dependence graphs for vector machines
Fri, 16 Apr 1993 17:18:52 GMT

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dependence graphs for vector machines (1993-04-16)
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Newsgroups: comp.compilers
Keywords: optimize, vector, question
Organization: Compilers Central
Date: Fri, 16 Apr 1993 17:18:52 GMT

We are two PhD students working on instruction scheduling for vector
processors. We would like to test different register allocators and
instruction scheduling algorithms on a set of FORTRAN vectorizable loops.

In order to do that we need to have the (low-level) dependence graph of each
loop, something like:

              VLD VLD VLD VLD
\ / \ /
\ / \ /
\ / \ /
\ /
\ /
\ /
\ /
\ /
\ /

Our problem is how to obtain these graphs from the Fortran loops. We are
thininkg on the following solutions:

  1) Compile the loops with a real vectorizing compiler ( the Convex one in
        our case) and get the assembler produced. Eliminate all the scalar code
        from the assembler and keep only the vector part. Write a little program
        that constructs the dependence graph from that ( renaming registers to
        eliminate artificial dependencies ).
        Problems with this approach:
          - Difficult to handle memory aliases (i.e. is VLD 300(a3),r1
          dependent on VLD 500(a3),r2 or not ? )
          - The graph produced is? (terribly?) biased by the compiler used

  2) Given that we are interested in loops, and usually the body of a loop is
        "simple" (mainly expressions and assignment), write a tiny compiler
        that parses the loops and generates the graph.
        Problems with this approach:
          - We won't be able to perform all the usual optimizations that a
          compiler does (cse, strength reduction ...) so we'll probably end up
          with a graph not very representative of real programs

  3) Is there any tool that does what we want ?
  4) Does the latest GNU gcc version (2.3.3?) vectorize for the convex ?
  4) Is there source code for a vectorizing compiler freely available ?
  5) Any other suggestions ?


Roger Espasa

Marta Jimenez

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