Re: More than you ever wanted to know about Fortran array indexing ;-)

[Jeffrey A Law <law at cygnus dot com>]

  In message <9710091754.AA05699@moene.indiv.nluug.nl>you write:
  > Pah, it probably means the PA is broken as an architecture :-) -  
:-) :-)  I've said this more than once myself.

  > I've tried it on machines as far apart as DEC Alpha and Motorola  
  > m68k - all Fortran code I throw at it gets compiled to better code  
  > *with* your `USE' patch; certainly all others (with possibly the x86  
  > as the sole exception) must fall between these extremes ...
Hmmm.  It's always possible I goof'd something during my benchmark runs.

Basically what I saw was inner loops improving, but one level out there
was _severe_ code explosion -- enough that improvements in the inner loops
were completely negated.

There's also the possibility the slowdown is PA specific due to quirks
in the PA's addressing modes.


  > Oh, BTW, making combine_givs_p accept all possible candidate givs  
  > for combining does not result in much improvement, because the  
  > return value of this function is but one of the (half a dozen)  
  > conditions that have to be fulfilled before giv G2 can be combined  
  > with giv G1 (see the routine combine_givs, i.e. without `_p').
Yup.  Found that out myself when I briefly looked at the PA slowdowns.


  > Obviously, this could work correctly if every giv could have a  
  > _list_ of other givs that could be combined with it, and  
  > combine_givs computed the transitive closure of the  
  > G2-combines-with-G1 relation (checking, in the mean time, that the  
  > computational relationship remained a valid addressing mode).
  > 
  > But that requires `some' more work ;-)
A "simple" matter of hacking :-) :-)

jeff