Re: PATCH for CSE

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

  In message <199807012334.QAA25625@smtp.earthlink.net>you write:
  > 
  > Here's a patch that improves execution time by a factor of three, for
  > one loop I've been looking at.  It reduces instruction count by a
  > similar factor, partly by enabling other optimizations.
  > 
  > When we had something like:
  > 
  >   (SET (REG 1) (ADDRESSOF (REG 2)))
  > 
  >   (SET (MEM (REG 1)) (REG 3))
  > 
  > CSE's find_best_addr decided that REG1 was a great address because it
  > was already in a register, and (ADDRESSOF (REG 2))) was lousy because
  > it had a higer ADDRESS_COST.  This patch introduces a new macro, for
  > use only in CSE because after that point ADDRESSOFs are purged, which
  > favors (ADDRESSOF (REG)).  That allows other mechanisms in CSE to
  > simplify (MEM (ADDRESSOF (REG 2))) to just plain REG 2.
  > 
I'm not sure if this is safe.  Consider if the ADDRESSOF was used
because we needed to take the address of a local variable and pass
it to a subroutine.  Then assume the subroutine stores into the the
location.

Later we have a read of that variable via (mem (adddressof (reg)) that
we turn into (reg) and miss the side effect of the subroutine call we
just made.  In fact, we've already got a bug of this nature that was
reported yesterday :(


Seems to me this is only safe if we can eliminate every addressof for
a particular register.

I think we can create (mem (addressof (reg)) expressions in cse, but
we can't collapse it into (reg) until we purge the addressof expressions..

jeff