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 <rguent...@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" } } */