Re: Rerunning CSE after GCSE

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

  In message <10102190018.AA15505@vlsi1.ultra.nyu.edu>you write:
  > I view this as a bit experimental, but when running a Benchmark (in Ada), I
  > noticed that gcse's cprop was propagating the constant into the beginning o
  > f
  > a basic block, but not further.  CSE is needed to propagate the rest of the
  > way.  But if the block in question is in a loop, loop.c may pull the
  > propagation that gcse did outside the loop and the propagation will never
  > occur.
  > 
  > So it's worthwhile to run cse again after gcse.  If if eliminates a jump,
  > further propagation may be possible, so cse is now rerun until it finished
  > propagating through jumps.  I think we'll be able to eliminate this if we
  > remove the skipping of blocks and following branch code from CSE and simpli
  > fy
  > it but we need measurements.
  > 
  > Some tests I did showed this did not slow things down too much, but further
  > investigation and measurements are clearly required.
  > 
  > Sun Feb 18 15:45:17 2001  Richard Kenner  <kenner@vlsi1.ultra.nyu.edu>
  > 
  > 	* toplev.c (note_deferral_of_defined_inlined_function): Argument
  > 	FNDECL may be unused.
  > 
  > 	* toplev.c (rest_of_compilation): Don't have CSE skip blocks or
  > 	follow jumps after first run.
  > 	If -fexpensive-optimizations rerun CSE after GCSE and iterate until
  > 	it doesn't change any jumps.
Please don't do this.  The two existing CSE passes already account for 
50% of the compilation time for many large programs (particularly those
in the plumhall testsuite).  Your change will have a *huge* impact on
our ability to run those tests in a reasonable timeframe.

We really need to speed up local CSE before running it anymore.

jeff