Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!ames!amelia!lemming.nas.nasa.gov!fouts
From: fouts@lemming.nas.nasa.gov.nas.nasa.gov (Marty Fouts)
Newsgroups: comp.lang.fortran
Subject: NEW TOPIC:  Consistency of execution environment
Message-ID: <995@amelia.nas.nasa.gov>
Date: 15 Sep 88 19:14:33 GMT
References: <13570@mimsy.UUCP> <3631@lanl.gov>
Sender: news@amelia.nas.nasa.gov
Reply-To: fouts@lemming.nas.nasa.gov.nas.nasa.gov (Marty Fouts)
Lines: 34


On a particular supercomputer I have access to, a certain C compiler
produces scalar code which is considerablly faster than the vector
code a certain Fortran compiler produces for some particular do loop.
Worse, the Fortran compiler produces *pathatic* scalar code when I
explicitly force it to not vectorize the loop in question.

I mentioned this to an analyst who mumbled some words about preserving
the execution environment, which vaugely reminded me of something, but
I'm not sure what. (;-(  So, a question:

Does the Fortran Standard require the consistency of in memory
representation of data items?  For instance, in the do loop:

      DO 10 I = 1, 100
        J = I
C
C other things which prevent assuming that J is an alias
C
  10  CONTINUE

I know that Fortran isn't required to keep a memory consistent version
of I around, but is it true that some part of the standard can be
interpretted as requiring that the value of J in memory be updated for
each pass through the loop?

Thanks,

Marty
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   |        fouts@lemming.nas.nasa.gov                    |
   |        ...!ames!orville!fouts                        |
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