applications which can have a large number of I-Cache conflict
misses and have a large Working Set as well e.g gcc, go, postscript etc.  I
really don't know how well LLVM handles I-Cache misses for these programs.
 I mentioned them because these are the benchmarks Gloy used to measure
performance of his transformation and they have interesting instruction
memory behaviour )

3.  Is this a good idea in terms of the complexity of implementing it? (To
be frank, I will be doing this work for my Master's thesis and I have just
more than a year in my hand)

Any comment on my idea (whether it is stupid / wise / can't tell without
actually implementing it) would be appreciated.  Also any pointers to how
I-Cache misses are handled (reduced) by LLVM will be good.

Thank you for your help;

** - I have seen that there is already PH's Basic Block Placement
transformation is implemented in LLVM but not Code Layout.
-- 
Arnamoy Bhattacharyya
Athabasca Hall 143
Department of Computing Science - University of Alberta
Edmonton, Alberta, Canada, T6G 2E8
780-680-7073
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