RFA: New optimization: Convert local statics into non-statics

[Nick Clifton <nickc at redhat dot com>]

Hi Guys,

  I would like to offer the attached patch consideration and I hope,
  approval to check into gcc.  It implements a new optimization that
  converts static variables local to a function into their non-static
  equivalents.  So for example it would convert this:

    int foo (void) { static int a = 1; return a; }

  into this:

    int foo (void) { int a = 1; return a; }

  The conversion is only done when it can be shown that it is safe to
  do so, which in practice means only when the variable is initialized
  before it is used.  It is only performed for simple types, so arrays
  and structures are not affected, and there is a new param provided
  to stop the optimization from triggering on functions which have
  lots of local variables already and adding some more would increase
  the pressure on the stack too much.

  The benefit of the optimization is that it allows gcc the chance to
  decide that the the converted variable can live entirely in a
  register.  Plus even if the variable is pushed onto the stack, it is
  often the case that accessing variables there is faster than
  accessing variables in global memory.

  The optimization could be performed by hand by editing the source
  code before compiling the program, but this option is not always
  available to the user.  (For example this optimization was motivated
  by the desire to improve the results for a well known benchmark.
  Since the benchmark code could not be altered, the optimization was
  added to the compiler).

  I have not added the optimization to those enabled by -O2 (or -O3)
  because:

    a) I do not know the current policy for deciding when an
    optimization is enabled by a given -Ox level.

    b) The optimization is new and unproven in the wide world of gcc
    users.  (Although we have been using it internally at Red Hat for
    a while and it has proved to be quite effective).

    c) There are some cases where the optimization results in worse
    performance for the compiled program.  (Usually when too many
    local variables end up on the stack).

  The patch includes a new gcc testcase, documentation and an update
  to the changes.html file for gcc 4.4.  Tested without any
  regressions with an x86 native toolchain.

  Please may I apply this patch ?

Cheers
  Nick

gcc/ChangeLog
2008-07-14  Nick Clifton  <nickc@redhat.com>

	* tree-optimize-local-statics.c: New file.  New optimization.
	Converts statics local to a function into non-statics.
	* tree-pass.h: Prototype pass_optimize_local_statics.
	* passes.c (init_optimization_passes): Add
	pass_optimize_local_statics.
	* common.opt (foptimize-local-statics): Add new command line
	option.
	* params.def (slo-local-variables-threshold): Define.
	* Makefile.in (OBJS-common): Add tree-optimize-local-statics.
	Add rule to compile tree-optimize-local-statics.c.
	* doc/invoke.texi: Document new command line option and param.

gcc/testsuite/ChangeLog
2008-07-14  Nick Clifton  <nickc@redhat.com>

	* gcc.dg/tree-ssa/static-local.c: New file.  Test the local
	statics optimization.

Attachment: slo.patch.bz2
Description: BZip2 compressed data