[PATCH][RFC] Move IVOPTs closer to RTL expansion

[Richard Biener]

On Sun, 8 Sep 2013, pinskia@gmail.com wrote:

> On Sep 8, 2013, at 7:01 PM, "Bin.Cheng" <amker.cheng@gmail.com> wrote:
> 
> > On Wed, Sep 4, 2013 at 5:20 PM, Richard Biener <rguenther@suse.de> wrote:
> >> 
> >> The patch below moves IVOPTs out of the GIMPLE loop pipeline more
> >> closer to RTL expansion.  That's done for multiple reasons.
> >> 
> >> First, the loop passes that at the moment preceede IVOPTs leave
> >> around IL that is in desparate need of basic re-optimization
> >> like CSE, constant propagation and DCE.  That puts extra load
> >> on IVOPTs and its cost model, increasing compile-time and
> >> possibly confusing it.
> >> 
> >> Second, IVOPTs introduces lowered memory accesses that it
> >> expects to stay as is, likewise it produces auto-inc/dec
> >> sequences that it expects to stay as is until RTL expansion.
> >> Passes such as DOM can break this expectation and make the
> >> work done by IVOPTs a waste.
> >> 
> >> I remember doing this excercise in the GCC 4.3 timeframe where
> >> benchmarking on x86_64 showed no gains or losses (but x86_64
> >> isn't very sensitive to IV choices).
> >> 
> >> Any help with benchmarking this on targets other than x86_64
> >> is appreciated (I'll re-do x86_64).
> >> 
> >> Bootstrapped and tested on x86_64-unknown-linux-gnu.
> >> 
> >> General comments are of course also welcome.
> >> 
> >> Thanks,
> >> Richard.
> >> 
> >> 2013-09-04  Richard Biener  <rguenther@suse.de>
> >> 
> >>        * passes.def: Move IVOPTs before final DCE pass.
> >>        * tree-ssa-loop.c (tree_ssa_loop_ivopts): Adjust for being
> >>        outside of the loop pipeline.
> >> 
> >>        * gcc.dg/tree-ssa/ivopts-3.c: Scan non-details dump.
> >>        * gcc.dg/tree-ssa/reassoc-19.c: Be more permissive.
> >> 
> >> Index: gcc/passes.def
> >> ===================================================================
> >> *** gcc/passes.def.orig 2013-09-04 10:57:33.000000000 +0200
> >> --- gcc/passes.def      2013-09-04 11:11:27.535952665 +0200
> >> *************** along with GCC; see the file COPYING3.
> >> *** 221,227 ****
> >>          NEXT_PASS (pass_complete_unroll);
> >>          NEXT_PASS (pass_slp_vectorize);
> >>          NEXT_PASS (pass_loop_prefetch);
> >> -         NEXT_PASS (pass_iv_optimize);
> >>          NEXT_PASS (pass_lim);
> >>          NEXT_PASS (pass_tree_loop_done);
> >>        POP_INSERT_PASSES ()
> >> --- 221,226 ----
> >> *************** along with GCC; see the file COPYING3.
> >> *** 237,242 ****
> >> --- 236,246 ----
> >>         opportunities.  */
> >>        NEXT_PASS (pass_phi_only_cprop);
> >>        NEXT_PASS (pass_vrp);
> >> +       /* IVOPTs lowers memory accesses and exposes auto-inc/dec
> >> +          opportunities.  Run it after the above passes cleaned up
> >> +        the loop optimized IL but before DCE as IVOPTs generates
> >> +        quite some garbage.  */
> >> +       NEXT_PASS (pass_iv_optimize);
> >>        NEXT_PASS (pass_cd_dce);
> >>        NEXT_PASS (pass_tracer);
> >> 
> >> Index: gcc/tree-ssa-loop.c
> >> ===================================================================
> >> *** gcc/tree-ssa-loop.c.orig    2013-09-04 10:57:32.000000000 +0200
> >> --- gcc/tree-ssa-loop.c 2013-09-04 11:11:27.536952677 +0200
> >> *************** make_pass_loop_prefetch (gcc::context *c
> >> *** 906,915 ****
> >>  static unsigned int
> >>  tree_ssa_loop_ivopts (void)
