gcc-8.0.0-alpha20180218 snapshot (r257797) takes indefinite time to compile the following snippet w/ -O2 -floop-nest-optimize -fno-tree-copy-prop -fno-tree-fre -fno-tree-loop-ivcanon: int cx; int e6 (int pj, int xe) { for (cx = 0; cx < 2; ++cx) while (xe < 1) { for (cx = 0; cx < 2; ++cx) pj *= 2; if (cx != 0) goto o3; ++xe; } o3: return pj; } % timeout 5 gcc-8.0.0-alpha20180218 -O2 -floop-nest-optimize -fno-tree-copy-prop -fno-tree-fre -fno-tree-loop-ivcanon -c v8gjilr7.c zsh: exit 124 timeout 5 gcc-8.0.0-alpha20180218 -O2 -floop-nest-optimize -fno-tree-copy-pro Two outstanding entries captured by perf top: 61.24% cc1 [.] degenerate_phi_result 38.49% cc1 [.] follow_copies_to_constant
Confirmed.
Probably related to us doing SCEV analysis w/o updating SSA form but follow_copies_to_constant and degenerate_phi_result not honoring that. The following would fix this: Index: gcc/tree-scalar-evolution.c =================================================================== --- gcc/tree-scalar-evolution.c (revision 258030) +++ gcc/tree-scalar-evolution.c (working copy) @@ -280,6 +280,7 @@ along with GCC; see the file COPYING3. #include "params.h" #include "tree-ssa-propagate.h" #include "gimple-fold.h" +#include "tree-into-ssa.h" static tree analyze_scalar_evolution_1 (struct loop *, tree); static tree analyze_scalar_evolution_for_address_of (struct loop *loop, @@ -1545,7 +1546,9 @@ follow_copies_to_constant (tree var) gimple *def = SSA_NAME_DEF_STMT (res); if (gphi *phi = dyn_cast <gphi *> (def)) { - if (tree rhs = degenerate_phi_result (phi)) + if (name_registered_for_update_p (gimple_phi_result (phi))) + break; + else if (tree rhs = degenerate_phi_result (phi)) res = rhs; else break;
So this is really a latent issue and not dependent on -floop-nest-optimize. It materializes during unrolling where we perform SCEV analysis on not up-to-date SSA form when analyzing.
Author: rguenth Date: Thu Mar 8 14:41:39 2018 New Revision: 258365 URL: https://gcc.gnu.org/viewcvs?rev=258365&root=gcc&view=rev Log: 2018-03-08 Richard Biener <rguenther@suse.de> PR middle-end/84552 * tree-scalar-evolution.c: Include tree-into-ssa.h. (follow_copies_to_constant): Do not follow SSA names registered for update. * gcc.dg/graphite/pr84552.c: New testcase. Added: trunk/gcc/testsuite/gcc.dg/graphite/pr84552.c Modified: trunk/gcc/ChangeLog trunk/gcc/testsuite/ChangeLog trunk/gcc/tree-scalar-evolution.c
GCC 6 branch is being closed
Author: rguenth Date: Tue Mar 26 11:18:26 2019 New Revision: 269939 URL: https://gcc.gnu.org/viewcvs?rev=269939&root=gcc&view=rev Log: 2019-03-26 Richard Biener <rguenther@suse.de> Backport from mainline 2019-01-08 Richard Biener <rguenther@suse.de> PR tree-optimization/86554 * tree-ssa-sccvn.c (visit_nary_op): When value-numbering to expressions with different overflow behavior make sure there's an available expression on the path. * gcc.dg/torture/pr86554-1.c: New testcase. * gcc.dg/torture/pr86554-2.c: Likewise. 2018-11-20 Richard Biener <rguenther@suse.de> PR tree-optimization/88105 * tree-ssa-dom.c (pass_dominator::execute): Do not walk backedges. * gcc.dg/gomp/pr88105.c: New testcase. 2018-03-08 Richard Biener <rguenther@suse.de> PR middle-end/84552 * tree-scalar-evolution.c: Include tree-into-ssa.h. (follow_copies_to_constant): Do not follow SSA names registered for update. * gcc.dg/graphite/pr84552.c: New testcase. Added: branches/gcc-7-branch/gcc/testsuite/gcc.dg/gomp/pr88105.c branches/gcc-7-branch/gcc/testsuite/gcc.dg/graphite/pr84552.c branches/gcc-7-branch/gcc/testsuite/gcc.dg/torture/pr86554-1.c branches/gcc-7-branch/gcc/testsuite/gcc.dg/torture/pr86554-2.c Modified: branches/gcc-7-branch/gcc/ChangeLog branches/gcc-7-branch/gcc/testsuite/ChangeLog branches/gcc-7-branch/gcc/tree-scalar-evolution.c branches/gcc-7-branch/gcc/tree-ssa-dom.c branches/gcc-7-branch/gcc/tree-ssa-sccvn.c
Fixed.