From 33031ee69097eeac734dfba91b717d5e9a583073 Mon Sep 17 00:00:00 2001 From: Richard Sandiford Date: Fri, 1 Jun 2018 12:49:44 +0000 Subject: [PATCH] Fix phi backedge detection in backprop (PR85989) This PR is a nasty wrong code bug due to my fluffing a test for a backedge in gimple-ssa-backprop.c. Backedges are supposed to be from definitions in the statement we're visiting to uses in statements that we haven't visited yet. However, the check failed to account for PHIs in the current block that had already been processed, so if two PHIs in the same block referenced each other, we'd treat both references as backedges. In more detail: The first phase of the pass goes through all statements in an arbitrary order, making optimistic assumptions about any statements that haven't been visited yet. The second phase then calculates a correct (supposedly maximal) fixed point. Although the first phase order is arbitrary in principle, we still use the CFG rpo to cut down on the backedges. This means that the only thing that's truly arbitrary is the order that we process the PHIs in a block. Any order should be OK and should eventually give the same results. But we have to follow whatever order we pick, and the pass wasn't doing that. 2018-06-01 Richard Sandiford gcc/ PR tree-optimization/85989 * gimple-ssa-backprop.c (backprop::m_visited_phis): New member variable. (backprop::intersect_uses): Check it when deciding whether this is a backedge reference. (backprop::process_block): Add each phi to m_visited_phis after visiting it, then clear it at the end. gcc/testsuite/ PR tree-optimization/85989 * gcc.dg/torture/pr85989.c: New test. From-SVN: r261064 --- gcc/ChangeLog | 10 +++++++++ gcc/gimple-ssa-backprop.c | 21 +++++++++++++---- gcc/testsuite/ChangeLog | 5 +++++ gcc/testsuite/gcc.dg/torture/pr85989.c | 31 ++++++++++++++++++++++++++ 4 files changed, 63 insertions(+), 4 deletions(-) create mode 100644 gcc/testsuite/gcc.dg/torture/pr85989.c diff --git a/gcc/ChangeLog b/gcc/ChangeLog index c9f581c0ed5..87305aebeeb 100644 --- a/gcc/ChangeLog +++ b/gcc/ChangeLog @@ -1,3 +1,13 @@ +2018-06-01 Richard Sandiford + + PR tree-optimization/85989 + * gimple-ssa-backprop.c (backprop::m_visited_phis): New member + variable. + (backprop::intersect_uses): Check it when deciding whether this + is a backedge reference. + (backprop::process_block): Add each phi to m_visited_phis + after visiting it, then clear it at the end. + 2018-06-01 Richard Biener * tree-vectorizer.h (vect_dr_stmt): New function. diff --git a/gcc/gimple-ssa-backprop.c b/gcc/gimple-ssa-backprop.c index 9ab655c43e8..bbc6311ff28 100644 --- a/gcc/gimple-ssa-backprop.c +++ b/gcc/gimple-ssa-backprop.c @@ -260,6 +260,11 @@ private: post-order walk. */ auto_sbitmap m_visited_blocks; + /* A bitmap of phis that we have finished processing in the initial + post-order walk, excluding those from blocks mentioned in + M_VISITED_BLOCKS. */ + auto_bitmap m_visited_phis; + /* A worklist of SSA names whose definitions need to be reconsidered. */ auto_vec m_worklist; @@ -496,8 +501,11 @@ backprop::intersect_uses (tree var, usage_info *info) { if (is_gimple_debug (stmt)) continue; - if (is_a (stmt) - && !bitmap_bit_p (m_visited_blocks, gimple_bb (stmt)->index)) + gphi *phi = dyn_cast (stmt); + if (phi + && !bitmap_bit_p (m_visited_blocks, gimple_bb (phi)->index) + && !bitmap_bit_p (m_visited_phis, + SSA_NAME_VERSION (gimple_phi_result (phi)))) { /* Skip unprocessed phis. */ if (dump_file && (dump_flags & TDF_DETAILS)) @@ -505,7 +513,7 @@ backprop::intersect_uses (tree var, usage_info *info) fprintf (dump_file, "[BACKEDGE] "); print_generic_expr (dump_file, var); fprintf (dump_file, " in "); - print_gimple_stmt (dump_file, stmt, 0, TDF_SLIM); + print_gimple_stmt (dump_file, phi, 0, TDF_SLIM); } } else @@ -629,7 +637,12 @@ backprop::process_block (basic_block bb) } for (gphi_iterator gpi = gsi_start_phis (bb); !gsi_end_p (gpi); gsi_next (&gpi)) - process_var (gimple_phi_result (gpi.phi ())); + { + tree result = gimple_phi_result (gpi.phi ()); + process_var (result); + bitmap_set_bit (m_visited_phis, SSA_NAME_VERSION (result)); + } + bitmap_clear (m_visited_phis); } /* Delete the definition of VAR, which has no uses. */ diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog index 12ed15f42d9..050e7545ac1 100644 --- a/gcc/testsuite/ChangeLog +++ b/gcc/testsuite/ChangeLog @@ -1,3 +1,8 @@ +2018-06-01 Richard Sandiford + + PR tree-optimization/85989 + * gcc.dg/torture/pr85989.c: New test. + 2018-06-01 Richard Biener PR middle-end/86017 diff --git a/gcc/testsuite/gcc.dg/torture/pr85989.c b/gcc/testsuite/gcc.dg/torture/pr85989.c new file mode 100644 index 00000000000..4dff5e69089 --- /dev/null +++ b/gcc/testsuite/gcc.dg/torture/pr85989.c @@ -0,0 +1,31 @@ +/* { dg-do run } */ + +#define N 9 + +void __attribute__((noipa)) +f (double x, double y, double *res) +{ + y = -y; + for (int i = 0; i < N; ++i) + { + double tmp = y; + y = x; + x = tmp; + res[i] = i; + } + res[N] = y * y; + res[N + 1] = x; +} + +int +main (void) +{ + double res[N + 2]; + f (10, 20, res); + for (int i = 0; i < N; ++i) + if (res[i] != i) + __builtin_abort (); + if (res[N] != 100 || res[N + 1] != -20) + __builtin_abort (); + return 0; +} -- 2.30.2