> >>  {
> >> !   if (number_of_loops (cfun) <= 1)
> >> !     return 0;
> >> 
> >> -   tree_ssa_iv_optimize ();
> >>    return 0;
> >>  }
> >> 
> >> --- 906,924 ----
> >>  static unsigned int
> >>  tree_ssa_loop_ivopts (void)
> >>  {
> >> !   loop_optimizer_init (LOOPS_NORMAL
> >> !                      | LOOPS_HAVE_RECORDED_EXITS);
> >> !
> >> !   if (number_of_loops (cfun) > 1)
> >> !     {
> >> !       rewrite_into_loop_closed_ssa (NULL, TODO_update_ssa);
> >> !       scev_initialize ();
> >> !       tree_ssa_iv_optimize ();
> >> !       scev_finalize ();
> >> !     }
> >> !
> >> !   loop_optimizer_finalize ();
> >> 
> >>    return 0;
> >>  }
> >> 
> >> Index: gcc/testsuite/gcc.dg/tree-ssa/ivopts-3.c
> >> ===================================================================
> >> *** gcc/testsuite/gcc.dg/tree-ssa/ivopts-3.c.orig       2013-09-04 10:57:33.000000000 +0200
> >> --- gcc/testsuite/gcc.dg/tree-ssa/ivopts-3.c    2013-09-04 11:11:27.559952952 +0200
> >> ***************
> >> *** 1,5 ****
> >>  /* { dg-do compile } */
> >> ! /* { dg-options "-O2 -fdump-tree-ivopts-details" } */
> >> 
> >>  void main (void)
> >>  {
> >> --- 1,5 ----
> >>  /* { dg-do compile } */
> >> ! /* { dg-options "-O2 -fdump-tree-ivopts" } */
> >> 
> >>  void main (void)
> >>  {
> >> *************** void main (void)
> >> *** 8,12 ****
> >>      f2 ();
> >>  }
> >> 
> >> ! /* { dg-final { scan-tree-dump-times "!= 0" 5 "ivopts" } }  */
> >>  /* { dg-final { cleanup-tree-dump "ivopts" } }  */
> >> --- 8,12 ----
> >>      f2 ();
> >>  }
> >> 
> >> ! /* { dg-final { scan-tree-dump-times "!= 0" 1 "ivopts" } }  */
> >>  /* { dg-final { cleanup-tree-dump "ivopts" } }  */
> >> Index: gcc/testsuite/gcc.dg/tree-ssa/reassoc-19.c
> >> ===================================================================
> >> *** gcc/testsuite/gcc.dg/tree-ssa/reassoc-19.c.orig     2012-12-18 14:24:58.000000000 +0100
> >> --- gcc/testsuite/gcc.dg/tree-ssa/reassoc-19.c  2013-09-04 11:13:30.895416700 +0200
> >> *************** void foo(char* left, char* rite, int ele
> >> *** 16,22 ****
> >>    }
> >>  }
> >> 
> >> ! /* { dg-final { scan-tree-dump-times "= \\\(sizetype\\\) element" 1 "optimized" } } */
> >>  /* { dg-final { scan-tree-dump-times "= -" 1 "optimized" } } */
> >>  /* { dg-final { scan-tree-dump-times " \\\+ " 1 "optimized" } } */
> >>  /* { dg-final { cleanup-tree-dump "optimized" } } */
> >> --- 16,22 ----
> >>    }
> >>  }
> >> 
> >> ! /* { dg-final { scan-tree-dump-times "= \\\(\[^)\]*\\\) element" 1 "optimized" } } */
> >>  /* { dg-final { scan-tree-dump-times "= -" 1 "optimized" } } */
> >>  /* { dg-final { scan-tree-dump-times " \\\+ " 1 "optimized" } } */
> >>  /* { dg-final { cleanup-tree-dump "optimized" } } */
> > 
> > Hi,
> > IVOPT transformation depends on loop invariant heavily, it generates
> > some loop invariants during rewriting iv uses and depends on
> > loop-invariant pass to hoist them outside of loop, so the position of
> > loop invariant pass may matter too if we move IVOPT.
> 
> Except other optimizations depend on lim before it too: vect is an example.

We already run LIM twice, moving the one that is currently after
IVOPTs as well should be easy.  But of course as you note IVOPTs
may introduce loop invariant code it also may introduce full
redundancies in the way it re-writes IVs.  And for both people may
claim that we have both CSE and LIM on the RTL level, too.

Richard.