Xref: utzoo comp.lang.fortran:869 comp.lang.c:11090
Path: utzoo!attcan!uunet!husc6!mailrus!ames!joyce!sri-unix!garth!smryan
From: smryan@garth.UUCP (Steven Ryan)
Newsgroups: comp.lang.fortran,comp.lang.c
Subject: Re: Should I convert FORTRAN code to C?
Summary: Fortran Follies.
Message-ID: <857@garth.UUCP>
Date: 2 Jul 88 21:06:35 GMT
References: <20454@beta.lanl.gov> <6067@megaron.arizona.edu>
Reply-To: smryan@garth.UUCP (Steven Ryan)
Organization: INTERGRAPH (APD) -- Palo Alto, CA
Lines: 32

>>                 An early step in the compiler usually turns the array
>> reference into a linear function on the indices - this throws out the
>> dependency information.
>
>This is not true.  How can information be lost by changing "a[i]" to
>"*(a+i)"?  In fact at least one very fancy FORTRAN compiler converts
>array references to exactly that form to extract dependency
>information that is difficult to recognize otherwise.

[at two fortran compilers--the vectorising compiler I worked on also
operated on an affine function of the subscripts.]

The C subscript, if taken literally, does lose dependency information.

    Strict C parse tree                   possible Fortran parse tree

    convert-*-to-variable                          subscript
              |                                   /         \
        integer-add                        symbol: a      integer-multiply
       /           \                                      /              \
symbol: a        integer-multiply                  element-size           i
                /                \
      element-size                i

By having an explicit subscript node, a Fortran optimiser can easily access
the array spans for each dimension, necessary for vectorisation. It also
simplifies determining what region is referenced or defined, necessary for
common subexpressions.

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