From: pugh@cs.umd.edu (Bill Pugh)
Subject: Release of v 3.0.0 of Omega test / Extended Tiny
Reply-To: pugh@cs.umd.edu (Bill Pugh)
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Date: Sat, 12 Dec 1992 20:57:45 GMT
Keywords: tools, optimize, FTP

Version 3.0.0 of our implementation of the Omega test is now available for
anonymous ftp from ftp.cs.umd.edu:pub/omega.  The Omega test is
implemented in an extended version of Michael Wolfe's tiny tool, a
research/educational tool for examining array data dependence algorithms
and program transformations for scientific computations.

New features include:

    Array & scalar expansion and privatization

    Reduction dependences

    Induction variable recognition

    Scalar forward substitution

    Storage classes

    A prototype partial implementation of a unified framework
    for reordering transformations. This framework reorders
    reordering transformations such as loop interchange, loop
    skewing, loop distribution, loop fusion and statement reordering.
    This framework can be used to optimize code fragments for 
    parallelism and/or locality.

    A FORTRAN to tiny translator (f2t) based on f2c


Features from version 2.1:

    Exact dependence analysis algorithms. 

    Array kill analysis (determining when intermediate writes kill
    a dependence). Array kill analysis is required in order to
    perform array privatization or expansion.

    Analysis of when "interesting" assertions can eliminate a dependence
    (a dependence is uninteresting if it is false only when the loop that
    carries the dependence executes less than 2 iterations).

This code is freely available for research or commercial use. The extended
version of tiny can be used as a educational or research tool.  Several
research groups are incorporating the Omega test dependence analyzer into
their compilers/programming environments, include groups at University of
Illinois, Urbana-Champaign and at Georgia Tech. Anyone interested in doing
this is strongly urged to stay in contact with us (at omega@cs.umd.edu) as
we are continuing to update and improve the interface that allows external
users to hook into our data dependence analyze.


This release consists of three components:

     *	The Omega test: A system for performing symbolic
	manipulations of conjunctions of linear constraints
	over integer variables. In particular, the operations
	supported include:
		* Checking if integer solutions exist

		* Eliminating an existential quantifier. For example,
		  we can transform 
			{ Exists b s.t. 0 <= a <= 5; b < a <= 5b}
		  to
			{ 2 <= a <= 5}

		* Checking to see if one set of constraints implies
		  another. For example, we can check to see if:
		
                   {0 <= x <= 3; -3 <= 2y <= 3} => {0 <= x+y, x-y <= 3}  

		The interface to the Omega test is described
		in the file doc/omega_interface.tex

     *	The Omega test dependence analyzer: A system built on top
	of the Omega test to analyze array data dependences. This
	system builds the appropriate problems and makes the appropriate
	queries of the Omega test to analyze array-based data dependences
	in programs. The analyzer computes data difference/direction
	vectors, recognizes reduction dependences, analyzes array kills
	(which enables array expansion and array privatization), 
	and determines when assertions can be used to eliminate dependences.

	We have attempted to define an interface that allows other users
        to make use of the Omega test dependence analyzer. This interface
        is described in include/lang-interf.generic and Lang-Interf3.0 
        (keep in touch if you plan to use this interface; we are continuing 
        to refine it).

     *  Extended tiny. We have extended Michael Wolfe's tiny tool. The
	major extensions are:
	
		* Improved user interface (scrolling, dependence browsing
		  windows)

		* Capabilities that improve the accuracy of data dependence
		  analysis (induction variable recognition, forward
		  substitution).

		* Features that demonstrate the additional information we
		  collect (scalar/array expansion and privatization,
		  interactive dependence zapping)

		* A semi-automatic procedure for converting FORTRAN programs
		  into tiny

		* A prototype implementation of a unified framework
		  for reordering transformations. The framework 
		  unifies loop interchange, skewing, distribution,
		  fusion, reversal, statement reordering, and some
		  cases of access normalization, loop alignment, loop
		  peeling and index set splitting.


