This should fix the endless PRE antic iterations for good. OK, it's somewhat of a hack but unless I fix another two or three parts of the antic algorithm to not prune values that may later re-appear I see no other way to guarantee this. And this isn't the time to fiddle with PRE further...
Bootstrapped on x86_64-unknown-linux-gnu, testing in progress. The assert I added previously is now defunct for CFG cycles which makes it useless, I'll rip it out as a followup. Richard. 2018-03-13 Richard Biener <rguent...@suse.de> PR tree-optimization/84830 * tree-ssa-pre.c (compute_antic_aux): Intersect the new ANTIC_IN with the old one to avoid oscillations. * gcc.dg/torture/pr84830.c: New testcase. Index: gcc/tree-ssa-pre.c =================================================================== --- gcc/tree-ssa-pre.c (revision 258480) +++ gcc/tree-ssa-pre.c (working copy) @@ -2154,6 +2154,35 @@ compute_antic_aux (basic_block block, bo /* clean (ANTIC_IN (block)) is defered to after the iteration converged because it can cause non-convergence, see for example PR81181. */ + /* Intersect ANTIC_IN with the old ANTIC_IN. This is required until + we properly represent the maximum expression set, thus not prune + values without expressions during the iteration. */ + if (was_visited + && bitmap_and_into (&ANTIC_IN (block)->values, &old->values)) + { + if (dump_file && (dump_flags & TDF_DETAILS)) + fprintf (dump_file, "warning: intersecting with old ANTIC_IN " + "shrinks the set\n"); + /* Prune expressions not in the value set. */ + bitmap_iterator bi; + unsigned int i; + unsigned int to_clear = -1U; + FOR_EACH_EXPR_ID_IN_SET (ANTIC_IN (block), i, bi) + { + if (to_clear != -1U) + { + bitmap_clear_bit (&ANTIC_IN (block)->expressions, to_clear); + to_clear = -1U; + } + pre_expr expr = expression_for_id (i); + unsigned int value_id = get_expr_value_id (expr); + if (!bitmap_bit_p (&ANTIC_IN (block)->values, value_id)) + to_clear = i; + } + if (to_clear != -1U) + bitmap_clear_bit (&ANTIC_IN (block)->expressions, to_clear); + } + if (!bitmap_set_equal (old, ANTIC_IN (block))) { changed = true; Index: gcc/testsuite/gcc.dg/torture/pr84830.c =================================================================== --- gcc/testsuite/gcc.dg/torture/pr84830.c (nonexistent) +++ gcc/testsuite/gcc.dg/torture/pr84830.c (working copy) @@ -0,0 +1,46 @@ +/* { dg-do compile } */ +/* { dg-additional-options "-fno-tree-ch -fno-tree-vrp" } */ + +int x0; + +void +br (int yp, int oo) +{ + int *qi = &yp; + + if (oo == 0) + { +g8: + if (x0 != 0) + x0 = yp; + else if (oo != 0) + x0 = yp; + + if (x0 == 0) + { + *qi = 0; + x0 = *qi; + } + + if (x0 != 0) + { + ++oo; + goto g8; + } + + if (yp == oo) + yp += !!oo; + } + else + { + x0 = 1; + while (x0 < 2) + { + qi = &oo; + ++oo; + x0 = 1; + } + } + + goto g8; +}