Also available are copies of several recent technical reports:

    William Pugh & David Wonnacott, Eliminating False Data Dependences using 
	the Omega Test, Tech. Report CS-TR-2993, Dept. of Computer Science,
	Univ. of Maryland, College Park (An earlier version of this paper
	appeared at the ACM SIGPLAN PLDI'92 conference).

                Abstract
      Array data dependence analysis methods currently in use generate 
      false dependences that can prevent useful program transformations. 
      These false dependences arise because the questions asked are
      conservative approximations to the questions we really should be 
      asking.  Unfortunately, the questions we really should be asking
      go beyond integer programming and require decision procedures for 
      a subclass of Presburger formulas.  In this paper, we describe how 
      to extend the Omega test so that it can answer these queries and 
      allow us to eliminate these false data dependences. We have 
      implemented the techniques described here and believe they are 
      suitable for use in production compilers.


    William Pugh, The Definition of Dependence Distance, Tech. 
	Report CS-TR-2992, Dept. of Computer Science, Univ. of 
	Maryland, College Park

                Abstract
      Data dependence distance is widely used to characterize data 
      dependences in advanced optimizing compilers.  The standard 
      definition of dependence distance assumes that loops are normalized 
      (have constant lower bounds and a step of 1); there is not a 
      commonly accepted definition for unnormalized loops.
      
      There are a number of subtleties involved in the definition of 
      dependence distance for unnormalized loops.  In particular, a number 
      of compilers and programming environments seem to be implicitly using
      a definition that can result in non-integral dependence distances.
      Since non-integral distances were not anticipated, the compilers 
      decide that such dependences do not exist. This results in the 
      compiler failing to recognize the existence of a dependence that 
      actually exists, which allows the compiler to make illegal program 
      transformations. This error existed in Parafrase-2, Parascope, and 
      the KAP compiler (this error has been reported and may be corrected 
      in these systems by now).
     

    William Pugh and Dave Wonnacott, Static Analysis of Upper Bounds 
    	on Parallelism, Tech Report CS-TR-2994, Dept. of Computer 
	Science, Univ. of Maryland, College Park

                Abstract
      Substantial parallelism exists in all of the Perfect Benchmarks 
      codes, although existing automatic techniques are able to find it
      in only a few cases.  Exposing much of this parallelism required a 
      careful, manual examination of all the dependences which appeared to 
      prevent parallelism, and the development and manual application of 
      new program transformations that have not yet been automated.

      We describe methods for statically analyzing the program to
      determine which code segments must either be run sequentially or
      rewritten using a new algorithm, and which might contain parallelism
      that automatic techniques are unable to expose. We can also provide
      some feedback about the kinds of additional, possibly manual, 
      analysis or transformations that may be required to expose the 
      parallelism.

      In order to determine upper bounds on parallelism, we describe
      methods for computing exact data dependence information, incorporating
      information about array kills and conditional dependences.  In cases 
      involving non-linear references, our information is inexact but we 
      calculate upper and lower bounds on the true dependences.  Since 
      we do not have to compute this more exact information for every array 
      reference pair in a program, but those for which an apparent 
      dependence threatens our ability to run part of the program in 
      parallel, the increased cost of performing this analysis should not 
      be prohibitive.


    Wayne Kelly and William Pugh, Generating Schedules and Code within a
    	Unified Reordering Transformation Framework, Tech
	Report CS-TR-2995, Dept. of Computer Science, Univ. of 
	Maryland, College Park

                Abstract
      We discuss using schedules as a framework for unifying reordering 
      transformations such as loop interchange, loop skewing, loop 
      distribution and loop alignment. This framework subsumes unimodular 
      loop transformations. We describe algorithms to align schedules
      and to produce the transformed code.

      This framework is capable of handling more complex schedules than we 
      currently generate, such as schedules that describe loop blocking and 
      interleaving. We are able to check the validity of these schedules 
      and produce appropriate transformed code.
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